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Brainfiller® Guide for:

Streamlined Electrical Design Based on the 2005 NEC®

STREAMLINED ELECTRICAL DESIGN

Edition 1.12

Jim Phillips, P.E.

t2g technical training group®

Copyright © 2007 Technical Training Group All Rights Reserved

brainfiller.com® ELECTRICAL DESIGN GUIDE

Jim Phillips, P.E. T2G Technical Training Group® of 37

DISCLAIMER This training notebook was developed by Technical Training Group / Jim Phillips, P.E., Canton, Ohio. Technical Training Group, has attempted to ensure that the information contained in this notebook is as accurate as possible. Information contained in this notebook is subject to change without notice and neither Technical Training Group nor Jim Phillips, P.E., assumes any responsibility for any errors, omissions or damages resulting from the use of the information contained within. This guide is only a compilation of data and information obtained from various sources and in no way constitutes an attempt by Technical Training Group or Jim Phillips, P.E. to render engineering or other professional services. If such services are required, a licensed professional engineer should be consulted. This information must be verified by the design professional in responsible charge of the design of the project before using it. The information contained in this training notebook is protected under United States Copyright laws. Much of the information is gathered from a variety of sources such as the National Electrical Code®. It must be noted that the tables apply to “typical” systems and may not account for issues such as but not limited to: elevated ambient temperatures, tap rules, special equipment requirements, and other exceptions or requirements that may be necessary. Permission is granted to reproduce this work provided it is not for sale and that the work is reproduced in its entirety with all pages including the cover pages, disclaimer, header and footers acknowledging Technical Training Group and Jim Phillips, P.E. Where trademarks or trade names are referenced, they are listed in the appendix in the back of this manual. The end user, by accepting this document or copy thereof, agrees to the conditions listed on this page.

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Table of Contents 1.0 Introduction ……………………………………………………………….5 2.0 Conductor Selection……………………………………………………..6 2.1 Conductor Selection Criteria 2.2 Load Current 2.3 Phase Conductors 2.4 Equipment Grounding Conductors 2.5 Overcurrent Protection 2.6 Additional Considerations 2.7 Conductor Schedules 3.0 Transformer Circuits ..…………………………………………………..11

3.1 Transformer Size 3.2 Transformer Protection 3.3 NEC® Table 450.3(B) 3.4 Magnetizing Inrush Current 3.5 ANSI Thru Fault Protection 3.6 Transformer Conductors 3.7 Transformer Schedules

4.0 Infinite Bus Short Circuit Calculations…………………….….……..19 4.1 NEC® Article 110.9 Requirements 4.2 Transformer Impedance 4.3 Infinite Bus Short Circuit Calculations 4.4 Motor Contribution 4.5 Calculation Worksheets

5.0 Calculation of Conduit Fill...….………….……….…………………….25 6.0 Motor Schedules………………………………………………………….27

6.1 Motor Circuits 6.2 Motor Full Load Current Ratings 6.3 Motor Feeder Sizing 6.4 Motor Short Circuit Protection

6.5 Motor Overload Protection 6.6 Motor Time Current Curves 6.7 Motor Schedules

7.0 Coordination Analysis…………………….……………………………33 7.1 Time Current Curves 7.2 Selective Coordination

References and Trademarks



1.0 INTRODUCTION Designing an electrical power system can be quite a complex task. Numerous codes and standards such as the National Electrical Code®, ANSI Standards, IEEE, U.L. and others, dictate many of the design requirements. The electrical power distribution system is actually made up of many “typical building blocks” such as feeders, motor circuits, panels, and transformers. Each of these “building blocks” generally has components such as cable and conduit selected based on a load current requirement. The experienced designer recognizes that a substantial portion of a design’s building blocks are always the same and can be standardized to some extent. As an example, there are only so many ways to size an 800 Amp circuit just like there are only so many ways to size a 460 Volt, 40 HP motor feeder. This guide provides many of the “typical” configurations of the building blocks in a series of tables that can be used in streamlining the electrical design process. This guide is in no way a substitute for having the design performed by a qualified electrical design professional. The electrical design must always be performed by or under the direction of a licensed professional. There are many exceptions and other considerations to the information provided in this guide such as “tap rules” as allowed by the NEC®, voltage drop, harmonics, conductor de-rating just to name a few. The designer must be sure that all of the special considerations have been addressed and that the design follows all appropriate codes and standards.

In addition to using streamlined electrical design tables, there are many commercially available computer programs available on the market that can assist in further accelerating the design process. Many of the programs also integrate a vast array of power system studies such as short circuit, coordination and arc flash into their packages allowing the use of the design data base for the study data base.



2.0 CONDUCTOR SELECTION 2.1 Conductor Selection Criteria Selecting the correct conductor requires considering many variables. The selection process usually begins with defining a load and then selecting a conductor with sufficient ampacity for that load. However the actual selection process also involves:

• Load requirements • Continuous / non-continuous loading • Insulation rating • Ambient temperature • Number of current carrying conductors in a raceway • Temperature rating of device terminals • Harmonics, voltage drop and many other factors

2.2 Load Current The NEC® considers electrical loads to be either continuous or non-continuous. A load that is expected to continue for three or more hours is defined as a continuous load. Conversely, a non-continuous load is expected to be operating less than three hours. The NEC® requires that circuits be designed to handle 125% of the continuous load and 100% of the non-continuous load. As an example, sizing a feeder for an 80 Amp continuous load would require the circuit be sized at least 125% of 80 Amps or 100 Amps. Looking at this in terms of the maximum circuit loading, conductors are only allowed to be continuously loaded up to 80% of their ampacity. Therefore the 100 Amp circuit can only be loaded to 80% X 100 Amps or 80 Amps continuously. 2.3 Phase Conductors The phase conductors listed in the tables are based on ampacities listed in NEC® Table 310.16 and are based on the 75ºC rating. Article 110.14(C) requires that the a conductor can not be loaded to a current that would produce a temperature greater than the device terminal rating which is typically 75ºC. For devices with terminal ratings other than 75ºC other considerations need to be made.



