ELECTRICAL RESIDENTIAL & COMMERCIAL CALCULATIONS

BONDING & GROUNDING & NEC GUIDELINES

Dwelling Unit Feeder/Service Conductor Calculations  

 

How do you size conductors for residential services and feeders?

Whether they're for journeymen, master electricians, or contractors, most electrical licensing exams require you to calculate residential loads and service or feeders sizes using one of two methods. The standard method, which is contained in Art. 220, Part II, involves more steps, but many people use it exclusively to avoid using the wrong method. However, most residential construction qualifies for the optional method in Art. 220, Part III, so it's prudent to understand both methods. In either case, you're free to exceed the NEC requirements — these are minimum requirements, not design specifications.

The standard method is where we'll start. It requires six sets of calculations for general lighting and receptacles, small-appliance, and laundry; air conditioning versus heat; appliances; clothes dryer; cooking equipment; and conductor size.

The following example should help illustrate how to apply these steps.

What size service conductor does a 1,500 sq ft dwelling unit need, if it contains the following loads? The service is 120/240V.

• Disposal (940VA)

• Dishwasher (1,250VA)

• Trash compactor (1,100VA)

• Water heater (4,500VA)

• Dryer (4,000VA)

• Cooktop (6,000VA)

• Two ovens (each 3,000VA)

• Air conditioning (5 hp, 230V)

• Three electric space heating units (each 3,000W)

General lighting and receptacles, small-appliance, and laundry. The NEC recognizes these circuits won't all be on (loaded) simultaneously. Thus, you may apply a demand factor to the total connected general lighting and receptacle load (220.16). To determine the service/feeder demand load, refer to Table 220.11 and follow these steps:

 

First, determine the total connected load for general lighting and receptacles (3VA per sq ft) [Table 220.3(A)], two small-appliance circuits each at 1,500VA, and one laundry circuit at 1,500VA (220.16) (Fig. 1).

Second, apply Table 220.11 demand factors to the total connected general lighting and receptacle load. Calculate the first 3,000VA at 100% demand and the remaining VA at 35% demand.

 

 

 

 

 

Fig. 1. In calculating conductor sizes for a dwelling unit, the NEC recognizes a homeowner won’t run all the appliances, turn on all the lights, and load all receptacles at the same time.

General lighting/receptacles: 1,500 sq ft× 3VA=4,500VA

Small-appliance circuits: 1,500VA×2 =3,000VA

Laundry circuit: 1,500VA×1= 1,500VA

Total connected load: 4,500VA+ 3,000VA+1,500VA=9,000VA

First 3,000VA at 100%=3,000VA× 1.00=3,000VA

Remainder at 35%=(9,000VA- 3,000VA)×0.35=2,100VA

Total demand load=5,100VA

Air-conditioning versus heat. Because air-conditioning and heating loads aren't on simultaneously, you may omit the smaller of the two loads (220.21). Calculate each of these at 100% (220.15) (Fig. 2).

Air conditioning: 5 hp, 230V

VA=E×I (Table 430.148)

28 FL×230V=6,440 VA

Heat: 3,000W×3 units=9,000W

Fig. 2. It should be obvious that the heating and cooling systems won’t be generated at the same time. Therefore, your calculations should be based on the larger of the two loads. The air conditioning load is smaller than the heat load, therefore it can be omitted.

Fig. 3. When determining load for appliances, the Code allows you to use a 75% demand factor when four or more fastened-in-place appliances are on the same feeder. Appliances. Per 220.17, you can use a 75% demand factor when four or more “fastened in place” appliances, such as a dishwasher or waste disposal, are on the same feeder. Don't include clothes dryers, cooking equipment, air conditioning, or heat in this category (Fig. 3).

Waste disposal: 940VA

Dishwasher: 1,250VA

Trash compactor: 1,100VA

Water heater: 4,500VA

Total connected appliance load: 7,790VA× 0.75=5,843VA

Fig. 3. When determining load for appliances, the Code allows you to use a 75% demand factor when four or more fastened-in-place appliances are on the same feeder.

Clothes dryer. Per 220.18, the feeder or service demand load for electric clothes dryers in a dwelling unit shall not be less than 5,000W. However, if the nameplate rating exceeds 5,000W, use that rating as the load. You can omit this calculation if the unit has no electric dryer provision. However, it's common to provide both gas and electric sources. If you see gas on the plans, verify electric will be omitted (Fig. 4).

Fig. 4. Electric clothes dryer demand must be accounted for as a separate load item.

The service and feeder demand load for a 4kW dryer is 5,000W.

Cooking equipment. For household-cooking appliances rated higher than 1.75kW, you can use the demand factors listed in 220.19, Table and Notes 1, 2, and 3.

All three of the units in the example are rated higher than 1.75kW and not higher than 8.75kW, so follow the instructions in Note 3. The two ovens are rated less than 3.5kW, so Table 220.19 Column A demand factor applies. The cooktop is 6kW, so Column B demand factor applies (Fig. 5).

Fig. 5. See Table 220.19 to determine demand load for cooling equipment rated higher than 1.75kW.

Column A demand: 3kW×2 units× 0.75 demand factor=4.5kW

Column B demand: 6kW×1 unit× 0.8=4.8kW

Demand load=4.5kW+4.8kW= 9.3kW=9,300W

Feeder and service conductor size. 400A and less: For 3-wire, 120/240V, single-phase systems, size the feeder or service conductors according to Table 310.15(B)(6). For all others, use Table 310.16. Size the grounded (neutral) conductor to the maximum unbalanced load (220.22) per Table 310.16.

Over 400A: Size the ungrounded and grounded (neutral) conductors per Table 310.16.

Now we can total up the demand loads from steps 1 through 5.

Step 1: 5,100VA

Step 2: 9,000VA

Step 3: 5,843VA

Step 4: 5,000VA

Step 5: 9,300W

Step 6: 34,243VA total demand load To determine the amperes need for the service use the formula: I5VA÷E.

I=34,243VA÷240V=143A

We can use 310.15(B)(6) for a 120/240V single-phase dwelling service and feeder up to 400A. This Table allows a smaller conductor size than Table 310.16.

A 143A demand load means this house requires at least a 150A service with 1 AWG conductors.

Optional method. You can use the easier optional method found in 220.30 only when the total connected load is served by a single 3-wire, 120/240V or 208Y/120V set of service or feeder conductors with an ampacity of 100A or greater. Because this condition describes the typical residential service, the optional method is likely to apply. Using it can simplify the design process and save you time because you have so many fewer sets of calculations.

General loads. The calculated load shall not be less than 100% for the first 10kW, plus 40% of the remainder of the following loads:

• Small-appliance and laundry branch circuits: 1,500VA for each 20A circuit.

• General lighting and receptacles: 3VA per sq ft

• Appliances: The nameplate VA rating of all appliances and motors fastened in place (permanently connected) or on a specific circuit. Be sure to use the range and dryer at nameplate rating.

HVAC. Include the largest of the following:

• 100% of the nameplate rating of the air-conditioning equipment.

• 100% of the heat-pump compressors and supplemental heating, unless the controller prevents simultaneous operation of the compressor and supplemental heating.

• 100% of the nameplate ratings of electric thermal storage and other heating systems where you expect the usual load to be continuous at the full nameplate value. Don't configure such systems under any other selection in this table.

• 65% of the nameplate rating(s) of the central electric space heating, including integral supplemental heating in heat pumps where the controller prevents simultaneous operation of the compressor and supplemental heating.

• 65% of the nameplate rating(s) of electric space heating, if there are less than four separately controlled units.

• 40% of the nameplate rating(s) of electric space heating of four or more separately controlled units.

Sizing service/feeder conductors. Now that we've seen how to determine residential loads, let's size the service/feeder conductors. We'll use the same specifications that we used for the standard method so we can compare apples to apples.

Step 1: Determine general loads [230.30(B)].

Small appliance: 1,500VA×2 circuits = 3,000VA

General lighting: 1,500 sq ft×3VA= 4,500VA

Laundry circuit=1,500VA

Now add up the appliance ratings.

• Disposal (940VA) • Dishwasher (1,250VA) • Trash compactor (1,100VA) • Water heater (4,500VA) • Dryer (4,000VA) • Ovens (3,000VA×2 units=6,000VA) • Cooktop (6,000VA)

The total connected load=32,790VA

Calculate the first 10,000VA at 100%=10,000VA× 1.00=10,000VA

Calculate the remainder at 40%= 22,790VA×0.40= 9,116VA

Demand load=10,000VA+9,116VA= 19,116VA

Step 2: Compare air conditioner at 100% vs. heat at 65% [220.30(C)].

Air conditioner: 230V×28A=6,440VA

Heat [220.30(C)(5)]: 9,000 W×0.65 = 5,850 W (omit)

Step 3: Calculate service/feeder conductors per 310.15(B)(6).

General loads=19,116VA

Air conditioning=6,440VA

Total demand load=25,556VA

I=VA÷E=25,556VA ÷ 240V = 106.5A

310.15(B)(6) requires at least a 110A service with 3 AWG conductors.

As you can see, in this case the optional method permitted a smaller service than the standard method of calculating a service for a dwelling.

Now that we've walked through the process of calculating residential services and feeders, you can see that doing so is fairly easy. You need to calculate the loads first, and then move on to the service and feeder size. The NEC provides the requirements in Art. 220 and 230. Doing these calculations correctly can save you money during design and construction, while providing safe homes for the families who occupy them.

Apply demand factors for correct load calculations A dwelling unit is a single unit that provides complete and independent living facilities, according to the NEC definition found in Art. 100 (Fig. 1 ).

Fig. 1. The definition of dwelling unit, as described above, is found in Art. 100.

Dwelling units have special requirements for load calculations. Although most of the actual load calculation requirements are in Art. 220, others are scattered throughout the Code and still come into play when making certain calculations (Where to Find Dwelling Unit Code Requirements Outside Art. 220). Keep the following considerations in mind when making dwelling unit calculations:

• Voltages. Unless other voltages are specified, calculate branch-circuit, feeder, and service loads using the nominal system voltage [220.5(A)]. For a single-family dwelling unit, the nominal voltage is typically 120/240V.

• Motor VA. Use motor table voltage and current values, such as 115V, 230V, or 460V — not 120V, 240V, or 480V [430.248 and 430.250]. A much more accurate VA rating is obtained by using the motor’s rated voltage and current, which were used in developing the Code Tables.

• Rounding. Where calculations result in a fraction of less than 0.50A, you can drop the fraction [220.5(B)].

• Receptacles. You can use 15A or 20A receptacles on 20A circuits as long as there is more than one receptacle on the circuit. For these purposes, a duplex receptacle is considered to be two receptacles [210.21(B)(3)].

• Continuous loads. A continuous load is one in which the maximum current is expected to continue for 3 hr or more, according to the Art. 100 definition. Fixed electric heating is one example of a continuous load [424.3(B)]. When sizing branch circuit conductors and overcurrent devices for a continuous load, multiply the load by 125% [210.19(A)(1) and 210.20(A)].

• Laundry rooms. A laundry area receptacle is required [210.52(F)], at least one of which must be within 6 ft of a washing machine [210.50(C)]. Any receptacle within 6 ft of the outside edge of a laundry sink must be GFCI protected [210.8(A)(7)].

Required circuits. In addition to the circuits required for dedicated appliances and those needed to serve the general lighting and receptacle load, a dwelling unit must have the following circuits:

• A minimum of two 20A, 120V small-appliance branch circuits for receptacles in the kitchen, dining room, breakfast room, pantry, or similar dining areas [220.11(C)(1)]. These circuits must not be used to serve other outlets, such as lighting outlets or receptacles from other areas [210.52(B)(2) Ex]. These circuits are included in the feeder/service calculation at 1,500VA for each circuit [220.52(A)].

