ee4503 electrical systems design

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EE4503Electrical Systems Design

Branch circuit and Feeder (Lighting and Appliances)

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Topics

• Characteristics of Loads

• Circuits Design

• Branch Circuit

• Feeder Circuit

• Load Schedule

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Characteristics of Loads

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• Continuous Load– More than 3 hrs. of continuous usage e.g.

lighting, air condition

• Non-continuous Load:– Less than 3 hrs. of continuous usage e.g.

electric oven, microwave

When not sure, add 25% more

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Characteristics of LoadsDefinition

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• Total Connected Load: total kVA or MVA listed

• Maximum Demand: the maximum kVA or MVA used at the same time

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Characteristics of LoadsDefinition

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• Demand Factor (D.F.): ratio of “Maximum Demand” to “Total Connected Load” ()

• Diversity Factor: summation of every branched circuits’ maximum demand to the system’s maximum demand (always greater than or equal to 1.0)

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Characteristics of LoadsDefinition

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• Peak Load (P): the maximum kVA or MVA at the given periond (e.g. 15 min)

• Load Factor (L.F.): ratio of “Average Load” to the “Peak Load” (). is total energy for period of time

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Characteristics of LoadsCommon Load

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• Lighting: about 20-50% of total load or 20-100VA/m2

• Receptacle: if no specific information, 180-200VA/set (1, 2 or 3 outlets)

• HVAC: 1TR (12,000 BTU) is about 1.5-1.8kVA

• Motor

• Escalator: manufacturer’s specification

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GroundingCommon Load - Lighting

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http://www.g-w.com/pdf/sampchap/9781605255880_ch12.pdf

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GroundingCommon Load - Escalator

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http://www.elevatorbooks.com/Content/Site108/ProductContent/April2012Articl_00000012203.pdf

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Circuit Design

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• Branch Circuit

• Feeder Circuit

• Main Circuit

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Branch Circuit

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• Connect from a distribution panel to load or receptacle

• Lighting or Appliance Branch Circuit– Lighting Branch Circuit

– Receptacle Branch Circuit

– Lighting & Receptacle Branch Circuit

– Individual Branch Circuit

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Branch CircuitCalculation

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• Load of a branch circuit (BC) at least the total load connected to that branch.

• Conductor Sizing Current Rating of CB

• Current Rating of CB Maximum Load

• Size of a BC goes by the current rating of CB: 15(16)A, 20A, 25A, 30(32)A, 40A, 50A, 63A

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Branch CircuitDesign Procedure

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• Find “Load”

• Calculate “Circuit Breaker” (% of BC)

• Calculate “Cable”

• Verify “Voltage Drop”: 1-2% of rated voltage

?? safety factor ??

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Branch CircuitDesign Recommendations

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• Lighting Circuit: continuous load– 50-70% of BC (no more than 80%) or

allowance of 10-30%

• Individual Branch Circuit: load is known– Should be no more than 80% of BC

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Branch CircuitDesign Recommendations

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• Receptacles Circuit: single, duplex, triplex– 180VA per set (200VA for ease)

• Individual Circuit with ELCB: heater, Jacuzzi

• Receptacles Circuit with ELCB: toilet, kitchen, basement, outdoor, < 1.5m from water

?? Some recommendations can be relaxed ??

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Branch CircuitDesign Recommendations

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• Separate the type of load: lighting on one BC

• Future Load:– 20% spare at least

– 40% if continuous load

• Cable size is no smaller than 2.5mm2

• BC for receptacles contains less than 10 sets

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Branch CircuitDrawing

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• # of hash marks is # of cables

• No hash mark means 2 cables

• One hash mark + number is the number of cables

• Arrow + Lx is connected to circuit breaker (phase) #x

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Branch CircuitPanelboard

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• No more than 42 circuit

• No longer than 50 m

• Easy to access (no higher than 1.8m)

• Rated is more than the rated of the cable

• At least one panel board per floor

• Must have protection equipment (CB)

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Branch CircuitPanelboard

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• Should be in the center: balance the voltage drop

• Should be close to the incoming: shortest feeder

• Three types– Active Branch Circuit (60-80%)

– Spare Branch Circuit with CB (10-20%)

– Spare Branch Circuit (10-20%)

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Branch CircuitFeeder

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• Rated current of feeder is the total load deducted by the “demand factor”

• Feeder Cable Rating of CB

• Rating of CB Feeder’s Rated Current

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Branch CircuitFeeder – Demand Factor

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• Demand Factor is the ratio of the maximum usage power at any certain time to the total connected load.

• There are tables for the demand factor, e.g. receptacles in kitchen, lighting. If not specify, 0.8 is a good number

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Branch CircuitFeeder – Neutral

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• In single phase system, neutral cable will be the same size and line cable.

• In three phase, neutral cable can be reduced.– Less than 200A, use the phase current

– More than 300A• Without Harmonics, • With Harmonics, use the phase current

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Branch CircuitFeeder - Design Procedure

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• Sum up total BCs’ load

• Calculate “Circuit Breaker”

• Calculate “Cable”: Phase then Neutral

• Verify “Voltage Drop”: 1-2% of rated voltage

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Branch CircuitLoad Schedule

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• List of connected load

• CB number start from 1(A), 3(B), 5(C), 2(A), 4(B), 6(C) then 7(A), 9(B), 11(C), 8(A), 10(B), 12(C)

• Consider the balance of load in all phases (less than 20% is recommended)

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Branch CircuitLoad Schedule – Design Recommendation

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• Load Calculation– Non-continuous Load, no multiplier

– Continuous Load, 1.25 multiplier (or more in case of future expansion)

– If not sure, treat as continuous load.

• Then choose the circuit breaker

• Then choose the cable

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Branch Circuit & FeederDesign Procedure

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• Make the Load Schedule– Design Branch Circuit

• Design Feeder (from the load schedule)

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Branch Circuit & FeederDesign Examples

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Branch Circuit & FeederDesign Examples

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Branch Circuit & Feeder!! 25% margin !!

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• If we are not sure of the load value, add extra 25%.

• After the load, when we choose the CB, add extra 25% (for the case there are some change in voltage/load/etc.)

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Branch Circuit & Feeder!! CB for “motor” load !!

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• CB is 250% bigger in rated.

• Cable is 125% bigger in rated.

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• Electrical Systems Design: ประสทิธิ ์พทิยพฒัน์

Reference

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