2.4 Equipment Grounding Conductors The equipment ground wire selection is based on NEC® table 250.122 and the overcurrent device rating listed in the table. This assumes the conductor is protected at it’s ampacity however the NEC® requires that the size of the equipment ground conductor be increased if the size of the phase conductor is increased such as in the case of solving a voltage drop problem. 2.5 Overcurrent Protection The rating of the overcurrent protection listed in the table is based on NEC Article 240.4 which requires that conductors other than flexible cords, flexible cables, and fixture wires be protected at their ampacity unless one of the numerous exceptions apply such as the “tap rules”. The basic concept of this protection article is that if you have a 50 Amp conductor, you protect it with a 50 Amp overcurrent device. The NEC provides many exceptions to this simple concept. Where the ampacity of the feeder conductor does not match a standard overcurrent device rating, the next higher device rating is selected as long as it is no greater than 800 Amps in accordance with NEC® requirements. Where the next larger standard device is selected, care must be given so the load does not exceed 80 percent of the conductors ampacity for continuous loads or 100 percent for non-continuous ratings. An example would be the common use of 500 kcm copper conductors for a 400 Amp circuit. The ampacity based on the 75 ºC rating of a 500 kcm conductor is 380 Amps but since a 380 Amp overcurrent device is not standard, the NEC allows the use of the next higher standard overcurrent device rating which is 400 Amps. In this case, the maximum allowable continuous loading is 304 Amps (80% x 380 Amps) rather than 320 Amps (80% x 400 Amps). Another example would be the use of 3 sets of 500 kcm conductor. The ampacity of the total of 3 sets would be 3 X 380 Amps or 1140 Amps total. In this case, since the ampacity is above the 800 Amp limit for using the next size standard overcurrent device a 1200 Amp device is not allowed. There are two options in this case. Option number one is to increase the circuit ampacity to meet or exceed 1200 Amps. Option number two is to use an overcurrent device rated 1140 Amps or less. Many solid state circuit breakers have an adjustable long time pick up setting that allows a 1200 Amp breaker to be set at 1140 Amps.



2.6 Additional Considerations The conductor ampacity is also based on no more than three current carrying conductors per raceway. When there are more than three current carrying conductors, the ampacities must be derated based on Table 2.1. The ampacity is also based on the conductors being applied within the rated ambient temperature of 30ºC. Additional derating must be applied to the ampacity for conductors used in higher ambient applications. 2.7 Conductor Schedules The tables are based on “Typical” feeders and must be used with care. There are numerous exceptions in the NEC that are not considered in the tables and should be addressed on a case by case basis. Such exceptions might include derating due to harmonics, over sizing the neutral due to harmonics, tap rules as well as other design factors. The table in this section provides a list of phase and ground conductor sizes, and overcurrent protection requirements for “typical” circuits. Both 3-wire circuits and 4-wire circuits with an individual ground conductor are included. The phase conductor selections listed in the conductor schedules are based on serving continuous loads. This requires that both the conductors and overcurrent protection (80% rated) would be subject to no more than 80% continuous load. As an example, a 100 Amp circuit would be loaded to no more than 80 Amps (80% x 100).

Excerpt from NEC® Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

Number of Current-Carrying Conductors

Percent of Values in Tables 310.16 through 310.19 as Adjusted for Ambient Temperature

if Necessary

4-6 80

7-9 70

10-20 50

Table 2.1 Adjustment for More than 3 Current Carrying Conductors in a Raceway or Cable



Table 2.2 Conductor Schedule - 3 Phase 3 Wire Circuits

Overcurrent Device Rating

No. Sets

Phase Conductor

Conductor Ampacity

Equipment Ground

30 1 3 - #10 35 A 10

60 1 3 - #6 65 A 10

100 1 3 - #2 115 A 8

125 1 3 - #1 130 A 6

150 1 3 - 1/0 150 A 6

175 1 3 - 2/0 175 A 6

200 1 3 - 3/0 200 A 6

225 1 3 - 4/0 230 A 4

250 1 3 - 250 kcm 255 A 4

300 1 3 - 350 kcm 310 A 4

400 1 3 - 500 kcm 380 A 2

400 1 3 - 600 kcm 420 A 2

500 2 3 - 250 kcm 510 A 2

600 2 3 - 350 kcm 620 A 1

800 2 3 - 500 kcm 760 A 1/0

800 2 3 - 600 kcm 820 A 1/0

1000 3 3 - 400 kcm 1005 A 2/0

1200 4 3 - 350 kcm 1240 A 3/0

1600 4 3 - 600 kcm 1680 A 4/0

1600 5 3 - 400 kcm 1675 A 4/0

1600 5 3 - 500 kcm 1900 A 4/0

2000 6 3 - 400 kcm 2010 A 250 kcm

2000 6 3 - 500 kcm 2280 A 250 kcm

1. Circuit ampacity is based on listed conductor ampacities from NEC® Table 310.16 based on 75ºC. 2. Conductors are assumed to be operating at rated ambient temperature with no correction applied. 3. No more than 3 current carrying conductors in each conduit with minimal non-linear loads. 4. Device terminals must be suitable for 75ºC. Circuit ampacity is based on copper conductors. 5. The table represents common circuit designs but does not represent all possible combinations. 6. Other NEC® rules and exceptions still may apply.



Table 2.3 Conductor Schedule - 3 Phase 4 Wire Circuits

Overcurrent Device Rating

No. Sets

Phase and Neutral

Conductor

Conductor Ampacity

Equipment Ground

30 1 4 - #10 35 A 10

60 1 4 - #6 65 A 10

100 1 4 - #2 115 A 8

125 1 4 - #1 130 A 6

150 1 4 - 1/0 150 A 6

175 1 4 - 2/0 175 A 6

200 1 4 - 3/0 200 A 6

225 1 4 - 4/0 230 A 4

250 1 4 - 250 kcm 255 A 4

300 1 4 - 350 kcm 310 A 4

400 1 4 - 500 kcm 380 A 2

400 1 4 - 600 kcm 420 A 2

500 2 4 - 250 kcm 510 A 2

600 2 4 - 350 kcm 620 A 1

800 2 4 - 500 kcm 760 A 1/0

800 2 4 - 600 kcm 820 A 1/0

1000 3 4 - 400 kcm 1005 A 2/0

1200 4 4 - 350 kcm 1240 A 3/0

1600 4 4 - 600 kcm 1680 A 4/0

1600 5 4 - 400 kcm 1675 A 4/0

1600 5 4 - 500 kcm 1900 A 4/0

2000 6 4 - 400 kcm 2010 A 250 kcm

2000 6 4 - 500 kcm 2280 A 250 kcm

1. Circuit ampacity is based on listed conductor ampacities from NEC® Table 310.16 based on 75ºC. 2. Conductors are assumed to be operating at rated ambient temperature with no correction applied. 3. No more than 3 current carrying conductors in each conduit with minimal non-linear loads. 4. Device terminals must be suitable for 75ºC. Circuit ampacity is based on copper conductors. 5. The table represents common circuit designs but does not represent all possible combinations. 6. Other NEC® rules and exceptions still may apply.