• One 20A, 120V branch circuit for the laundry receptacle(s). It can’t serve any other outlet(s), such as lighting, and can serve only receptacle outlets in the laundry area [210.52(F) and 210.11(C)(2)]. In your feeder/service load calculation, include 1,500VA for the 20A laundry receptacle circuit [220.52(B)], as shown in Fig. 2.

Fig. 2. Per Sec. 210.11(C)(2), one 20A, 120V branch circuit is required for the laundry area receptacles.

Feeder and service calculations. Occupants don’t use all loads simultaneously under normal living conditions, so “demand factors” can be applied to many of the dwelling unit loads in order to size the service. Some demand factors provided in the Code are intended for use in dwellings only; others are allowed only in non-dwellings. Therefore, be careful to apply demand factors only as allowed by the NEC.

The NEC provides two dwelling service load calculation methods: the standard method and the optional method.

Standard method for feeder and service load calculations

The standard method consists of three calculation steps:

1. General lighting VA load. When calculating branch circuits and feeder/service loads for dwellings, include a minimum 3VA per sq ft for general lighting and general-use receptacles [220.12]. When determining the area, use the outside dimensions of the dwelling. Don’t include open porches, garages, or spaces not adaptable for future use.

2. Small appliance and laundry circuits. The 3VA per sq ft rule includes general lighting and all 15A and 20A, 125V general-use receptacles, but doesn’t include small-appliance or laundry circuit receptacles. Therefore, you must calculate those at 1,500VA per circuit. See 220.14(J) for details.

3. Number of branch circuits. Determine the number of branch circuits required for general lighting and general-use receptacles from the general lighting load and rating of the circuits [210.11(A)]. Although this is explained in Annex D, Example D1(a) of the NEC, let’s look at an another example.

Question: What’s the general lighting and receptacle load for a 2,000-sq-ft dwelling unit that has 34 convenience receptacles and 12 luminaires rated 100W each (Fig. 3)?

Fig. 3. Sample calculation showing how to follow the rules in Sec. 220.12 regarding general lighting and receptacles for a 2,000-sq-ft dwelling unit.

The calculation is pretty simple.

2,000 sq ft x 3VA = 6,000VA.

No additional load is required for general-use receptacles and lighting outlets because they are included in the 3VA per sq ft load specified by Table 220.12 for dwelling units. See 220.14(J).

Now let’s work through an example to determine the number of circuits required.

Question: How many 15A circuits are required for a 2,000-sq-ft dwelling unit?

Step 1: General lighting VA = 2,000 sq ft x 3VA = 6,000VA

Step 2: General lighting amperes: I = VA ÷ E I = 6,000VA ÷ 120V* I = 50A *Use 120V, single-phase unless specified otherwise.

Step 3: Determine the number of circuits: Number of circuits = General lighting amperes ÷ circuit amperes Number of circuits = 50A ÷ 15A Number of circuits = 3.30, or 4 circuits. Any fraction of a circuit must be rounded up.

Optional method for feeder and service load calculations

You can use the optional method [Art. 220, Part IV] only for dwelling units served by a single 120/240V or 120/208V 3-wire set of service or feeder conductors with an ampacity of 100A or larger [220.82]. The optional method consists of three calculation steps:

1. General loads [220.82(B)] 2. Heating and air-conditioning load [220.82(C)] 3. Feeder/service conductors [310.15(B)(6)]

Step 1: General loads [220.82(B)]

The general calculated load must be at least 100% for the first 10kVA, plus 40% of the remainder of the following loads:

1. General lighting and receptacles: 3VA per sq ft 2. Small-appliance and laundry branch circuits: 1,500VA for each 20A, 120V small-appliance and laundry branch

circuit specified in 220.52. 3. Appliances: The nameplate VA rating of all appliances and motors that are fastened in place (permanently

connected) or located on a specific circuit, not including heating or air-conditioning.

Be sure to calculate the range and dryer at their nameplate ratings.

Step 2: Heating and air-conditioning load [220.82(C)]

Include the larger of (1) through (6):

1. Air-conditioning equipment: 100% 2. Heat-pump compressor without supplemental heating: 100% 3. Heat-pump compressor and supplemental heating: 100% of the nameplate rating of the heat-pump

compressor and 65% of the supplemental electric heating for central electric space-heating systems. If the control circuit is designed so that the heat-pump compressor can’t run at the same time as the supplementary heat, omit the compressor from the calculation.

4. Space-heating units (three or fewer separately controlled units): 65%. 5. Space-heating units (four or more separately controlled units): 40%. 6. Thermal storage heating: 100%.

Step 3: Feeder/service conductors [310.15(B)(6)]

• 400A and less. For individual dwelling units of one-family, two-family, and multi-family dwellings, use Table 310.15(B)(6) to size 3-wire, single-phase, 120/240V service or feeder conductors (including neutral conductors) that serve as the main power feeder. Feeder conductors aren’t required to have an ampacity rating greater than the service conductors [215.2(A)(3)]. Size the neutral conductor to carry the unbalanced load per Table 310.15(B)(6). Table 310.15(B)(6) can’t be used for sizing the feeder or service conductors that supply more than a single dwelling unit.

• Over 400A. Size ungrounded conductors and the neutral conductor using Table 310.16 for feeder/services over 400A and those that do not fill all of the requirements for using Table 310.15(B)(6). Let’s try a calculation example.

Question: What size service conductor is required for a 1,500-sq-ft dwelling unit containing the following loads? Cooktop: 6,000VA Disposal: 900VA Dishwasher: 1,200VA Dryer: 4,000VA Ovens (two each): 3,000VA Water heater: 4,500VA A/C: 17A, 230V Electric heating (one control unit): 10kVA

Step 1: General loads [220.82(B)] General lighting: 1,500 sq ft x 3VA = 4,500VA Small-appliance circuits: 1,500VA x 2 circuits = 3,000VA Laundry circuit: 1,500VA Appliances (nameplate): Cooktop: 6,000VA Disposal: 900VA Dishwasher: 1,200VA Dryer: 4,000VA Ovens (each 3 kW): 6,000VA Water heater: 4,500VA

Total connected load: 31,600VA

First 10kW at 100%: 10,000VA x 1.00 = 10,000VA

Remainder at 40%: 21,600VA x 0.40 = 8,640VA

Calculated general load: 10,000VA + 8,640VA

Calculated general load: 18,640VA

Step 2: Air-Conditioning versus heat [220.82(C)]

Air-conditioning at 100% [220.82(C)(1)] vs. electric space heating at 65% [220.82(C)(4)]

Air conditioner [Table 430.248]: A/C VA = V x A A/C VA = 230V x 17A A/C VA = 3,910VA (omit)

Electric space heat: 10,000VA x 0.65 = 6,500VA

Step 3: Feeder/service conductors [310.15(B)(6)]

Calculated general load (Step 1): 18,640VA

Heat calculated load (Step 2): 6,500VA

Total calculated load = 18,640VA + 6,500VA = 25,140VA

I = VA ÷ E

I = 25,140VA ÷ 240V = 105A

Therefore, the feeder/service ungrounded conductor is sized to 110A, 3 AWG [310.15(B)(6)].

The Code doesn’t explain how demand factors were derived, and it’s not essential that you understand this in order to apply them correctly. Be sure to work on some practice calculations so you understand how to apply the various demand factors to a dwelling unit calculation.

The standard calculation and the optional calculation methods were both discussed in this article. These are two distinctly different calculation methods, so be careful not to mix them. Remember that the standard method is in Part III of Art. 220, and the optional method is contained in Part IV. When you are evaluating the necessary loads in either type of calculation method, follow the requirements for specific loads covered in other Articles outside of Art. 220. Which method is better to use? On an exam, you’ll likely be told which method to use on a specific question. However, if the question doesn’t specify a method, use the standard calculation. The optional method is usually faster and easier to apply, so it has a natural advantage for daily use on the job.

Where to Find Dwelling Unit Code Requirements Outside Art. 220 Branch circuits — Art. 210

Areas supplied by small appliance circuits — 210.52(B)(1)

Feeders — Art. 215

Services — Art. 230

Overcurrent protection — Art. 240

Wiring methods — Art. 300

Conductors — Art. 310

Appliances — Art. 422

Electric space-heating equipment — Art. 424

Motors — Art. 430

Air-conditioning equipment — Art. 440

 

Multifamily Dwelling Unit Service and Feeder Calculations

 

 

How to calculate the service for a single-family or a multifamily dwelling

The NEC defines a dwelling unit as a single unit that provides complete and independent living facilities for one or more persons that must include permanent provisions for living, sleeping, cooking, and sanitation (Fig. 1 on page xx). A dwelling becomes "multifamily" when it contains three or more dwelling units [Art. 100 Definitions] (Fig. 2 on page xx).

Fig. 1. A dwelling unit is defined as a single unit that provides permanent provisions for living, sleeping, cooking, and sanitation.

Fig. 2. A multifamily dwelling is defined as a building with three or more dwelling units. Examples include apartment buildings, condominiums, some hotels and motels.

When you size the service for a single-family dwelling, you calculate the load and apply the appropriate demand factors. For a multifamily dwelling, you do the same thing, except you apply the appropriate demand factors to the sum of the individual dwelling units of that multifamily dwelling. You are allowed to use the standard method from Part III of Art. 220 or the optional method from Part IV of Art. 220.

Standard method. The same method used for single dwellings can be applied to multifamily dwellings. The NEC allows some additional demand factors for multifamily dwellings, on the presumption that there will be diversity of usage between the various units. For example, it's very unlikely that four families will run their clothes dryers, ranges, and small appliances at exactly the same time.

The following steps can be used to determine feeder and service sizes for a multifamily dwelling using the standard method contained in Art. 220, Part III:

Step 1: General Lighting, Small Appliance, and Laundry Demand [Table 220.42]

• 3VA per sq ft for general lighting and general-use receptacles [Table 220.12]. • 1,500VA for each small-appliance circuit (minimum of 2 circuits) [220.52(A)]. • 1,500VA for each laundry circuit [220.52(B)].

Step 2: Air-Conditioning versus Heat [220.51]

The larger of the air-conditioning load or the space-heating load.

Step 3: Appliance Calculated Load [220.53]

Nameplate ratings of all appliances (except heating, air-conditioning, cooking equipment, and dryers) are taken times a 75% multiplier if there are four or more on the feeder.

Step 4: Household Dryer Calculated Load [220.54]

Dryers are allowed the demand factors of Table 220.54, but this table does not allow less than 100% demand until there are five units or more. The 5kW minimum per dryer applies to all dwelling units [220.54]. A laundry circuit isn't required for an individual dwelling unit if the multifamily unit has common laundry facilities.

Step 5: Household Cooking Equipment Calculated Load [220.55]

Perhaps one of the most confusing tables in the NEC is Table 220.55 for household ranges. This table is confusing because the first two columns are percentage multipliers, while the third column is a final kVA value. The notes to this table further complicate matters. Be sure you study this table carefully and pay close attention to how to properly apply each column.

Step 6: Service Conductor Size [Table 310.16]

When sizing the service or feeder conductors for a single-family dwelling, you can use Table 310.15(B)(6), but that is not the case when sizing conductors for the service or feeder to a two-family or multifamily dwelling. For sizing those conductors, use Table 310.16 instead. Use Table 310.15(B)(6) for the feeders to an individual dwelling unit within the building.

Optional method. When should you use the optional method instead of the standard one? If you have the necessary information, you'll probably want to use the optional method, because it's faster and easier to calculate.

The optional method for multifamily dwellings is different from the one for single-family dwellings. That's because with multifamily dwellings, you apply demand factors in recognition of the diversity of usage of all the loads in all the separate units.

Let's make sure this is clear. In a single family unit, you have diversity among the various types of loads. Although you have that in multifamily units as well, you have diversity among the units that make up the multifamily dwelling — all of the families in a multifamily dwelling aren't using identical loads at identical times.