3.0 TRANSFORMER CIRCUIT TABLES 3.1 Transformer Size There are various methods that can be used for sizing transformers depending on factors such as capacity for future load growth, demand factors, load diversity and others. Most NEC® calculations for a facility’s load are based on total Volt-Amperes or VA. Taking the total VA and dividing by 100 to convert it to kVA is one method for sizing a transformer. Another method is if the total load is given in Amps, the total load current can be converted from Amps to kVA as in the example below: To calculate the total 3 phase kVA given a three phase current on each phase we use the following formula:

kVA3phase = Amps3phase X kVLine-Line X √3 Where:

kVA3phase = total three phase kVA of load kVLine-Line = Voltage line-line in kV i.e. 480 Volts = 0.48 kVA Amps3phase = Current on each phase √3 = Square root of three i.e. 1.732

As an example if the load is 1804 Amps per phase at 480 Volts, the three phase kVA would be:

1804 Amps X 0.48kV X 1.732 = 1500 kVA To calculate the transformer’s ampacity given a transformers three phase kVA rating:

FLA = kVA3phase / [ kVLine-Line x √3 ] As an example, if a three phase transformer is rated 1500 kVA at 480 Volts, the current rating for each phase would be:

FLA = 1500kVA / [ 0.48kV x √3 ] = 1804 Amps



3.2 Transformer Protection Transformers are a very critical component in the electrical power distribution system since a failure often means losing a significant portion of the load creating costly downtime. Proper sizing of the transformer, selection of its conductors and adequate protection are essential to a sound design. Many factors such as load, compliance with the NEC® Article 450, consideration of the magnetizing inrush and short circuit protection all play an essential role in the design. Sizing the transformer is generally based on the load to be served. For example, if the transformer was sized to serve a 225 Amp panel rated 208 Volts, a 75 kVA transformer is commonly used. This transformer’s secondary full load current rating known as FLA (as shown in Table 3.3) is 208 Amps which is sufficient for the panel’s continuous current rating of 80% of it’s rated load ( 80% X 225 Amp = 180 Amps). Although this may be the most common selection, it is not the only selection and some may select a different size for a variety of reasons. The tables in this section do not reflect every scenario or consider every variable and therefore, the final design must be reviewed by an electrical design professional. 3.3 NEC Table 450.3(B) NEC® Table 450.3(B) defines the maximum ratings or settings of overcurrent protection for transformers rated 600 volts and less. The transformer schedules in this section address secondary protection based on 125% of the full load current rating of the transformer. Where 125% of the transformer’s current rating does not correspond to a standard overcurrent device rating, the next higher standard device rating may be used. If the ampacity rating is 9 amps or less, 167% can be used. The primary overcurrent device can be sized as large as 250% of the primary full load current as long as the secondary device is sized based on the 125% rule. Sizing the primary device at it’s maximum of 250% can create special protection problems since it may not adequately protect the transformer against secondary short circuits. When a fault occurs between the transformer primary and secondary overcurrent device, the primary device must respond. If it is too large, it will typically be less sensitive creating insufficient protection of the transformer. In addition, a larger primary device may also require using larger primary conductors depending on how the tap rule exceptions are used.



3.4 Magnetizing Inrush Current NEC® Table 450.3 (B) provides the upper limits for sizing transformer protection. Generally the smaller the device the better the protection since smaller devices tend to be more sensitive offering better protection. However, this does not necessarily mean that lower rated devices should always be used when protecting transformers. When a transformer is energized, it can draw a magnetizing inrush current through the primary device that can be as large as 8 to 12 times its primary full load current. This inrush current can last approximately 6 cycles or 0.1 seconds so the overcurrent devices should be sized large enough and with enough time delay to not nuisance trip during transformer energization. 3.5 ANSI Thru Fault Protection When a short circuit occurs downstream of a transformer, the high currents associated with the fault will flow through the transformer possibly subjecting it to damaging magnetic forces and heat. ANSI C57 defines a transformers short circuit withstand characteristic that can be used in selecting the primary protective device. The withstand characteristic is defined as a time current curve and is based on the transformer’s size, design, winding configuration and impedance. The withstand characteristic will vary depending on the type of transformer, i.e. liquid filled, dry type, etc. and defines the upper limit for protection. For the 45 kVA dry type transformer in Figure 3.1, the withstand characteristic is shown as two diagonal lines. The line to the right is labeled “ANSI Withstand” and represents the standard protection limit. The parallel line just to the left is marked “Adjusted ANSI Withstand” and represents the withstand characteristic of a delta - wye grounded transformer. When a secondary line to ground fault occurs on the secondary of a delta - wye grounded transformer, the primary device only “sees” 58% of it’s normal current due to unbalances in the transformer windings. With the lower short circuit current, the

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CURRENT IN AMPERES X 10 AT 480 VOLTS


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TRANSFORMER45 kVA5%

TRANSFORMER45 kVAINRUSH

TRANSFORMERFLA

PRIMARY C/BSQD F FrameFAFrame = 100A (70AT)Trip = 70

PRIMARY C/B17969A

PRIMARY

SECONDARY

TRANSFORMER45 kVA0.48 - 0.208 kV5%

PRIMARY C/BSQD FA100/70

Adj. ANSI Withstand

Minimum FLA

Maximum per NEC

ANSI Withstand

45 kVA 480 Volt Delta Primary

70 Amp C/B Inrush 10 X FLA at 0.1 Sec.

Primary 70 A C/B



primary device would operate more slowly than expected allowing damage to the secondary winding that is experiencing the short circuit current. Adjusting the curve to the left by 58% means a smaller / faster primary device must be selected to satisfy the adjusted curve. There are similar adjustments for delta-delta connections as well. In the example, a 70 Amp circuit breaker was selected which is large enough to carry the load current, small enough to comply with the NEC® limits and slow enough to clear the magnetizing inrush current. Although the circuit breaker is fast enough to pass to the left of the ANSI withstand characteristic, it does not completely clear the adjusted ANSI characteristic. Because of this situation, some may choose to select a 60 Amp device, however a smaller device may cause the inrush to become an issue. Selecting overcurrent devices quite often requires compromising between competing objectives. Since each transformer can have a unique withstand characteristic and each overcurrent device can have a unique time current curve, the tables in this section represent general device selections. For optimal device selection, the individual transformer’s withstand characteristic and the selected overcurrent device’s time current curve based on the specific manufacturer should be used. 3.6 Transformer Conductors Transformer primary and secondary conductors listed in the table are sized according to the ampere rating of the primary and secondary overcurrent protection. The NEC allows for various exceptions to this sizing method based on numerous “tap rules”, therefore the conductor selections listed in the table may not be the only acceptable choices. The conductor ampacity is based on NEC® Table 310.16 copper conductors 75ºC ratings operating in an ambient temperature not exceeding 30ºC. The equipment grounding conductor(s) are sized based on NEC® Table 250.122 and the overcurrent protection listed in the transformer schedule. In a few cases

For a more detailed explanation of ANSI transformer protection, visit:

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where you can order the video series on Protective Device Coordination Studies that contains an in depth example of ANSI curves and graphs.



the NEC® requires a #3 equipment ground however this was changed to the more commonly used #2 conductor in the transformer schedules. There are also exceptions where the size of the grounding conductor may need to be increased due to considerations such as very high short circuit current or an increase in the size of the phase conductor. 3.7 Transformer Schedules The more common 3 phase 480 Volt Delta - 208Y/120 Volt transformers sizes are listed in the Transformer Schedules. The schedules are organized according to kVA rating, maximum (not necessarily optimum) overcurrent protection based on NEC Table 450.3 (B), typical overcurrent protection (more commonly used sizes) based on thermal magnetic circuit breakers and time delay fuses. The typical device sizes are based on a survey of breaker and fuse manufacturer’s data as well as various designs. Although the typical selection may be a more common size that is frequently used, it may not always be the most optimal selection for a given design. Other considerations such as a transformer’s maximum magnetic inrush current, ANSI through fault protection and various other exceptions may require a different device rating. The more detailed analysis usually requires developing time current curves which is beyond the scope of this guide. Therefore, the table is only a general guide and the final design must be reviewed by the electrical design professional in responsible charge of the project. There are 3 Transformer Schedules listed in this section.

Table 3.1 - Transformer Schedule - Primary Circuit Breaker Protection Table 3.2 - Transformer Schedule - Primary Time Delay Fuse Protection Table 3.3 - Transformer Schedule - Secondary Overcurrent Protection



Table 3.1 Transformer Schedule - Primary Circuit Breaker Protection

1. Maximum primary overcurrent protection is based on NEC Table 450.3(B) which allows the primary overcurrent protection to be sized as large as 250% of the primary full load current as long as the secondary protection is sized no larger than 125% of the secondary full load current or next larger device rating.

2. The “typical “ circuit breaker size is based on a survey of many designers and manufacturers.

Many factors influence the final selection such as transformer magnetic inrush current, ANSI C57 short circuit protection and the use of the many exceptions found in the NEC such as the “tap rules”.

3. The final selections must be reviewed and approved by the design professional in responsible

charge of the design.

4. Transformer inrush current can be between 8 to 12 times the primary FLA and last approximately 6 cycles.

5. The phase and ground conductors are copper and sized based on the transformer primary full load

current and the ampacities in table 310.16 from the 75ºC column. It is assumed no derating is required. Other conductor sizes may be selected in accordance with other NEC articles and exceptions.

Transformer Primary - 480 Volts 3 Phase Delta Molded Case Circuit Breakers

kVA FLA Maximum OCP per

NEC®

Typical Circuit

Breaker Phase Wire Ground

Wire

15 18 45 30 #10 #10

30 36 90 50 #6 #10

45 54 125 70 #4 #8

75 90 225 125 #1 #6

112.5 135 300 175 2/0 #6

150 180 450 225 4/0 #4

225 271 600 350 350 kcm #2

300 361 800 450 2 sets - 4/0 #2

500 601 1200 800 2 sets - 500 kcm 2 - 1/0

750 902 2000 1200 4 sets - 350 kcm 4 - 3/0

1000 1203 3000 1600 5 sets - 500 kcm 5 - 4/0



Table 3.2 Transformer Schedule - Primary Time Delay Fuse Protection

1. Maximum primary overcurrent protection is based on NEC Table 450.3(B) which allows the primary overcurrent protection to be sized as large as 250% of the primary full load current as long as the secondary protection is sized no larger than 125% of the secondary full load current or next larger device rating.

2. The “typical “ circuit breaker size is based on a survey of many designers and manufacturers.

Many factors influence the final selection such as transformer magnetic inrush current, ANSI C57 short circuit protection and the use of the many exceptions found in the NEC such as the “tap rules”.




5. The phase and ground conductors are copper and sized based on ampacities in table 310.16 from

the 75ºC column. It is assume no derating is required. Other conductor sizes may be selected in accordance with other NEC articles and exceptions.

Transformer Primary - 480 Volts Time Delay Fuses


NEC®

Typical Time Delay

Fuse Phase Wire Ground

Wire

15 18 45 25 #10 #10

30 36 90 45 #6 #10

45 54 125 70 #4 #8

75 90 225 125 #1 #6

112.5 135 300 175 2/0 #6

150 180 450 225 4/0 #4

225 271 600 350 350 kcm #2

300 361 800 400 500 kcm #2

500 601 1200 800 2 sets - 500 kcm 2 - 1/0

750 902 2000 1200 4 sets - 350 kcm 4 - 3/0

1000 1203 3000 1600 5 sets - 500 kcm 5 - 4/0



Table 3.3 Transformer Schedule - Secondary Overcurrent Protection

Transformer Secondary Protection -208Y / 120 Volts


NEC®

Typical Secondary

Device Rating Phase Wire Ground Wire

15 42 60 60 #6 #8

30 83 110 100 #2 #8

45 125 175 150 1/0 #8

75 208 300 225 4/0 #4

112.5 312 400 400 500 kcm #3

150 416 600 600 2 sets - 350 kcm 2 - #1

225 625 800 800 2 sets - 500 kcm 2 - 1/0

300 833 1200 1000 3 sets - 500 kcm 3 - 2/0

500 1389 2000 1600 5 sets - 500 kcm 5 - 4/0

750 2083 3000 2500 7 sets - 500 kcm 7 - 350

1000 2778 4000 3000 8 sets - 500 kcm 8 - 400

1. The maximum secondary overcurrent device rating is based on NEC 450.3(B) which allows the secondary device to be sized up to 1.25 X secondary FLA and where this does not correspond to a standard overcurrent device rating, the next larger device can be used. The “Typical” device rating corresponds to sizes typically used. Other design considerations may affect the final size such as various NEC® exceptions.




4. The phase and ground conductors are copper and sized based on ampacities in table 310.16 from

the 75ºC column. It is assumed no derating is required. Other conductor sizes may be selected in accordance with other NEC articles and exceptions.



4.0 INFINITE BUS SHORT CIRCUIT CALCULATIONS 4.1 NEC Requirements NEC® Article 110.9 requires that equipment intended to interrupt short circuit current must have a sufficient interrupting rating. During the preliminary design stage of a project, simplified short circuit calculations are often used to verify equipment’s adequacy prior to specification. These simplified calculations are often referred to as “infinite bus” calculations and are based on a minimal amount of data including the transformer’s three phase kVA rating (self cooled without fans), the nameplate percent impedance and the secondary line-line voltage. 4.2 Transformer Impedance The transformer percent impedance is derived from a factory test where a short circuit is applied on the secondary windings of the transformer. The test begins with the transformer de-energized but then a very small percentage of rated

primary voltage is applied. The primary voltage is slowly raised while monitoring the current circulating in the shorted secondary. When the current in the shorted secondary is equal to the transformer’s full load current rating, the test is complete. The percent of rated primary voltage that it took to produce rated full load current in the shorted secondary is the transformer percent impedance and is stamped on the nameplate.