You can use the optional method [220.84] for multifamily dwelling unit feeder and service calculations only if each dwelling unit is equipped with electric cooking equipment and electric heating and/or air-conditioning, and is supplied by no more than one feeder. Follow these rules:

1. Use the demand factors of Table 220.84, based on the number of dwelling units. 2. Determine the feeder/service neutral calculated load per 220.61. 3. Calculate house loads for common areas per Art. 220, Part III and then add them to the Table 220.84

calculated load.

House loads are those not directly associated with the individual dwelling units of a multifamily dwelling. Some examples include landscape and parking lot lighting, hall and stairway lighting, common recreation areas, and common laundry facilities.

Follow these steps:

1. Determine total connected load. 2. Calculate the load. 3. Size feeder and service conductors.

Let's look at these three steps in a bit more detail by walking through an example. In practice, you may see NEC-compliant variations of executing these steps.

Step 1: Determine total connected load [220.84(C)].

Add the following loads (from all the dwelling units) together, then apply the Table 220.84 demand factor:

• 3VA per sq ft for general lighting and general-use receptacles. • 1,500VA for each small-appliance circuit (minimum of two circuits). • 1,500VA for each laundry circuit. • The nameplate rating of all appliances. • The nameplate rating of all motors. • The larger of the air-conditioning load or the space-heating load.

A laundry circuit isn't required for an individual dwelling unit if the multifamily unit has common laundry facilities.

Step 2: Calculate the load.

Apply the demand factor from Table 220.84 to the total connected load (Step 1). You can convert the calculated load (kVA) to amperes by:

Single-Phase Formula: I = VA ÷ E

Three-Phase Formula: I = VA ÷ (1.732 x E)

Step 3: Size feeder and service conductors.

Size the ungrounded conductors per Table 310.16, based on the calculated load.

Example Problem A 120/240V, single-phase system supplies a 12-unit multifamily building (Fig. 3 on page xx). Each 1,500 sq ft unit contains:

Fig. 3. Sample problem of how to size the ungrounded conductors for a 12-unit multifamily building.

Dishwasher — 1.5kVA

Water Heater — 4.0kVA

Washing Machine — 1.2kVA

Dryer — 4.5kVA

Range — 14.4kVA

A/C (230V x 17A) — 3.91kVA

Electric Space Heating — 5.0kVA

Question: What size conductor is required if the service is rated 120/240V, single-phase and the conductors are installed in parallel in two separate raceways?

Step 1: Total Connected Load

Step 1a: Determine the General Lighting Load:

General Lighting (1,500 sq ft x 3VA = 4,500VA)

Small-Appliance Circuits (2 circuits x 1,500VA = 3,000VA)

Laundry Circuit (1,500VA)

4,500 + 3,000 + 1,500 = 9,000

9,000VA x 12 units = 108,000VA

Step 1b: Determine the Appliance Calculated Load:

Dishwasher (1,500VA)

Water Heater (4,000VA)

Dryer [nameplate] (4,500VA)

Range [nameplate] (14,400VA)

1,500 + 4,000 +4,500 + 14,400 = 24,400

24,400VA x 12 units = 292,800VA

Step 1c: Compare the Air-Conditioning versus Heat Load:

A/C = 3,910VA (omit)

Heat = 5,000VA x 12 units = 60,000VA

Step 2: Total Connected Loads

General Lighting, Receptacles (108,000VA)

Appliances Connected Load (292,800VA)

Heat (60,000VA)

108,000 + 292,800 + 60,000 = 460,800

Total Connected Load = 460,800VA

Total Calculated Load = Total Connected Load x Demand Factor [Table 220.84]

Total Calculated Load = 460,800VA x 0.41 [Table 220.84]

Total Calculated Load = 188,928VA

Step 3: Service Conductor Size (Fig. 4 on page xx)

I = VA ÷ E

I = 188,928VA ÷ 240V

I = 787A

Fig. 4. Sample calculation of how to determine service conductor size on a multifamily dwelling.

Conductor size if paralleled in two raceways [240.4(B)]:

787A ÷ 2 raceways = 393A per conductor

Feeder/Service Conductors:

Parallel 600kcmil conductors rated 420A at 75ºC [Table 310.16] would meet these load requirements.

Grounding electrode conductor sizing. What size grounding electrode conductor is required if the service ungrounded conductors are 600kcmil with two conductors in parallel in two separate raceways?

600kcmil x 2 = 1,200kcmil (equivalent area of the ungrounded conductors) [Table 250.66, Note 1].

Over 1,100kcmil for ungrounded conductors requires a grounding electrode conductor of 3/0 AWG [Table 250.66].

Feeder installation. When installing feeders, include an equipment grounding conductor in each raceway. Section 250.118 lists allowable equipment grounding conductors.

To size this conductor using a wire-type equipment grounding conductor, go to Table 250.122 and select the equipment grounding conductor based on the overcurrent device protecting the conductors in the raceway [250.122(F)]. For instance, the equipment grounding conductor in each raceway of an 800A feeder, which is paralleled using two 600kcmil conductors per phase, will require a 1/0 AWG equipment grounding conductor in each raceway.

Two more samples. Working these two additional sample problems will reinforce what we've learned thus far.

Size the grounding electrode conductor.

Question: What size grounding electrode conductor is required if the service ungrounded conductors are 300kcmil with three conductors in parallel in three separate raceways?

300kcmil x 3 = 900kcmil equivalent area of the ungrounded conductors [Table 250.66, note 1]

900kcmil for ungrounded conductors requires a grounding electrode conductor of 2/0 AWG [Table 250.66]

Size the parallel service conductor.

Question: What size conductor is required if the service with a calculated load of 787A is rated 120/240V, single-phase and the conductors are installed in parallel in four separate raceways?

787A ÷ 4 raceways = 196.75A

3/0 AWG copper is rated 200A at 75ºC [Table 310.16], so four 3/0 AWG conductors can be paralleled for this service.

A word about two-family dwellings. The feeder for a two-family dwelling unit is calculated using the standard method in Part III of Art. 220. When that calculated load exceeds the calculation for three identical units using the optional method of 220.84, the lesser of the two calculations is permitted to be used [220.85].

Avoiding confusion. The sizing of branch circuits, feeders, and service conductors for multifamily dwellings is similar to the sizing for single-family dwellings. You size the feeders to individual dwelling units in the same manner, whether that dwelling unit is a single-family dwelling or an individual unit of an apartment building.

The NEC allows the use of Table 310.15(B)(6) for sizing the feeders or service conductors to an individual dwelling unit. However, to size the conductors that provide the service to a two-family or multifamily dwelling you must use Table 310.16.

Whether calculating the service for a single-family or a multifamily dwelling, be sure to follow the Code rules for the specific calculation you are working on and do not intermix the standard method with the optional method. Follow the steps outlined in this article and apply the demand factors allowed for each method carefully, and you will be successful.

 

Commercial Electrical Load Calculations

Knowing how to correctly size loads in commercial applications is an essential skill for electricians

Even if you work with stamped drawings, you'll eventually need to do commercial load calculations in the field or on a licensing exam. The NEC covers commercial calculations in Art. 220, but other articles also apply. For example, you must know the definitions in Art. 100, be familiar with what Art. 210 says about continuous loads, and understand the overcurrent protection requirements set forth in Art. 240.

Two items associated with this type of calculation repeatedly need clarification:

• Voltage

The voltage to use for your calculations depends on the system design voltage. Thus when you calculate branch-circuit, feeder, and service loads, you must use a nominal system voltage of 120V, 120/240V, 208Y/120V, 240V, 347V, 480Y/277V, 480V, 600Y/347V, or 600V unless otherwise specified (220.2) (Fig. 1 below).

• Rounding

Refer to 200.2(B) to end the rounding mystery. When the ampere calculation exceeds a whole number by 0.5 or more, round up to the next whole number. If the extra is 0.49 or less, round down to the next whole number. For, example, round 29.5A up to 30A, but round 29.45A down to 29A.

Specific loads. Art. 220 doesn't cover all specific loads. For example, you'll find motors in Art. 430 and air conditioners in Art. 440. To know if you should look in another Article, use the NEC index.

Art. 220 has specific requirements for most loads, including the following:

Dryers. Size the branch-circuit conductors and overcurrent protection device for commercial dryers to the appliance nameplate rating. Calculate the feeder demand load for dryers at 100% of the appliance rating. If the dryers run continuously, you must size the conductor and protection device at 125% of the load [210.19(A), 215.3, and 230.42]. Table 220.18 demand factors don't apply to commercial dryers.

Fig. 1. Don’t make the mistake of using actual field measurements of system voltage in your calculations. Unless specified otherwise, loads shall be computed using the nominal system voltage such as 120V, 120/240V, 208Y/120V, 240V, 347V, 480Y/277V, 480V, 600Y/347V or 600V.

Let's apply what we've just learned. What size branch-circuit conductor and overcurrent protection does the NEC require for a 7kW dryer rated 240V when the dryer is in a multi-family dwelling laundry room (Fig. 2)?

I=P÷E

7,000W÷240V=29A

The ampacity of the conductor and overcurrent device must be at least 29A (240.4). Per Table 310.16, a 10 AWG conductor at 60°C is rated 30A. Therefore, you must use a 30A breaker with a 10 AWG conductor.

Electric heat[424.3(B)]. Size branch-circuit conductors and the overcurrent protection device for electric heating to not less than 125% of the total heating load, including blower motors. Calculate the feeder/service demand load for electric heating equipment at 100% of the total heating load.

Kitchen equipment.Size branch-circuit conductors and overcurrent protection for commercial kitchen equipment per the appliance nameplate rating.

Fig. 2. When determining proper branch-circuit protection and conductor size for a commercial clothes dryer, you must use a demand load of 100%. The reduced demand factors for multiple dryers (Table 220.18) don’t apply in a commercial setting.

To determine the service demand load for commercial kitchen equipment that has thermostatic control or intermittent use, apply the demand factors from Table 220.20 to the total connected kitchen equipment load. The feeder or service demand load can't be less than the sum of the two largest appliance loads. The demand factors of Table 220.20 don't apply to space-heating, ventilating, or air-conditioning equipment.

Laundry equipment. Size these circuits to the appliance nameplate rating. You can assume a laundry circuit isn't a continuous load and that commercial laundry circuits are rated 1,500VA — unless noted otherwise in the project drawings or exam question.

Lighting. The NEC requires a minimum load per square foot for general lighting, depending on the type of occupancy [Table 220.3(A)]. For the guestrooms of hotels, motels, hospitals, and storage warehouses, you can apply the general lighting demand factors of Table 220.11 to the general lighting load.

Assume the general lighting load for commercial occupancies other than guestrooms of motels, hotels, hospitals, and storage warehouses is continuous. Calculate it at 125% of the general lighting load listed in Table 220.3(A).

Receptacles. You don't do all receptacle load calculations the same way. The NEC has separate requirements, depending on the application.

Multi-outlet receptacle assembly. For service calculations, consider every 5 feet (or less) of multi-outlet receptacle assembly to be 180VA. When you can reasonably expect a multi-outlet receptacle assembly to power several appliances simultaneously, consider each foot (or less) as 180VA for service calculations. Normally, a multi-outlet receptacle assembly isn't a continuous load [220.3(B)(8)].

Receptacle VA load. The minimum load for each commercial or industrial general-use receptacle outlet is 180VA per strap [220.3(B)(9)]. Normally, receptacles aren't continuous loads.

Number of receptacles permitted on a circuit. The maximum number of receptacle outlets permitted on a commercial or industrial circuit depends on the circuit ampacity. To calculate that number, divide the VA rating of the circuit by 180VA for each receptacle strap.

Let's work a sample problem. How many receptacle outlets are permitted on a 15A, 120V circuit (Fig. 3)?

Total circuit VA load for a 15A circuit: 120V×15A=1,800VA Number of receptacles per circuit: 1,800VA÷180VA=10 receptacles

Receptacle sizing.The NEC permits 15A circuits in commercial and industrial occupancies, but some local codes require a minimum 20A rating (310.5).

Receptacle service demand load. In other than dwelling units, you can add — to the lighting loads — receptacle loads computed at not more than 180VA per outlet per 220.3(B)(9). You can also add fixed multi-outlet assemblies computed per 220.3(B)(8). Both of these must adhere to the demand factors given in Table 220.11 or in Table 220.13.

Fig. 3. The minimum load for each commercial general-use receptacle outlet is 180VA per strap. In this example, the 15A, 120V breaker could accommodate 1,800VA of load (120V x 15A = 1,800VA). Therefore, you could install a total of 10 receptacles on this circuit.

Bank and office general lighting and receptacles. Calculate the receptacle demand load at 180VA for each receptacle strap [220.3(B)(9)] if the number of receptacles is known, or 1VA for each square foot if the number of receptacles is unknown [Table 220.3(A) Note b].

Signs. The NEC requires each commercial occupancy that's accessible to pedestrians to have at least one 20A branch circuit for a sign [600.5(A)]. The load for the required exterior signs or outline lighting must be at least 1,200VA [220.3(B)(6)]. A sign outlet is a continuous load. You must size the feeder load at 125% of the continuous load [215.2(A)(1) and 230.42].

The following question will allow you to practice what we've just covered. What's the demand load for one electric sign?

1,200VA×1.25=1,500VA

Neutral calculations. The neutral load is the maximum unbalanced demand load between the grounded (neutral) conductor and any one ungrounded (hot) conductor — as determined by the calculations in Art. 220, Part B. This means you don't consider line-to-line loads when sizing the grounded (neutral) conductor. What about load reduction? That depends on certain factors, which we'll look at next.

Reduction over 200A. You can reduce the feeder/service net computed load for 3-wire, single-phase or 4-wire, 3-phase systems that supply linear loads for that portion of the unbalanced load over 200A, by a multiplier of 70%.

To see how this would work for an actual installation, determine the neutral demand load for a balanced 400A, 3-wire, 120/240V feeder.

Total neutral load for 400A service: First 200A at 100%: 200A×1.00=200A Remainder at 70%: 200A×0.70=140A Total demand load: 200A×140A=340A

Reduction not permitted. You can't reduce the neutral demand load for 3-wire, single-phase, 208Y/120V or 480Y/277V circuits that consist of two line wires and the common conductor (neutral) of a 4-wire, 3-phase wye system. This is because the common (neutral) conductor of a 3-wire circuit connected to a 4-wire, 3-phase wye system carries about the same current as the phase conductors [310.15(B)(4)(b)].

As proof of this theory, see the example in Fig. 4.

In addition, you can't reduce the neutral demand load for nonlinear loads supplied from a 3-phase, 4-wire, wye-connected system, because they produce triplen harmonic currents that add on the neutral conductor. This situation can require the neutral conductor to be larger than the ungrounded conductor load (220.22 FPN 2).

Knowing the correct way to do commercial load calculations makes you more valuable because you can play a key role in the field design, inspection, and implementation process. It's one more skill that helps you do the job right the first time.

Fig. 4. Sizing the grounded (neutral) conductor can be tricky. Just remember that you can’t reduce the neutral demand load for 3-wire, single-phase, 208Y/120V or 480Y/277V circuits that consist of two line wires and the common conductor (neutral) of a 4-wire, 3-phase system.

Commercial Loads — Part 2

Calculating commercial receptacle loads and understanding the optional calculation method

Last month in Code Basics, “Commercial Loads — Part 1,” on page 24 of the January issue, we discussed when it’s okay to apply demand factors when calculating electrical load requirements for commercial installations. Because different sets of demand factors apply for different types of electrical loads (and even for different types of commercial buildings), this article demonstrated why it’s important to determine what kinds of loads you have before starting your commercial load calculations.

In part two of this installment, it’s time to address how to calculate receptacle loads and introduce the optional calculation method for commercial occupancies.

The basics The multioutlet receptacle assembly, such as a plug strip, is common in commercial applications. For each 5 ft (or fraction thereof) of multioutlet receptacle assembly, use 180VA in your feeder/service calculations. This is assuming that it's unlikely for the appliances plugged into this assembly to operate simultaneously [220.14(H)].

If you expect several appliances to operate simultaneously from the same multioutlet receptacle assembly, consider each foot (or fraction of a foot) as 180VA for feeder/service calculations. A multioutlet receptacle assembly isn't generally considered a continuous load.

Try this sample problem to determine the feeder/service load. What's the calculated load for 10 workstations (Fig. 1), each of which has 10 ft of multioutlet receptacle assembly (not used simultaneously) and 3 ft of multioutlet receptacle assembly (used simultaneously)?

Fig. 1. This example shows how to determine the multioutlet assembly calculated load on 10 workstations, which could be found in a typical office setting.

10 stations with 10 ft per station = 100 ft of multioutlet assembly (not simultaneously used)

10 stations with 3 ft per station = 30 ft of multioutlet assembly (simultaneously used)

100 ft ÷ 5 ft per section = 20 sections x 180VA = 3,600VA

30 ft ÷ 1 ft per section = 30 sections x 180VA = 5,400VA

Calculated load = 3,600VA + 5,400VA = 9,000VA

This next example shows how to size the branch circuits using 20A branch circuits.

First, find the VA allowed per circuit:

120V x 20A = 2,400VA for noncontinuous loads

Then, divide by 180VA to find how many 180VA sections a 20A circuit can serve:

2,400VA ÷ 180VA = 13

Each work bench requires 2 – 180VA sections for the 10 ft section, and 3 – 180VA sections for the 3 ft section, which is 5 – 180VA sections per workbench. At 13 sections per circuit, a 20A branch circuit can serve two tables.

Receptacle VA load Receptacles are generally not considered continuous loads. The load for a general-use receptacle outlet in a non-dwelling occupancy is 180VA per strap [220.14(I)]. The maximum number of receptacle outlets permitted on a commercial or industrial circuit depends on the circuit ampacity. Calculate the number of receptacles per circuit by dividing the VA rating of the circuit by 180VA for each receptacle strap (also called a yoke), as shown in Fig. 2

Fig. 2. Each 15A or 20A, 125V general-use receptacle outlet is considered as 180VA per mounting strap.

Based on the Art. 100 definition, a duplex receptacle is two receptacles on the same yoke. For the purposes of this calculation, a single receptacle or a duplex receptacle each count as 180VA [220.14(I)].

Receptacle feeder/service calculated load Calculate receptacle loads at not less than 180VA per outlet (strap) per 220.14(I) and fixed multioutlet assemblies per 220.14(H). According to 220.44, you can add these calculated loads to the lighting loads and apply the lighting load demand factors given in Table 220.42. Alternatively, you can use the demand factors for receptacles given in Table 220.44, which are as follows:

• First 10kVA at 100% demand factor. • Remainder over 10kVA at 50% demand factor.

Calculate the receptacle load using 180VA for each single or multiple receptacle on one yoke or strap [220.14(I)].

The receptacle calculated load for office buildings and banks is the larger calculation of (1) or (2):

1. Determine the receptacle calculated load at 180VA per receptacle yoke [220.14(I)], then apply the demand factor from Table 220.44.

2. Determine the receptacle calculated load at 1VA per sq ft.

It's common not to know the exact number of receptacles that will eventually be installed in an office building or bank. The main structure is built first, then individual office space that's rented out to each tenant will often have a custom installation — or a new tenant will remodel the space to fit his or her needs. A calculation of 1VA per square foot allows a generic feeder/service demand for general receptacles.

What is the receptacle calculated load for an 18,000-sq-ft bank/office building containing 160 15A and 20A, 125V receptacles (straps)? [220.14(K)(1)], as shown inFig. 3.

Fig. 3. This example demonstrates how to calculate the receptacle load for an 18,000-sq-ft office building containing 160 receptacles.

160 receptacles (straps) x 180VA [220.14(I)] = 28,800VA

Total receptacle load = 28,800VA

First 10,000VA at 100% (10,000VA x 1.00 = 10,000VA)

Remainder at 50% (18,800VA x 0.50 = 9,400VA) [Table 220.44]

Receptacle calculated load = 19,400VA

Compare this to the 1VA per sq ft method [220.14(K)(2)]

18,000 x 1VA per sq ft = 18,000VA (smaller answer, omit)

Sign circuits The NEC requires each commercial occupancy accessible to pedestrians to have at least one 20A branch circuit for a sign [600.5(A)]. The load for the required exterior sign or outline lighting must be a minimum of 1,200VA [220.14(F)]. A sign outlet is a continuous load, and the feeder/service conductor must be sized at 125% of the continuous load [215.2(A)(1) and 230.42].

What is the feeder/service conductor calculated load for one electric sign (Fig. 4)?

Feeder/service calculated load = 1,200VA x 1.25 = 1,500VA

Fig. 4. Follow the rules in Sec. 220.14(F) for guidance on how to calculate the feeder/service conductor load for an electric sign.

Commercial/industrial vs. residential You've probably noticed that receptacle calculations for commercial/industrial applications differ from those that apply to residential applications. The differences exist because in residential locations, the receptacles are generally placed much closer together for convenience purposes but used in a diverse manner — so that all receptacles are not heavily loaded during all periods of time. In commercial occupancies, there are fewer rules governing receptacle placement, so they may be placed as needed, but may be called into use more often and for longer periods of time. In dwelling units, the receptacle load is included in the general lighting load VA calculated according to Table 220.12 and is then subject to the demand factors of Table 220.42. The 180VA per receptacle strap allowance does not apply to dwelling unit calculations.

In Part 1, we looked at how to calculate commercial loads using the standard method. You can save time by using the optional method. The optional method calculations are located in Part IV of Art. 220. The optional method calculations vary according to the type of building:

• New dwelling units (220.82) • Existing dwelling units (220.83) • Multifamily dwellings (220.84) • Schools (220.86) • New restaurants (220.88)

All-electric restaurant If a restaurant has electric space heating, electric air-conditioning, or both, you can use the optional method, which consists of the following two steps:

1. Determine the total connected load. Add the nameplate rating of all loads at 100%, including both the air-conditioning and heating load [Table 220.88 Note].

2. Apply the demand factors from Table 220.88 to the total connected load calculated in Step 1.

An example will help illustrate how the optional method works for restaurants. What's the calculated load for an all-electric restaurant (120/208V, 3-phase) that has a total connected load of 300kVA?

Total connected load = 300kVA

First 200kVA at 80% (200kVA x 0.80 = 160kVA)

Next 201kVA to 325kVA at 10% (100kVA x 0.10 = 10kVA

Total calculated load = 170kVA

I = VA ÷ (E x 1.732)

I = 170,000VA ÷ (208V x 1.732)

I = 170,000VA ÷ 360V = 472A

Paralleling two conductors per phase: 472A ÷ 2 raceways = 236A

250kcmil is rated 255A at 75ºC: 255A x 2 conductors (in parallel) = 510A

The minimum neutral size allowed when paralleling conductors is 1/0 AWG [250.24(C)(2) and 310.4].

What size grounding electrode conductor do you need if the service uses two sets of 250kcmil conductors in parallel?

First, find the equivalent area of the parallel conductors:

250kcmil x 2 conductors = 500kcmil [Table 250.66, Note 1]

500kcmil requires a 1/0 AWG grounding electrode conductor [Table 250.66].

The largest grounding electrode conductor to a ground rod is 6 AWG. The largest to a concrete encased electrode (Ufer) is 4 AWG [250.66(A) and 250.66(B)].