Of course %Z does not equal %V according to Ohm’s Law. A very long time ago, transformers used to have %IZ stamped on the nameplate and I X Z is a voltage. Eventually the “I” was dropped since it represented 100% of the full load current from the test. Multiplying Z X 100% = Z. For the short circuit calculations to be accurate, the actual transformer nameplate impedance must be used and not an assumed value. If the transformer has been specified but has not been delivered at the project site for inspection, the transformer’s actual final tested impedance can vary by as much as +/- 7.5% of the specified value. To be even more conservative, the calculations can be based on using 0.925 X the specified impedance to account for the possibility of the transformer’s tested impedance being low by 7.5%. As an example: A transformer is specified with a 5.75% impedance. The actual tested impedance can vary by +/- 7.5% of the specified 5.75% therefore the



actual impedance could ultimately be as low as 92.5% (0.925) of the specified value:

5.75 X (100% - 7.5%) = 5.75 X 0.925 = 5.32%

4.3 Infinite Bus Short Circuit Calculation Since we know that applying the rated percent impedance (voltage) on the primary of a transformer will result in rated full load current circulating in the shorted out secondary winding, what would happen if 100% voltage was applied. You would have the maximum short circuit current. These two relationships are proportional.

% Zxfmr / FLAsecondary = 100 % / SCAinfinite Where:

% Zxfmr = transformer nameplate impedance FLAsecondary = transformer secondary full load current SCAinfinite = maximum short circuit current assuming an infinite source.

We can rearrange the two ratios and solve for SCAinfinite

SCAinfinite = (FLAsec X 100) / % Zxfmr This method is commonly referred to as the “infinite bus” method because it is based only on the transformer impedance and ignores any other impedance on the primary such as the utility company. If the utility impedance is ignored, that means we assumed it was “0” and the current that would be available if limited by 0 ohms would be infinite. This method leads to conservative results since utility systems do not have infinite short circuit current. Although if you are located next to a 2000 MW power plant the short circuit current could become extremely high (although not infinite). Example: What is the worst case infinite bus short circuit current on the secondary of a transformer rated 1500 kVA with a nameplate impedance of 5.75% and a secondary voltage of 480 Volts? This problem can be solved in only two steps. The first step is to calculate the secondary full load current rating based on the transformer’s kVA rating. Where a transformer has a base kVA rating and a fan cooled rating, use the base rating. The second step is to calculate the maximum three phase short circuit current based on the full load current and the nameplate percent impedance. This calculation ignores any other impedance such as the source of conductor.



Step One:

FLAsecondary = kVA / (kVline-line X √3)

FLAsecondary = 1500 kVA ( 0.48 kV X 1.732 )

FLAsecondary = 1804 Amps Step Two:

SCAinfinite = (1804 Amps X 100 ) / 5.75%

SCAinfinite = 31,374 Amps 4.4 Motor Contribution During a short circuit, all directly connected running motors can contribute short circuit current that flows towards the fault. The magnitude of the motor contribution is based on the motor’s sub-transient reactance Xd’’ and can be as high as each motors locked rotor (starting) current. This means that a typical motor can contribute short circuit current of approximately 4 to 6 times it’s full load current. Identifying all motors during the preliminary design stage can be a difficult task. A commonly used practice for a worst case approximation is to include a multiple of the transformer’s full load current in the calculations as a “cushion” to account for motor contribution. For voltages of 240 Volts through 600 Volts, motor contribution can be approximated as 4 X transformer FLA. This very large and conservative approximation is based on the bulk of the transformer capacity serving motor load - not very likely! For voltages less than 240 Volts, 2 X transformer FLA is often used. This represents 50 % of the transformer capacity is serving motor load. i.e. 50% x 4 x transformer FLA.

Attend Jim’s One Week Power System Engineering Class to learn how to conduct a detailed Short Circuit Study. This course also includes electrical design, protective device coordination studies and harmonic analysis. See the detailed agenda, locations and dates at:

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4.5 Calculation Worksheets The short circuit calculation worksheets in this section are for approximations only. They generally lead to very conservative results that can be much greater than the actual short circuit current based on more exact data. Therefore, it may also be desirable to perform a detailed short circuit study or detailed calculations for more exact results. Although the worst case maximum short circuit calculations are conservative for selecting equipment interrupting ratings, it might not be conservative for use in other studies such as voltage drop, arc flash and harmonics studies. For these types of studies, lower short circuit currents could possibly yield more significant problems. For a free training video clip where Jim explains transformer percent impedance in detail, visit:

www.brainfiller.com



Table 4.1 Transformer Infinite Bus Short Circuit Calculation Worksheet

Steps

INFINITE BUS SHORT CIRCIUT CALCULATION

Step 1 Enter Data

Transformer kVA3 Phase Rating (AA) = _______kVA

Transformer % Impedance = _______%Z

Secondary VoltageLine-Line in kV = _________kV

Step 2

Determine FLAsecondary

FLA = kVA3 phase ÷ (√3 x kVline-line) FLA = ___________kVA ÷ (√3 x __________kV) ________Amps

Step 3

Determine Short Circuit Amps (SCA)

SCAsecondary = (FLA x 100) ÷ %Z SCAsecondary = (___________Amps x 100) ÷ _____% _________SCA

If Secondary Voltage < 240V SCAmotor = FLAsecondary X 2

Step 4

Determine Approximate

Motor Contribution

SCAmotor

If Secondary Voltage > 240V

< 600V SCAmotor = FLAsecondary X 4

SCAmotor = ________ A X _____

Select either X “2” or “4” _______SCAmotor

Step 5

Determine Approximate Total Short

Circuit Amps

Total Short Circuit Amps = Step 3 + Step 4 Step 3 + Step 4 ________SCAtotal

FLA = Transformer secondary full load amps SCAsecondary = Short Circuit Amps on the transformer secondary SCAmotor = Short circuit motor contribution SCAtotal = Total short circuit current at secondary bus



Table 4.2 Example Problem Using Transformer Infinite Bus Short Circuit Calculation Worksheet

Steps

INFINITE BUS SHORT CIRCIUT CALCULATION

Step 1 Enter Data

Transformer kVA3 Phase Rating (AA) = 1500 kVA

Transformer % Impedance = 5.75 %Z

Secondary VoltageLine-Line in kV = 0.48 kV

Step 2

Determine FLAsecondary

FLA = kVA3 phase ÷ (√3 x kVline-line) FLA = 1500 kVA ÷ (√3 x 0.48 kV) 1804 Amps

Step 3

Determine Short Circuit Amps (SCA)

SCAsecondary = (FLA x 100) ÷ %Z SCAsecondary = ( 1804 Amps x 100) ÷ 5.75 % 31,374 SCA

If Secondary Voltage < 240V SCAmotor = FLAsecondary X 2

Step 4

Determine Approximate

Motor Contribution

SCAmotor

If Secondary Voltage > 240V

< 600V SCAmotor = FLAsecondary X 4

SCAmotor = 1804 A X 4

Select either X “2” or “4” 7,216 SCAmotor

Step 5

Determine Approximate Total Short

Circuit Amps

Total Short Circuit Amps = Step 3 + Step 4 Step 3 + Step 4 38,590 SCAtotal

Example: Use this worksheet to determine the worst case 3 phase short circuit current at the 480 Volt secondary bus assuming an infinite source. The transformer is rated 1500 kVA with a 5.75% nameplate impedance and a secondary voltage of 480 Volts. The worksheet on this page is filled out to illustrate it’s use in performing the calculations for this problem.