Not all-electric restaurant What if the restaurant isn't all-electric? Try calculating the load for a not all-electric restaurant (120/208V, 3-phase) that has a total connected load of 300kVA.

Total connected load = 300kVA

First 200kVA at 100% (200kVA x 1.00 = 200kVA

201kVA to 325kVA at 50% (100kVA x 0.50 = 50kVA

Total calculated load = 250kVA

I = VA ÷ (E x 1.732)

I = 250,000VA ÷ (208V x 1.732)

I = 250,000VA ÷ 360V = 694A

Paralleling two conductors per phase: 694A ÷ 2 raceways = 347A

500kcmil is rated 380A at 75°C: (380A x 2 = 760A).

When a calculation result does not correspond to a standard overcurrent protection size, we are allowed to round up to the next standard size, as long as it does not exceed 800A [240.4(B)]. This would allow the use of an 800A overcurrent device [240.6(A)]

The minimum neutral size when paralleling conductors is 1/0 AWG [250.24(C)(2) and 310.4].

Fortunately, you don't have to use both the standard and optional methods and then pick the one that is the larger. You are allowed to use either approach, so you can save time by using the optional method.

Article 225: Outside Branch Circuits

Article 225 provides installation requirements for outside branch circuits and feeders that run on (or between) structures or poles (Fig. 1). The NEC differentiates between buildings and structures, but for convenience we'll refer to both of themas structures.  

Fig. 1. Article 225 contains the installation requirements for outside branch circuits and feeders run on or between buildings, structures, or poles.

Other articles also pertain to outside branch circuits (see Other Articles Related to Outside Branch Circuitson page 71); most of these are application-specific. Lighting is the most common application. Make sure you don't place 277V luminaires within 3 feet of platforms, fire escapes, or windows that open [225.7(C)] (Fig. 2 on page 69). See 210.6(C) for the types of luminaires permitted on 277V or 480V branch circuits.

Fig. 2. Luminaires on a 227V or 480V branch circuit cannot be located within 3 feet of platforms or windows that open.

Festoon lighting is a string of outdoor lights suspended between two points [Art. 100]. It's commonly used at carnivals and similar functions [525.20(C)]. Festoon lighting conductors must be at least 12 AWG, unless messenger wires support them. Overhead festoon lighting conductors must be supported by messenger wire (with strain insulators) when spans exceed 40 feet [225.6(B)]

Conductor size and support For overhead spans up to 50 feet, the minimum conductor size for outside branch circuits is 10 AWG. For longer spans, it's 8 AWG [225.6(A)].

When these spans are installed over a building, they must be securely supported by substantial structures [225.15]. Where practicable, supports must be independent of the building [230.29]. If you use a mast for support, it must have adequate mechanical strength, braces, or guy wires to withstand the strain caused by the conductors [225.17]. Don't use trees or other vegetation for conductor support [225.26].

Attachments The points of attachment for overhead conductors must be at least 10 feet above finished grade. Maintain the minimum conductor clearance required by 225.18, even if that means raising the points of attachment (Fig. 3 on page 70).

Fig. 3. The point of attachment must not be less than 10 feet above the finished grade and must be located so the conductor clearance contained in 225.18 is maintained.

When attaching open conductors, use fittings identified for use with conductors. Alternatively, you can use noncombustible, nonabsorbent insulators securely attached to the structure. You can attach branch conductors to the service mast, but don't attach aerial communications cables or antennas to it [810.12].

Vertical clearances For overhead conductors of 600V or less, maintain the following clearances: [225.18]:

• 10 feet above finished grade, sidewalks, platforms, or projections from which they might be accessible to pedestrians if circuits are 150V to ground or less. Article 225 doesn't provide a clearance for over 150V.

• 12 feet above residential property and driveways, and commercial areas not subject to truck traffic if circuits of 300V to ground or less. It's 15 feet for circuits over 300V to ground.

• 18 feet over public traffic ways, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles (e.g., those used for cultivation, grazing, or forestry).

Observe the clearance requirements in 680.8 for any conductor that runs above pools, outdoor spas, outdoor hot tubs, diving structures, observation stands, towers, or platforms.

Overhead clearances Conductors must maintain a vertical clearance of 8 feet above the surface of a roof, for least 3 feet from the edge of the roof. Four exceptions exist, and they're listed in 225.19(A).

Conductors must maintain a vertical, diagonal, and horizontal clearance of at least 3 feet from signs, chimneys, radio and television antennas, tanks, and other structures (buildings and bridges are excluded) [225.19(B)].

Final span clearance Remember earlier we said you have to keep luminaires at least 3 feet from platforms and similar locations? The same thing applies to overhead conductors. But conductors that run above a window aren't required to maintain the 3 feet distance.

Conductors must maintain a vertical clearance of at least 10 feet above platforms, projections, or surfaces from which they might be reached. This vertical clearance must be maintained for 3 feet, measured horizontally from those surfaces.

Don't install conductors under an opening through which materials might pass, or where conductors will obstruct building openings [225.19(D)(3)] (Fig. 4 on page 70). Arrange raceways on exterior surfaces so they drain. In wet locations, they must be rain-tight [225.22].

Fig. 4. Overhead conductors must not be installed under an opening through which materials might pass, and they must not obstruct an entrance or building opening.

Multiple structures Where more than one structure is on the same property, each must be served by no more than one feeder or branch circuit [225.30]. As you might expect, the NEC provides several exceptions.

The first one is “Special Conditions.” You can provide additional circuits for:

• Fire pumps,

• Emergency systems,

• Legally required standby systems,

• Optional standby systems,

• Parallel power production systems, and

• Systems designed for connection to multiple sources of supply for the purpose of enhanced reliability.

You can also, by special permission, provide additional feeders for:

• Multiple-occupancy buildings where there's no available space for supply equipment accessible to all occupants.

• A structure so large that two or more feeder supplies are necessary.

The three other exceptions are:

1. The capacity requirements exceed 2,000A.

2. Different voltages, frequencies, or uses. For example, control of outside lighting from multiple locations.

3. Documented safe switching procedures are established and maintained for disconnection.

Disconnects Provide a disconnecting means for all conductors that enter a structure [225.31]. Install it at a readily accessible location nearest the point of entrance of the conductors [225.32]. You can locate it elsewhere:

• Where documented safe switching procedures are established and maintained. But it must be monitored by qualified persons (see Art. 100).

Also:

• A disconnecting means isn't required within sight of poles that support luminaires.

• The disconnecting means for a sign doesn't need to be readily accessible if installed per the requirements for signs. Each sign must be controlled by an externally operable switch or circuit breaker that opens all ungrounded conductors to the sign. The sign disconnecting means must be within sight of the sign, or the disconnecting means must be capable of being locked in the open position [600.6(A)].

The structure disconnecting means can consist of no more than six switches (or circuit breakers) in a single enclosure, or separate enclosures for each supply permitted by 225.30. Group all disconnects in one location [225.34], and mark each one to indicate the loads served [110.22].

To minimize accidental interruption of the critical power systems, 225.30(A) requires the disconnecting means for a fire pump [695.4(B)(2)] or standby power [701.11(E)] to be located remotely from the normal power disconnect. First responders to a fire can shut down power to the facility without shutting off the fire pump.

In a multiple-occupancy building, each occupant must have access to the disconnecting means for the occupancy [225.35]. The occupant disconnect can be accessible to building management, if management provides electrical maintenance under continuous supervision.

You can use a snap switch (or a set of 3-way or 4-way snap switches) as the disconnecting means for garages and outbuildings on residential property without having a “service equipment” rating.

Where more than one feeder supplies a structure, a permanent plaque or directory must be installed at each feeder disconnect location and denote all other feeders supplying that structure and the area served by each [225.37].

The structure disconnecting means can consist of either a manually operated switch (or circuit breaker) or a power-operated one that's capable of being operated manually [225.38]. If you use a shunt-trip push button as the means of opening a power-operated circuit breaker, the breaker is the disconnecting means and the push button is not.

The feeder or branch-circuit disconnecting means for a structure must have an ampere rating not less than the calculated load determined per Art. 220 [225.39]. But observe the following:

• One-circuit installation

The disconnecting means must have a rating at least 15A.

• Two-circuit installation

The feeder disconnecting means must be rated at least 30A.

• One-family dwelling

The feeder disconnecting means must be rated at least 100A, 3-wire.

For all other installations, the feeder or branch-circuit disconnecting means must be rated at least 60A.

You may have noticed that Art. 225 is primarily concerned with clearances, support, and disconnects. If you address those issues before starting your outside branch-circuit installations, you should have no problem complying with Art. 225. Be sure to review Table 225.2 for other applicable Articles and comply accordingly.

Sidebar: Other Articles Related to Outside Branch Circuits

• Branch circuits (Art. 210)

• Class 1, Class 2, and Class 3 remote control, signaling, and power-limited circuits (Art. 725)

• Communications circuits (Art. 800)

• Community antenna television and radio distribution systems (Art. 820)

• Conductors for general wiring (Art. 310)

• Electric signs and outline lighting (Art. 600)

• Feeders (Art. 215)

• Floating buildings (Art. 553)

• Grounding (earthing) and bonding (Art. 250)

• Marinas and boatyards (Art. 555)

• Radio and television equipment (Art. 810)

• Services (Art. 230)

• Solar photovoltaic systems (Art. 690)

• Swimming pools, fountains, and similar installations (Art. 680)

 

Bonding and Grounding

Bonding and Grounding Transformers Clearing up confusion on bonding and grounding solidly grounded transformers

After a national arc-flash hazard analysis project was performed at eight recently constructed parts distribution warehouse sites for a Global 100 company as part of an OSHA Voluntary Protection Program (VPP), management found the results to be somewhat shocking.

During the data gathering process, Electrical Service Solutions, Inc., discovered more than 35 violations of the NEC involving improper bonding and grounding of transformers. Violations ranged from system bonding jumpers that were missing, undersized, improperly terminated, and installed in two locations to grounding electrode conductors that were either missing, undersized, improperly terminated to the electrode, and/or connected to the separately derived system in a location other than where the system bonding jumper was connected. These findings reiterate the fact that a significant amount of confusion still remains in the industry on the topic of bonding and grounding of transformers. Let’s take a closer look at the areas where most of the misconceptions arise.

The effective ground-fault current path To understand the concept of bonding and grounding for safety, the installer must know that for normal load current, short circuit current, or ground-fault current to flow, there must be a continuous circuit or path — and a difference of potential. The 2011 NEC defines the effective ground-fault current path as “an intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors on high-impedance grounded systems.” An effective ground-fault current path is an essential part of the overcurrent protection system.

Normal load current, short circuit current, or ground-fault current will use any and all completed paths, dividing in opposite proportion to the impedance in each path, to return to its source and then back to the origin of the fault. The unintentional ground-fault current flow in these completed paths facilitates the sure instantaneous operation of the overcurrent device, rapidly interrupting the energy source supplying the ground fault. The ground-fault current path must be complete and meet three important criteria:

1. The path for ground-fault current must be electrically continuous and reliable. 2. It must have adequate current-carrying capacity to conduct safely (both in magnitude and duration) any fault

likely to be imposed on it. 3. It must be of low impedance to facilitate the instantaneous operation of the overcurrent device in the ground-

fault current path.

A ground-fault current path for a grounded separately derived system/transformer that doesn’t meet these criteria becomes a silent and often lethal source of electrical shock when a ground fault occurs. If an effective ground-fault current path isn’t established and a ground-fault occurs on the derived ungrounded circuit conductors of a transformer, ground-fault current will not flow; therefore, the operation of the overcurrent protection device in the ground-fault current path won’t be initiated. Electrical raceways, enclosures, and equipment will become energized with dangerous energy, continually searching for a path back to its source. When a human body completes the ground-fault current path, it results in electrical shock or electrocution. Unlike obvious indications of faulty wiring of branch or feeder circuits, defective high-impedance ground-fault current paths are difficult to detect, because these circuits are predominantly called upon when a ground fault occur.

Five key components Following is an overview of essential areas related to bonding and grounding single, solidly grounded, 480V – 208Y/120V, delta-to-wye, 3-phase transformers.

1. System bonding jumper — The 2011 NEC defines the system bonding jumper as “the connection between the grounded circuit conductor and the supply-side bonding jumper, or the equipment grounding conductor, or both, at a separately derived system.” The objective of the system bonding jumper is to connect the grounded conductor (neutral), supply-side bonding jumper, and the equipment grounding conductors of the separately derived system/transformer, which is required to create an effective ground-fault current path.

This path allows unintentional ground-fault current to flow from the point of a ground fault on the derived ungrounded circuit conductors, to the derived source, then back to the origin of the ground fault. This unintentional ground-fault current flow elevates the current in the transformer primary winding for ground faults between the derived source of the transformer and the first overcurrent protection device — or facilitates the operation of the transformer secondary overcurrent protection devices if the ground fault is on the load side of these devices. The system bonding jumper is one of the key elements that forms the effective ground-fault current path from the furthermost downstream point in the electrical system back to the derived source, the secondary winding of the transformer. If the system bonding jumper isn’t properly installed (Photo 1 and Photo 2), an effective ground-fault current path will not be established.

Photo 1. This 8 AWG bare copper system bonding jumper is undersized for this installation. NEC Table 250.66 requires a 4 AWG or larger copper system bonding jumper for 3/0 AWG copper derived ungrounded circuit conductors.

Photo. 2. Note in the photo how the 3/0 AWG bare copper grounding electrode conductor originates from the neutral point XO and pierces through the mesh in the bottom of the transformer enclosure. Because the grounding electrode conductor is not solidly bonded to the transformer enclosure, this is no compliant system bonding jumper for this separately derived system.

Table 250.66 of the 2011 NEC is used to size the system bonding jumper based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. Because the system bonding jumper is part of the ground-fault current path, it’s necessary to maintain a proportional size relationship between the derived ungrounded circuit conductors and the system bonding jumper. Where the derived ungrounded circuit conductors are larger than the maximum sizes given in this table, 250.28(D)(1) requires the system bonding jumper be not less than 12.5% of the area of the largest derived ungrounded circuit conductor. For purposes of this article, this requirement will be designated the “12.5% rule.”

2. Grounding electrode and grounding electrode conductor — The 2011 NEC defines grounding electrode as “a conducting object through which a direct connection to earth is established,” and the grounding electrode conductor as “a conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.” The purpose of the grounding electrode and grounding electrode conductor is to connect the separately derived system/transformer grounded conductor or equipment to ground (earth), to limit the voltage imposed by line surges and to stabilize the transformer secondary voltage to ground during normal operation (Photo 3).

Photo 3. This separately derived system/transformer has a compliant system bonding jumper; however, the required grounding electrode conductor is absent.

 

 

 

 

 

 

 

The grounding electrode makes the earth connection for the transformer secondary circuit. It must be an effective connection, and all grounding paths must be connected to it. To prevent objectionable current flow, the grounding electrode conductor connection to the grounded conductor must be made at the same point on the separately derived system where the system bonding jumper and supply-side bonding jumper are connected, as specified in Sec. 250.30(A)(5).

Section 250.66 and Table 250.66 are used to size the grounding electrode conductor based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer; however, because the maximum current in a grounding electrode conductor is limited by the impedance path through the grounding electrode and earth — and is not intended to be part of the effective ground-fault current path — the 12.5% rule does not apply.

3. Bonding of metal water piping system(s) and exposed structural metal — Section 250.104(D) of the 2011 NEC requires that where a separately derived system/transformer supplies power to an area, the grounded conductor shall be bonded to the nearest available point of the metal water piping system(s) and the exposed building frame structural metal in the area served by the transformer. This bonding jumper connection effectively eliminates any possible difference of potential that can exist between the grounded conductor of the transformer derived source, the metal water piping system(s), and the exposed building frame structural metal. It also provides a ground-fault current path for ground-fault current likely to be imposed on the metal water piping system(s) or exposed building frame structural metal in the area served by the transformer.

Table 250.66 is used to size these bonding jumper conductors based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. Because the metal water piping system(s) or exposed building frame structural metal in the area served by the transformer will predominantly be used as a grounding electrode as identified in 250.30(A)(4), the rules for grounding electrode conductors apply. Therefore, the 12.5% rule does not apply. To prevent objectionable current flow, this bonding jumper conductor connection shall be made at the same point on the separately derived system where the grounding electrode conductor is connected, as stated in 250.104(D).

A separate bonding jumper from the grounded conductor to the metal water piping system(s) and exposed building frame structural metal is not required when either is used as a grounding electrode, as identified in 250.30(A)(4), and if a bonding jumper is installed between the exposed building frame structural metal and metal water piping system in the area served by the transformer.

4. Supply-side bonding jumper — The 2011 NEC defines supply-side bonding jumper as “a conductor installed on the supply side of a service or within a service equipment enclosure(s), or for a separately derived system, that ensures the required electrical conductivity between the metal parts required to be electrically connected.” Specific to this article, the supply-side bonding jumper is the conductor of the wire type, run with the derived circuit conductors from the source/transformer enclosure to the first system disconnecting means. The objective of the supply-side bonding jumper is to connect the equipment grounding conductors of the transformer derived source to the system bonding jumper/equipment grounding conductor connection, which is required to create an effective ground-fault current path. If a ground fault occurs on the derived ungrounded circuit conductors, ground-fault current will flow from the point of the ground fault on the derived ungrounded circuit conductors to the system bonding jumper/equipment grounding conductor connection by means of the supply-side bonding jumper to the derived source and then back to the origin of the fault. This unintentional ground-fault current flow elevates the current in the transformer primary for ground faults between the derived source of the transformer and the first overcurrent protection device — or facilitates the operation of the transformer secondary overcurrent protection devices if the ground fault is on the load side of these devices.

Table 250.66 is used to size the supply-side bonding jumper based on the size of the derived ungrounded circuit conductors supplied by the secondary of the transformer. The supply-side bonding jumper is part of the ground-fault current path. Therefore, the 12.5% rule does apply.

When the system bonding jumper is not located at the derived source of the separately derived system, the grounded conductor (neutral) serves as part of the supply-side bonding jumper during a ground-fault condition. Thus, in addition to the existing requirements for grounded conductor sizing, the grounded conductor must comply with the same minimum sizing requirements as the supply-side bonding jumper per 250.30(A)(3).

5. Equipment grounding conductor — The 2011 NEC defines an equipment grounding conductor as “the conductive path(s) installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.” The purpose of the equipment grounding conductor for the transformer primary circuit is to connect all conductive material that encloses the transformer’s primary ungrounded circuit conductors or electrical equipment, which is required to create an effective ground-fault current path. This path allows unintentional ground-fault current to flow from the point of a ground fault on the transformer primary ungrounded circuit conductors to the transformer metallic enclosure to the building main service or source of the transformer primary circuit and then back to the origin of the fault, facilitating the operation of the transformer primary circuit overcurrent protection devices. The equipment grounding conductors also prevent an objectionable potential above ground (earth) on raceways and equipment enclosures.

Section 250.122 and Table 250.122 are used to size the equipment grounding conductor based on the rating or setting of automatic overcurrent devices in the circuit ahead of electrical raceways, enclosures, and equipment.

The Table summarizes the components described in this article, applicable 2011 NEC sections, and sizing table headings. It provides component functions and outlines minimum requirements as identified in the NEC related to bonding/grounding for safety of single, solidly grounded, 480V – 208Y/120V, delta-to-wye, 3-phase transformers.

 

 

 

 

 

 

 

 

Grounding and Bonding — Part 1

New 2011 NEC revisions clear up confusion between bonding and grounding

Most power quality and safety issues in electrical installations arise from misapplication of the grounding and bonding requirements of Art. 250. One common problem is installers ground where they should bond.

While the NEC provides clear descriptions of grounding and bonding in Art. 100, the words are often misused in the various articles. Typically, the error involves saying “grounding” instead of “bonding.” This error is even in nomenclature such as “equipment grounding conductor.” You should not be grounding your load side equipment. You should be bonding it.

More Precise The 2008 NEC heavily revised the requirements for bonding limited energy systems. Unfortunately, the new wording created confusion. The 2011 NEC is more precise. For example, it changed “bonding and grounding conductors” to “bonding and grounding electrode conductors.”

Bonding is a means of providing electrical continuity between metallic objects. Simple definition, right? What’s not always simple is correctly applying the NEC requirements, some of which changed with the 2011 revision. That will be the focus of this article. Many of these changes were for the sake of clarity (see More Precise).

Service equipment Bond all metal raceways and enclosures that contain (or support) service conductors [250.92]. Interestingly, the NEC requires raceways and enclosures that contain feeder or branch conductors to connect to the circuit “equipment grounding conductor” [250.86], which is actually a bonding conductor [Art. 100].

If a panel knockout is oversized, concentric, or eccentric, or uses reducing washers, bond around that opening. Use a bonding jumper, not a standard locknut (Fig. 1).

Fig. 1. If a panel knockout is oversized, concentric, or eccentric — or uses reducing washers — use a bonding jumper, not a standard locknut.

The NEC gives you the choice of four methods for ensuring electrical continuity at service equipment, service raceways, and service conductor enclosures [250.92(B):

1. Bonding jumpers. Bond metal parts to the service neutral conductor. This requires a main bonding jumper [250.24(B) and 250.28]. Because the service neutral conductor provides the effective ground-fault current path to the power supply [250.24(C)], you don’t have to install an equipment grounding conductor within PVC conduit containing service-entrance conductors [250.142(A)(1) and 352.60 Ex 2] (Fig. 2).

2. Threaded fittings. Terminate metal raceways to metal enclosures by threaded hubs on enclosures (if made wrenchtight).

3. Threadless fittings. Terminate metal raceways to metal enclosures by threadless fittings (if made tight). 4. Other listed devices. These include bonding-type locknuts, bushings, wedges, or bushings with bonding

jumpers.

Fig. 2. An SSBJ isn’t required within nonmetallic conduit because the service neutral conductor serves as the effective ground-fault current path.

This last method needs more discussion. To bond one end of the service raceway to the service neutral conductor, you must use a listed bonding wedge or bushing with a bonding jumper. Size it per Table 250.66, based on the area of the largest ungrounded service conductors within the raceway [250.102(C)].

When a metal raceway containing service conductors terminates to an enclosure without a ringed knockout, you can use a bonding-type locknut. Bonding one end of a service raceway to the service neutral provides the low-impedance fault current path to the source (Fig. 3).

Fig. 3. Bonding one end of a service raceway to the service neutral provides the low-impedance fault current path to the source.

Other systems

You can’t have “separate grounds” between communications systems and your service. You must provide an external intersystem bonding terminal (for connecting communications systems bonding conductors at service equipment) [250.94]. For structures supplied by a feeder, do this at the metering equipment enclosure and disconnecting means (Fig. 4).

Fig. 4. For structures supplied by a feeder, you must provide an external intersystem bonding terminal at the metering equipment enclosure and disconnecting means.

The resulting termination must:

• Be accessible for connection and inspection. • Consist of a set of terminals with the capacity for connecting at least three intersystem bonding conductors. • Not interfere with opening the enclosure for a service, building/structure disconnecting means, or metering

equipment. • Be securely mounted and electrically connected to service equipment, the meter enclosure, or exposed

nonflexible metallic service raceway — or it must be mounted at one of these enclosures and connected to the enclosure or grounding electrode conductor. Use at least a 6 AWG copper conductor.

• Be securely mounted to the structure disconnecting means — or it must be mounted at the disconnecting means and connected to the enclosure or grounding electrode conductor. Use at least a 6 AWG copper conductor.