5.0 CALCULATION OF CONDUIT FILL Pulling conductors in an overloaded conduit can result in an increased risk of insulation damage. NEC® Chapter 9 Table 1 states that the maximum percentage of the cross sectional area of conduit that can be filled by conductors shall not exceed:

From NEC Chapter 9 Table 1 Percent of Cross Section of Conduit and Tubing for Conductors

Number of Conductors All Conductor Types

1 53%

2 31%

Over 2 40%

When sizing conduit where all conductors are the same size and have the same insulation, it is easiest to look up the conduit size in Annex C of the NEC®. The NEC® tables list the maximum number of conductors permitted in various types and trade sizes of conduit. If the conductors are of different sizes, look up the conductor areas Chapter 9, Table 5 of the NEC®. Multiply the conductor area by the number of conductors of that size. Repeat for each size conductor and total all areas. Once the total area is calculated, select the conduit size from the Chapter 9, Table 4 of the NEC®. Example: To calculate the conduit size for four 500 kcm and one #2 conductor with THHN insulation, use Table 5 from Chapter 9 of the NEC® and look up the area of 500 kcm THHN and multiply by four. Then look up the area of #2 THHN and multiply by one. Add these numbers to calculate the total area. Select the conduit size using the appropriate conduit from Chapter 9, Table 4 of the NEC® (500 kcm THHN area x 4) = (0.7073 x 4) = 2.8292

(#2 THHN area x 1) = (0.1158 x 1) = 0.1158

Total Area 2.9450

From the 40% column in Table 5.1 on the next page, select a 3 inch minimum size for rigid steel conduit.



Table 5.1 Rigid Metal Conduit Percent Area

Excerpt from NEC Chapter 9 Table 4 - Article 344 - Rigid Metal Conduit (RMC)

Trade Size Inches

Trade Size Metric

Total Area 100% (IN2)

1 – Wire 53% (IN2)

2 – Wires 31% (IN2)

> 2 – Wires 40% (IN2)

½”

¾”

1”

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1 ½”

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52.9

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78.5

90.7

102.9

128.9

154.8

0.314

0.549

0.887

1.526

2.071

3.408

4.866

7.499

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12.882

20.212

29.158

0.166

0.291

0.470

0.809

1.098

1.806

2.579

3.974

5.305

6.828

10.713

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0.170

0.275

0.473

0.642

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1.508

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3.103

3.994

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9.039

0.125

0.220

0.355

0.610

0..829

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1.946

3.000

4.004

5.153

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6.0 MOTOR SCHEDULES 6.1 Motor Circuits NEC® Article 430.22 defines the requirements for sizing motor feeders for single motors (although other requirements and exceptions exist in other articles as well). This Section contains motor schedules to assist in motor circuit design. These schedules provide data that can be used to determine the size of the phase conductors, short circuit protection based on circuit breakers and time delay fuses as well as NEMA starter sizes. Like other sections in this guide, the schedules contain the more common selections based on a survey of manufacturers and designs. Factors such as voltage drop, motor starting characteristics, ambient conditions and other design considerations could ultimately affect the final selection, therefore the final design must be performed under the direction of a licensed design professional. 6.2 Motor Full Load Current Ratings NEC® Table 430.250 provides typical full load current values for three phase AC motors operating at speeds usual for belted motors with normal torque characteristics. These currents are listed in the Motor Schedules and are also

the basis for selecting motor feeders and sizing the short circuit protection in accordance with the NEC®. Motor overloads are not sized based on the currents of Table 430.250 and must be sized based on the motor’s actual nameplate current as well as the motor’s service factor, temperature rating and thermal damage limits. The motor overload must also have sufficient time delay to allow the motor to start.

6.3 Motor Feeder Sizing Motor feeder conductor sizes are based on a minimum of 125% of the motor full load amps per NEC® Article 430.22. The conductor ampacity is based on NEC® Table 310.16, 75ºC copper conductors based on a 30ºC ambient temperature and no more than three current carrying conductors in a raceway. The conductor size may need to be adjusted for variations from these selection factors. In addition, long motor feeders may result in an undesirable amount of voltage drop. A common solution is to increase the size of the feeder.



6.4 Motor Short Circuit Protection Short circuit protection is based on a multiple of the motor full load current based on Table 430.250. Since different protective devices will have time current characteristics, the NEC provides multipliers that are unique for each type of device. Some of these multipliers are listed in the table below and are from NEC Table 430.52. Table 6.1 Motor Short Circuit Protection

6.5 Motor Overload Protection Motor overload protection is designed to protect the motor against damaging currents that result from the motor stalling or significantly slowing down. This can often be due to a problem with the process or load, such as a faulty bearing or other type of jam. During a stall condition, the motor can draw a current equal to it’s locked rotor (starting) current that can typically range from 4 to 6 times the motor’s normal running current. Motors can only survive this current for a specific amount of time (referred to as the safe stall time) and this current must be removed before the motor can sustain damage. The motor overload is designed with a time delay to allow the motor to start yet interrupt if the starting current lasts too long. Sizing the motor overload is based on the actual motor nameplate data rather than Table 430.250. For motors with a service factor 1.15 or greater or a marked temperature rise of 40ºC or less, the overload can be sized up to 125% of the motor nameplate full load current. The overload for other motors is 115% unless it falls under one of the exceptions in the NEC®.