• Use terminals listed as grounding and bonding equipment.

Bonding conductors and jumpers The 2011 revision helps distinguish between the rules for bonding jumpers upstream from an overcurrent device versus bonding jumpers downstream from an overcurrent device.

The NEC now clarifies that bonding jumpers on the load side of an overcurrent device must comply with all of Sec. 250.122, not just Table 250.122. It also:

• Clarifies the rules for bonding jumpers installed in a raceway versus those installed outside a raceway. • Adds provisions for protecting aluminum bonding jumpers against corrosion. • Addresses physical protection for all bonding jumpers.

Equipment bonding jumpers must:

• Be copper. • Terminate by listed pressure connectors, terminal bars, exothermic welding, or other listed means [250.8(A)].

Supply-side bonding jumpers:

• Size these per Table 250.66, based on the largest ungrounded conductor within the raceway. • If the ungrounded supply conductors are larger than 1,100kcmil copper or 1,750kcmil aluminum, size the

bonding jumper at least 12.5% of the area of the largest set of ungrounded supply conductors. • If the ungrounded supply conductors and the supply-side bonding jumper are of different materials, size the

supply-side bonding jumper on the assumed use of the same material.

Size bonding jumpers on the load side of feeder and branch circuit overcurrent devices per 250.122, based on the rating of the circuit overcurrent device. The equipment bonding jumper doesn’t have to be larger than the largest ungrounded circuit conductors [250.122(A)].

If you use a single equipment bonding jumper to bond two or more raceways, size it per 250.122, based on the rating of the largest circuit overcurrent device.

You can install equipment bonding jumpers, bonding jumpers, or bonding conductors inside or outside of a raceway.

• If inside a raceway, these conductors must be identified per 250.119. If circuit conductors are spliced or terminated on equipment within a metal box, then the equipment grounding conductor associated with those circuits must be connected to the box per 250.148.

• If outside of a raceway, these conductors can’t be longer than 6 ft and must be routed with the raceway.

Piping systems and exposed structural metal Metal-piping systems, such as sprinkler, gas, or air, that are likely to become energized must be bonded to the electrical system. This bonding prevents a difference of potential that can produce flashover and ignition.

The equipment grounding conductor (for the circuit that’s likely to energize the piping) can serve as the bonding means [250.104]. Via an Informational Note, the NEC now alerts the reader that the National Fuel Gas Code, NFPA 54, Sec. 7.13 contains further information about bonding gas piping.

If it’s likely to become energized, exposed structural metal that forms a metal building frame must be bonded to the one of the following:

• Service equipment enclosure. • Service neutral conductor. • Structure disconnecting means (structures supplied by a feeder or branch circuit). • Grounding electrode conductor (if sufficiently sized). • Grounding electrode system.

Size the bonding jumper per Table 250.66, based on the area of the ungrounded supply conductors. The bonding jumper must be copper if within 18 in. of the earth [250.64(A)], securely fastened to the surface on which it’s carried [250.64(B)] and adequately protected if exposed to physical damage [250.64(B)]. All points of attachment must be accessible, except as permitted in 250.68(A).

Separately derived systems You must bond a separately derived system (SDS) to either the:

• Nearest available point of the metal water piping system in the area served by the SDS, or • Structural metal building frame — but only if it serves as the grounding electrode [250.52(A)(1)] for the SDS.

You must bond the SDS to exposed structural metal (interconnected to form the building frame), unless the structural frame serves as the grounding electrode [250.52(A)(2)] for the SDS.

In all three of the above cases:

• Bond to the neutral point of the SDS at the grounding electrode conductor connection point [250.104(D)(1)]. • Size the bonding jumper per Table 250.66, based on the area of the largest ungrounded conductor of the

derived system.

Previous NEC revisions required you to bond structural metal (if likely to become energized) to the service equipment enclosure. But what about a structure supplied by a feeder or branch circuit?

The 2011 revision makes it clear that you must bond the structural metal (if likely to become energized) to the disconnecting means of the structure, regardless of the type of circuit feeding the premises.

No pure-play Figure 250.1 lays out Art. 250 into three informational blocks (plus a fourth that’s off to the side). We’ve now addressed bonding, which is off in a block by itself. But contrary to what 250.1 may indicate, the other blocks are not grounding pure-plays. In our next issue, we’ll see where grounding and bonding collide

Grounding and Bonding — Part 2

One of the revisions to the 2011 NEC involves a new definition for a common term: bonding jumper, supply-side. A supply side bonding jumper is a conductor on the supply side or within a service or separately derived system to ensure the electrical conductivity between metal parts required to be electrically connected (Fig. 1).

Fig. 1. A supply-side bonding jumper is a conductor on the supply side or within a service to ensure the electrical conductivity between metal parts required to be electrically connected.  

 

 

 

The method used to size bonding jumpers depends on their location in the circuit, which can be a point of confusion. Generally speaking, if bonding conductors are:

1. On the load side of the service main overcurrent device, they are sized according to Table 250.122, based on the rating of the overcurrent device.

2. On the supply side of a service main overcurrent device or ahead of the overcurrent protection device on the secondary of a separately derived system, they are sized using Table 250.66 and the 12.5% rule in 250.102(C).

Because of the deletion of the definition “grounding conductor” from Art. 100, a revision was required in the note to 250.4(A)(1) that had previously used this term. The revised reference to bonding and grounding electrode conductors now provides a much more specific Code application. This note now makes it clear that the grounding conductor being referred to is the grounding electrode conductor, which should not be any longer than necessary.

The description of currents that aren’t considered to be objectionable has been changed in 250.6(C). Temporary currents from “abnormal conditions, such as ground faults,” aren’t to be classified as objectionable current.

The NEC previously used the term “accidental” to describe conditions that resulted in currents not deemed objectionable. Ground faults are listed in this section as an example of a condition that results in this current, but only under abnormal conditions.

Objectionable current Objectionable neutral current occurs because of improper neutral-to-case connections or wiring errors that violate 250.142(B). The reason this current is considered objectionable is because when it flows on or through metal parts, it can cause improper operation of electronic equipment, fires, electric shock, and even death.

When a system is properly bonded, the voltage between all metal parts will be zero. When properly grounded, the voltage between ground and those parts is also zero. However, the absence of a zero volt reference may very well be an indication of the presence of objectionable neutral current (Fig. 2).

Fig. 2. The absence of a zero volt reference may be an indication of the presence of objectionable neutral current.

Objectionable neutral current will flow when the neutral conductor:

• Is connected to the metal case of a panelboard that’s not used as service equipment. • Is connected to the metal case of a disconnecting means that’s not part of the service equipment. • Is connected from one system to a circuit of a different system due to wiring errors (Fig. 3).

 

 

 

 

 

 

 

 

 

 

Fig. 3. Danger: The 120V/208V panelboard (de-energized) can have dangerous voltage from the 277V lighting circuit because of the crossed neutrals.

It will also flow on metal parts if:

• The circuit equipment grounding conductor is used as a neutral conductor.

• The neutral conductor is connected to the circuit equipment grounding conductor on the load side of the system bonding jumper for a separately derived system.

The removal from Art. 100 of the term “grounding conductor” also had an effect in 250.8(A), which now makes it clear that its conductor termination requirements apply to bonding jumpers, equipment grounding conductors, and grounding electrode conductors.

Part II. System grounding and bonding The 2011 NEC addresses the location of ground detection sensing equipment for ungrounded systems as well as adding marking requirements for ungrounded systems.

Ground detectors are required for ungrounded systems that operate between 120V and 1,000V [250.21]. The location of the sensing equipment has never been explicitly stated in the NEC until now, leaving some people confused as to the intent of the requirement.

With this revision, these sensing devices must be installed as close as practicable to the point where the system receives its supply. Previously, you could install this equipment at the branch circuit level. Doing so would leave all of the feeder circuits without the detection, allowing them to operate under a ground fault without any indication.

Several revisions to 250.24(C) make it easier to navigate. The Code now spells out what the rules are for sizing the grounded conductor when dealing with service conductors in a single raceway and when parallel service conductors are installed in multiple raceways. A new subsection (3) was also added to clarify that the grounded conductor of a delta-connected, corner-grounded 3-phase, 3-wire service must have an ampacity at least equal to that of the ungrounded conductors.

Separately derived systems Section 250.30 has been reorganized and includes many revisions and notes to clarify the grounding and bonding requirements of separately derived systems (SDS). Following are some highlights:

• A neutral-to-case connection must not be made on the load side of the system bonding jumper, except as permitted by 250.142(B).

• An unspliced system bonding jumper must be installed at the same location where the grounding electrode conductor terminates to the neutral terminal of the SDS — either at the SDS or the system disconnecting means, but not at both locations [250.30(A)(5)].

• Where the system bonding jumper is installed at the source of the SDS, the jumper must connect the neutral conductor of the SDS to the supply-side bonding jumper and the metal enclosure of the source (transformer case).

• If the system bonding jumper is installed at the first disconnecting means of an SDS, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the disconnecting means (Fig. 4). 

• If the SDS and the first disconnecting means are in separate enclosures, a supply-side bonding jumper must run to the SDS disconnecting means. The supply-side bonding jumper can be a nonflexible metal raceway, a wire, or a bus. 

Fig. 4. If the system bonding jumper is installed at the first disconnecting means of an SDS, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the disconnecting means. 

 

 

 

If the supply-side bonding jumper is:

• Of the wire type, size it per Table 250.66, based on the area of the largest ungrounded SDS conductor in the raceway or cable.

• A bus with a cross-sectional area no smaller than required by Table 250.66. • Installed at the disconnecting means instead of the source, the requirements of 250.30(A)(3) apply.

Install the grounding electrode as near as practicable — preferably in the same area as the system bonding jumper [250.30(A)(4)]. Size its conductor per 250.66, based on the area of the largest ungrounded conductor of the derived system [250.30(A)(5)].

Buildings or structures supplied by a feeder or branch circuit The exception dealing with existing installations has been clarified, and a new subsection addresses buildings supplied by an SDS.

The 2008 NEC contained a significant change to 250.32 by requiring that an equipment grounding conductor be installed for buildings or structures supplied by feeders or branch circuits. With this change, an exception added to the 2008 Code clarified that existing premises wiring systems need not comply with this new rule.

Because most NEC changes don’t come with an exception that gives exemption for existing installations, confusion ensued. For example, when the AFCI requirements were expanded, there wasn’t an exception for existing buildings, because the Code isn’t retroactive. Why not? Because, as 90.2(A) tells us, the NEC is an installation standard, not a maintenance standard.

So when the exception was added, many Code users had no idea what it really pertained to. The change in this NEC revision cycle, however, clarifies that this exception applies only to existing structures that met the previous Code requirements and continue to meet the previous requirements, such as not having continuous grounded metal paths between the two buildings or structures.

Previous NEC editions required a building or structure supplied by a feeder or branch circuit to be supplied with an equipment grounding conductor. When a building or structure is supplied by an SDS, with no overcurrent protection at the source, there’s no equipment grounding conductor by definition. This revision clarifies that when this occurs, a supply-side bonding jumper must be run to the building or structure and installed per 250.30(A).

Some other key points from all these revisions include:

• To quickly clear a ground fault and remove dangerous voltage from metal parts, the building or structure disconnecting means must be connected to the circuit equipment grounding conductor, which must be one of the types described in 250.118 [250.32(B)(1)].

• If the supply circuit equipment grounding conductor is of the wire type, size it according to 250.122, based on the rating of the overcurrent device.

• You can’t connect the supply circuit neutral conductor to the remote building or structure disconnecting means [250.142(B)]. However, the neutral conductor can continue to serve as the ground-fault return path for the structure disconnecting means for existing installations in compliance with previous editions of the Code where there are no continuous metallic paths between structures, and some other conditions are met [250.32(B)(1)Ex].