Maximum Rating or Setting of Motor Branch Circuit Short Circuit and Ground Fault Devices Excerpts from Table 430.52 of the NEC

Type of Motor Non Time Delay Fuse

Dual Element Time Delay

Fuse

Instantaneous Trip Breaker

Inverse Time Breaker

Squirrel Cage

Induction - Other than Design B

Energy Efficient

300 175 800 250

Design B Energy

Efficient

300 175 1100 250



6.6 Motor Time Current Curves The operation of a motor and its protection can best be described with the use of time current curves. Figures 6.1 and 6.2 Illustrate the protection of a 75 Hp motor. The motor starting characteristic shows the locked rotor current (starting current) lasting for approximately 5 seconds and then lowering to the rated full load current. The motor stall time is shown as a “+” on each graph and the overload (green crosshatched band) must have enough time delay for the motor

to start yet operate quickly enough to protect against stall outs. Figure 6.1 and 6.2 are identical with only one difference. Figure 6.1 uses an instantaneous only circuit breaker, commonly referred to as a motor circuit protector for short circuit protection. The NEC® allows this device to be set as high as 8 X the motor full load current for standard squirrel cage induction motors, excluding Design B energy efficient motors which can have a much larger starting current. The typical motor can draw a locked rotor current often between 4 to 6 X (higher for premium efficiency and some other designs). The 8 X factor allows the motor sufficient

margin for starting without operating the short circuit protective device. The black vertical band in Figure 1 represents the time current characteristic of the motor circuit protector. The starting current is shown as 6 times the Full Load Current. The motor circuit protector can be set as high as 8 X the Full Load Current to allow the motor to start.

MS

0.5 1 10 100 1K 10K0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

75HP Start.tcc Ref. Voltage: 480 Current Scale x10^0 EQUIP1.drw

TIME

IN S

EC

ON

DS

Figure 6.1



Figure 6.2 shows a time delay fuse that has a slope that is inversely proportional to the current, i.e. more current - less time, less current - more time. The NEC allows this type of device to be sized as large as 175% of the motor full load current. As an example, a 40 HP motor rated 460 Volts has a full load current rating of 40 Amps according to NEC Table 430.250. 175% X 40 Amps would allow a maximum fuse size of 70 Amps. The curve on the right is the same as Figure 6.1 on the previous page but instead of a motor circuit protector, a time delay fuse is used. The fuse in Figure 2 can be sized up to 175% X the motor’s full load current. The NEC® allows the next higher standard rating. Care must be given so the fuse will allow the motor to start and preferably allow the motor’s overloads to trip first in the event of a stalled motor or overload condition. 6.7 Motor Schedules Motor schedules are provided for 460 Volt and 230 Volt standard induction motors. The selections are typical and can vary depending on many other variables and design conditions. For motors with long starting times, larger devices may be required. For circuits with significant voltage drop, larger conductors might be used. Correct overload protection must also be provided in accordance with the NEC®. The final selection should be reviewed by a licensed design professional responsible for the project, and include reviewing time current curves to verify correct protection and operation for the specific application.

MS

0.5 1 10 100 1K 10K0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

75HPFUSE.tcc Ref. Voltage: 480 Current Scale x10^0 EQUIP1.drw

TIME

IN S

EC

ON

DS

Figure 6.2



Table 6.1 460 Volt Motor Circuit Schedules

Short Circuit Protection

Hp

FLA

Typical Dual

Element Fuse

Maximum Dual

Element Fuse

Typical Inverse Time

Breaker

Maximum Inverse Time

Breaker

Phase Conductor

Size

Minimum

NEMA Starter Size

5 7.6 10 25 15 20 12 0

7.5 11 15 35 25 30 10 1

10 14 20 45 30 35 10 1

15 21 30 70 45 60 10 2

20 27 40 90 60 70 10 2

25 34 50 110 70 90 8 2

30 40 60 125 80 100 8 3

40 52 70 175 100 150 6 3

50 65 100 200 110 175 4 3

60 77 125 250 150 200 3 4

75 96 150 300 175 250 1 4

100 124 175 400 200 350 2/0 4

125 156 200 500 225 400 3/0 5

150 180 225 600 250 450 4/0 5

200 240 300 800 350 600 350 kcm 5

1. The selections in the table are “typical” for motors operating at normal speed and torque. 2. Long motor starting times may require alternate device selections. Motor full load current rating (FLA) is based

on NEC table 430.250. 3. Actual motor nameplate data must be used for the selection of overload protection. 4. Motors that have special starting characteristics such as high torque, low speed, high starting currents and

other special conditions may require additional consideration in the selection of protection. 5. Typical Dual Element Fuse - based on survey of several manufacturer’s recommended RK-5 time delay fuse. 6. Maximum Dual Element Fuse - based on 175% of FLA or next standard rating based on NEC 430.52. 7. Typical Inverse Circuit Breaker - based on a survey of several manufacture’s recommended circuit breakers. 8. Maximum Inverse Breaker - based on 250% of FLA or next standard rating based on NEC 430.52. 9. It is possible that smaller devices than those shown can be used and still allow motor to start. 10. Phase conductor size is based on 1.25 X FLA per NEC 430.22 for single motors, continuous duty applications.. 11. Conductor ampacity is based on no more than three current carrying copper conductors in a raceway, 75ºC

insulation and 75ºC device terminal rating. Other conductor sizes may be necessary for special conditions such as voltage drop or conductor derating.

12. Although No. 14 conductors can be used with some smaller motors, it is common to use No. 12 instead.



Table 6.2 230 Volt Motor Circuit Schedules

Short Circuit Protection

Hp

FLA

Typical Dual

Element Fuse

Maximum Dual

Element Fuse

Typical Inverse Circuit

Breaker

Maximum Inverse Circuit

Breaker

Phase Conductor Size

Minimum

NEMA Starter Size

2 6.8 10 15 15 20 12 0

3 9.6 15 20 20 25 12 0

5 15.2 20 30 30 40 12 1

7.5 22 35 40 45 60 10 1

10 28 40 50 60 70 10 2

15 42 60 80 80 110 6 2

20 54 80 100 90 150 4 3

25 68 100 125 100 175 4 3

30 80 125 150 125 200 3 3

40 104 150 200 150 300 1 4

50 130 175 250 200 350 2/0 4

60 154 225 300 225 400 3/0 5

75 192 250 350 300 500 250 kcm 5

100 248 350 450 400 700 350 kcm 5

125 312 450 600 500 800 2 sets - 3/0 6

150 360 500 700 600 1000 2 sets - 4/0 6

200 480 600 1000 800 1200 2 sets -350 kcm 6

1. The selections in the table are “typical” for motors operating at normal speed and torque. 2. Long motor starting times may require alternate device selections. Motor full load current rating (FLA) is based

on NEC table 430.250. 3. Actual motor nameplate data must be used for the selection of overload protection. 4. Motors that have special starting characteristics such as high torque, low speed, high starting currents and

other special conditions may require additional consideration in the selection of protection. 5. Typical Dual Element Fuse - based on survey of several manufacturer’s recommended RK-5 time delay fuse. 6. Maximum Dual Element Fuse - based on 175% of FLA or next standard rating based on NEC 430.52. 7. Typical Inverse Circuit Breaker - based on a survey of several manufacture’s recommended circuit breakers. 8. Maximum Inverse Breaker - based on 250% of FLA or next standard rating based on NEC 430.52. 9. It is possible that smaller devices than those shown can be used and still allow motor to start. 10. Phase conductor size is based on 1.25 X FLA per NEC 430.22 for single motors, continuous duty applications.. 11. Conductor ampacity is based on no more than three current carrying copper conductors in a raceway, 75ºC

insulation and 75ºC device terminal rating. Other conductor sizes may be necessary for special conditions such as voltage drop or conductor derating.

12. Although No. 14 conductors can be used with some smaller motors, it is common to use No. 12 instead.



7.0 Coordination Analysis 7.1 Time Current Curves A coordination study is an “attempt” to select devices and determine optimum setting adjustments so that during an overload or short circuit, only the device closest to the problem trips and the other devices remain closed. This allows clearing the problem circuit while maintaining service continuity to the remainder of the system. To evaluate coordination, time current curves are used. The curves are logarithmic in nature which represents orders of magnitude so the graph scale increases by factors of 10 rather than just single digits such as 1, 2, 3 etc. The graph’s horizontal scale is defined as current in Amps. This scale is frequently multiplied by 10 or 100 to allow graphing higher magnitudes of current.

The current scale for the graphs in this section are all multiplied by 10. Every overcurrent device has a time current curve that defines how it will respond to various levels of current. Graph 1 at the left represents a 20 Amp circuit breaker. The upper left portion of the curve aligns at “2” which when multiplied by 10, is 20 Amps. Look at the bottom horizontal scale for the number “1”. Since this is also multiplied by 10, the “1” represents 10 Amps. Following the 10 Amp line up to the top of the graph, it never intersects the circuit breaker

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TIME-CURRENT CURVEST2G TechnicalTraining Group TCC-1

FAULT:DATE: Jan 31, 2007BY :REVISION: 1

Graph 1 20 Amp Circuit Breaker

10 Amp line never intersects the breaker’s curve

100 Amp line intersects the

breaker’s curve between

2.5 and 7 Seconds

Larger currents cause the breaker’s instantaneous to trip

Overload / Thermal Part of the Curve

Instantaneous / Magnetic Part of the

Curve

2 X 10 = 20 Amps

Brainfiller.com®



curve. This means the breaker does not trip which is what you would expect when a 20 Amp breaker only carries 10 Amps. Now look at 10 on the horizontal current scale. It actually represents 10 Amps X 10 or 100 Amps. Moving up the vertical scale on this line, the circuit breaker curve is intersected between 2.5 and 7 seconds. This means that if the 20 Amp breaker carries 100 Amps, it will trip between 2.5 and 7 seconds. If the breaker sees a larger current such as several thousand Amps, it will trip instantaneously in about 1 cycle. The instantaneous region of the breaker’s curve is represented by the long flat horizontal portion of the curve on the right side of the graph. 7.2 Selective Coordination To evaluate selective coordination between two or more devices, the time current curves of all of the devices in a particular circuit must be drawn together. Graph 2 illustrates the time current curve of an adjustable 225 Amp main breaker and a 20 Amp branch breaker. In this example, both curves begin to overlap at 900 Amps. This indicates that if a current above 900 Amps flows through both the 20 Amp and 225 Amp devices, both might possibly trip. The curves for both devices completely overlap for currents above 1500 Amps. This indicates that both devices will very likely trip together for all currents above 1500 Amps. A short circuit or overload above 1500 Amps on the 20 Amp circuit, would not

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Both curves begin to

overlap at 900 Amps

Both curves completely overlap at 1500 Amps

Graph 2 20 A and 225 A Breakers 225 A Breaker Set “Low”

Instantaneous set “Low”

Brainfiller.com®



only trip the 20 Amp circuit, but the 225 Amp main would also trip removing the entire panel from service instead of just the individual 20 Amp circuit. Since the 225 Amp main breaker is adjustable, it is possible to “move” it’s instantaneous trip setting which is defined by the vertical part of the curve. Graph 2 illustrates the instantaneous setting set “low” which means the overlap with the 20 Amp breaker occurs at a 900 Amps. Changing the setting to “high” as illustrated in Graph 3, moves the beginning of the overlap from 900 Amps to 1800 Amps. This setting change would increase coordination so the devices now coordinate with each other for currents up to 1800 Amps. Although not perfect, this increases the likelihood that only the 20 Amp breaker would respond to the fault. Graph 3 illustrates the preferred coordination between the 20 Amp circuit breaker and the main 225 Amp breaker. Coordination is improved somewhat with the higher setting since there is less overlap between the two curves. Overlapping of the curves is an indication that both devices could trip together. A comprehensive coordination study requires taking these basic concepts and applying them to other devices in the power distribution system. There will always be trade offs in determining the optimum settings. Slower settings for upstream devices often improve coordination with downstream devices. However, faster settings are typically more sensitive and may provide better overall protection.

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Graph 3 20 A and 225 A Breaker

225 Amp Breaker Set “High”

With the higher instantaneous,

now the overlap does not begin

until 1800 Amps

Instantaneous set “High”

Brainfiller.com®



REFERENCES AND TRADEMARKS

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Hazard Calculations” and is a regular contributor for NEC Digest® the NFPA’s Official NEC® Magazine. He is nationally known for his power system analysis background and ability to analyze complex problems. Jim consistently receives excellent reviews for his "unique" teaching style and ability to answer the tough questions in an easy to understand manner. Early in his career he worked for Ohio Edison Company and was a Project Engineer for Square D Company's Power System Analysis Group. He is also the founder of Phillips Engineers + Consultants, Inc. He has taught classes at the college level and is a Senior Member of IEEE where he has been a distinguished lecturer, past member of the Energy Policy Committee, Illumination Engineering Society, NFPA, and many other organizations. He has also written for Consulting-Specifying Engineer Magazine and holds a BSEE from The Ohio State University. He is a licensed Professional Engineer in many states.

JIM PHILLIPS, P.E. For over 25 years, Jim has conducted more than 1600 live seminars for tens of thousands of people from the United States and around the globe. His vast experience in the electrical industry makes him a highly sought after speaker on every facet of electric power systems. He is a member of the IEEE 1584 working group “IEEE Guide for Performing Arc Fl h

Brainfiller.com Your source for training by Jim Phillips, P.E.

• Technical Articles • Continuing Education Courses • Distance Learning • On Site Training • Training Seminars Scheduled Nationwide • Work Sheets • Training Videos

Tap into Jim’s experience with any of these programs that he has developed:

• How to Perform an Arc Flash Study - !EEE 1584 • Electrical Safety / Arc Flash - NFPA 70E • Power Distribution Equipment • Design of Electric Power Systems • National Electrical Code ® • Short Circuit Studies • Protective Device Coordination Analysis • Emergency and Standby Power Systems • Protective Relaying • Medium Voltage Power Systems • Symmetrical Components and Per Unit Analysis • Grounding and Power Quality • Harmonic Analysis and Power Factor

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