When applying the revised requirements of Part II, it’s critical that you don’t confuse the grounding path with the bonding path — and that you watch those neutral connections carefully. As a rule, the neutral should never be connected to the enclosure or equipment grounding conductor anywhere except in the service disconnects and the secondary side of separately derived systems. Objectionable neutral current presents a real danger and can damage equipment as well as cause fires and electric shock or electrocution.

In the final part of this series, we’ll turn our attention to Parts III, IV, and VI of Art. 250.

 

 

 

Guidelines for Grounding and Bonding Telecom Systems

Updates and changes to codes and standards that affect the way you design and install telecommunications systems

Because bonding and grounding systems within a building are intended to have one electrical potential, coordination between electrical and telecommunications bonding and grounding systems is essential during design and installation. One way to coordinate these efforts is to follow industry-established codes and standards. But how do you know which ones to follow? This article presents a brief history and overview of the relevant codes and standards you should be familiar with as well as discusses recent developments that affect all designers and installers.

Of course, the first relevant code is the National Electrical Code (NEC), which addresses bonding and grounding as minimum requirements for life safety. While ensuring public safety is the highest priority, the industry began to realize in the late 1980s and early 1990s that the electrical grounding requirements, while protecting end-users, were not protecting the end-user’s expensive electronic (IT) equipment.

The industry addressed this concern by developing standards. While similar to a code, which is adopted by local states and municipalities into law, the use of a standard is voluntary. However, it may become a requirement for any given project, if the owner or designer/engineer lists it as such in the construction documents.

One of the first standards to address bonding and grounding was IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (the Green Book) when Chapter 5 was added in the 1991 edition – Electronic Equipment Grounding. This addition primarily featured improved bonding and grounding practices for the power systems serving information technology (IT) equipment.

The IEEE followed up with IEEE Standard 1100, Recommended Practice for Powering and Grounding Electronic Equipment (the Emerald Book) in 1992. IEEE 1100 expanded on the Green Book, explaining the issues with poor power quality, lightning, and other surges and different ground potentials on metallic data cabling.

In 1994, the Telecommunications Industry Association (TIA) and the Electronic Industries Association (EIA) published ANSI/TIA/EIA-607, Commercial Building Grounding and Bonding Requirements for Telecommunications, which established the need for a dedicated telecommunications grounding and bonding system (see Main Components of a Telecommunications Grounding and Bonding System on page C27). This standard specified requirements for a ground reference (ground busbar) in each telecommunications space, including the telecommunications entrance room(s), telecommunications closets, and IT equipment rooms. It also established the bonding of telecommunications system pathways within the telecommunications spaces to the ground reference.

The ANSI/TIA/EIA standard was revised in 2002 to become ANSI-J-STD-607-A, Commercial Building Grounding (Earthing) and Bonding Requirements for Telecommunications. The standard was developed jointly by TIA/EIA Working Group 41.7.2 in close coordination with the Alliance for Telecommunications Industry Solutions (ATIS) T1E1.5 and T1E1.7. The term “earthing” was used as the internationally accepted term for “grounding.” The major changes from ANSI/TIA/EIA-607 were greater grounding busbar detail, the addition of tower and antenna bonding and grounding recommendations, work area and personal operator-type equipment position grounding and bonding recommendations, and harmonized international terminology (although terminology from the NEC was retained). Nevertheless, the standard only addressed the connections from the electrical ground to the busbar in each telecommunications space — the connection from the busbar to the equipment was still missing.

One of the glaring omissions from the standard was ensuring a quality installation. To address this issue, NECA and BICSI developed a joint standard, ANSI/NECA/BICSI-607, Standard for Telecommunications Bonding and Grounding Planning and Installation Methods for Commercial Buildings. Published in 2011, this standard provides limited planning information, but excels in eliciting installation requirements. For example, it clearly states how to ensure a bond to the busbar by using an antioxidant compound to the connection point using compression 2-hole lugs. From a planning perspective, the standard specifies bonding to a telecommunications rack and includes bonding and grounding information for data centers. However, one of the drawbacks is it was based upon ANSI-J-STD-607-A while the updated ANSI/TIA-607-B standard was in development and nearing publication.

ANSI/TIA-607-B, Generic Telecommunications Bonding and Grounding (Earthing) for Customer Premises is by far the most encompassing telecommunications standard for bonding and grounding. Its requirements are for “generic” premises rather than for just a commercial building, and its purpose is to enable and encourage the planning, design, and installation of generic telecommunications bonding and grounding systems within premises. This is to address the basic grounding and bonding requirements without prior knowledge of the specific telecommunication or IT system that will be installed. While primarily intended to provide direction for the design of new buildings, this standard may be used for existing building renovations or retrofits. Design requirements and choices are provided to enable the designer to make informed design decisions.

The ANSI/TIA-607-B standard covers regulatory requirements, an overview of a bonding and grounding system, the components involved, and design requirements. Additionally, performance and test requirements are covered, although simplistically. The TR-42.16 Subcommittee recognized this shortfall, and is already developing an addendum to this standard on:

• soil resistivity testing using a 4-point method, • grounding electrode system design, • grounding electrode system resistance testing including the fall of potential method and use of the clamp-on

test meter.

Through these updates, there have been many changes to the components of a telecommunications bonding and grounding system. Busbar requirements were modified to be made of copper or copper alloys having a minimum of 95% conductivity when annealed, as specified by the International Annealed Copper Standard (IACS). While some manufacturers are offering a less expensive product made of other metals, copper is still the preferred material. Regardless of the material, the busbars must still be listed, and the dimensions of the telecommunications main grounding busbar (TMGB) and the telecommunications grounding busbar (TGB) have not changed. As for the connectors to these busbars, the surface of all bonding and grounding connectors used on a TMGB and TGB shall be of a material that provides an electrochemical potential of less than 300mV between connector and grounding busbar. Essentially, with these requirements, the standard is ensuring a well-performing bonding and grounding system.

Conductor sizing is still 2kcmil per linear foot of conductor length, but the maximum size is now 750kcmil (Table). The previous edition of this standard sized the TBB conductor up to 3/0 AWG. The standard also states that the grounding conductors shall not decrease in size as the grounding path moves closer to earth, and the size of the conductor is not intended to account for the reduction or control of electromagnetic interference (EMI). The sizing of this conductor affects the:

• BCT (bonding conductor for telecommunications) • TBB (telecommunications bonding backbone) • GE (ground equalizer) • Supplementary bonding network (found in computer rooms) • TEBC (telecommunications equipment bonding conductor) • UBC (unit bonding conductor)

Design requirements for this standard include the entire system from the entrance point to the equipment in the rack within the telecommunications room. It also specifies that ANSI/NECA/BICSI-607 is to be used for bonding and grounding installation information. In order to limit the potential difference between telecommunications conduits or between telecommunications conduits and power conduits, the standard specifies that the telecommunications conduits shall be bonded to the TMGB/TGB.

Additionally, to achieve the objectives of potential equalization, ensure that cable runway/ladder sections are bonded together to the TMGB/TGB. When the electric panelboard serving that telecommunications room is located in the same room or space as the TMGB/TGB, that panelboard’s alternating current equipment ground (ACEG) bus (when equipped) or the panelboard enclosure shall be bonded to the TMGB/TGB. A qualified electrician should install this bond.

Informative annexes included with this standard have information on grounding electrodes, towers and antennas, telecommunications electrical protection, electrical protection for operator-type equipment positions, and a cross reference of more commonly used terms.

For a designer of telecommunications bonding and grounding systems, the ANSI/TIA-607-B standard is the most encompassing standard to follow for premises buildings. Although there are many other guides (see Resources at a Glance below), standards are developed so that a consensus must be reached among industry expert volunteers. Although best practices may have valuable information, they typically have not been vetted among a large subset of industry experts.

What does this mean for the electrical contractor? For anyone with a telecom division, it’s important to stay current on the telecommunications grounding standards. A well-designed system would include a telecommunications grounding detail and riser diagram (click here to see Figure), and specifications would list the most recent edition of the applicable standards.

For drawings and specifications that are silent — or that may reference outdated standards or conflicting guidelines — the contractor should ask the design team for clarification (during the bid window, if possible) as to what sizing to follow for the TBB, because a change in conductor size may greatly affect cost.

For electrical contractors subcontracting out the telecommunications work, the demarcation point for work between electrical and telecommunications contractors should be carefully coordinated. A recommended practice is for the electrical contractor to provide the grounding conductor and connection from the main electrical ground to the TMGB, as well as from an electrical panel in a telecommunications room to the grounding busbar in that room. The telecommunications contractor would then provide all of the grounding busbars and bonding conductors within and between the telecommunications rooms, as well as make all final connections to the TMGB, TGBs, and telecommunications infrastructure/equipment.

Peterworth works in the Information Technology Services - Networking & Telecommunications department at the University of Texas at Austin in Austin, Texas. He can be reached at: mpeterworth@austin.utexas.edu.

Sidebar: Main Components of a Telecommunication Grounding and Bonding System

The Telecommunications Main Grounding Busbar (TMGB) is typically located in the telecommunications entrance facility — where the telecommunications cables enter the building and need to transition to indoor-rated cables per Sec. 800.48 of the NEC, which limits unlisted cables to 50 ft or less. This busbar is pre-drilled and made of copper with a minimum of 4-in.-wide by ¼-in. -hick material of varying length. It is to be connected to the main electrical ground with an appropriately sized copper conductor. The entrance protectors from the outside cables and conduits are to be connected to this TMGB, as well as any other telecommunications equipment co-located in the entrance facility. (It is common practice for the entrance facility to also be the main network room for IT equipment.)

In all other telecommunications rooms, there is to be a telecommunications grounding busbar (TGB), to be made of copper with pre-drilled holes and minimum dimensions of 2-in.-wide by ¼-in.-thick material of varying length. These are to be connected back to the TMGB through appropriately sized copper conductors that form the telecommunications bonding backbone (TBB), as shown in the Figure (click here to see Figure).

In each telecommunications room, the ladder rack, equipment rack, entrance (lightning) protectors for the telecommunications lines, and even IT equipment are connected back to the TMGB or TGB with a bonding conductor. If either building steel or the electrical panel is available in the telecommunications room, they need to be connected back to the TMGB or TGB as well, with a minimum of 6 AWG copper conductor or larger.

Sidebar: Resources at a Glance There are many standards and guidelines a designer may choose to specify. Here are a list of several that may be referenced for more specific project and installation applications:

• ANSI/ATIS-0600334, Electrical Protection of Communications Towers and Associated Structures • ATIS 0600318, Electrical Protection Applied to Telecommunications Network Plant at Entrances to Customer

Structures or Buildings

• ATIS 0600321, Telecommunications – Electrical Protection for Network Operator-Type Equipment Positions • ATIS-0600313, Electrical Protection for Telecommunications Central Offices and Similar Type Facilities • EN 50310, Application of Equipotential Bonding and Earthing in Buildings with Information Technology

Equipment • MIL-HDBK-419A, Grounding, Bonding, and Shielding for Electronic Equipments and Facilities Basic Theory • ANSI/IEEE 1100, 2005, Recommended Practice for Powering and Grounding Electronic Equipment • ANSI/IEEE C2, 2007, National Electrical Safety Code (NESC) • ANSI/ATIS 0600333, Grounding and Bonding of Telecommunications Equipment • ANSI/ATIS 0600334, 2008, Electrical Protection of Communications Towers and Associated Structures • ANSI/TIA/EIA-606-A, 2007, Administration Standard for the Telecommunications Infrastructure of Commercial

Buildings • FIPS PUBS 94, 1983, Guideline on Electrical Power for ADP Installations, 1983 (USA Federal Information

Processing Standards Publications) • ITU-T K.27, 1996, Bonding Configuration and Earthing Inside a Telecommunication Building