1. EE4503 Electrical Systems Design
Branch circuit and Feeder (Lighting and Appliances)

2. Topics Characteristics of Loads Circuits Design Branch Circuit
Feeder Circuit
Load Schedule

3. Characteristics of Loads
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

4. Characteristics of Loads Definition
Total Connected Load: total kVA or MVA listed
Maximum Demand: the maximum kVA or MVA used at the same time

5. Characteristics of Loads Definition
Demand Factor (D.F.): ratio of “Maximum Demand” to “Total Connected Load” (𝐷.𝐹. = 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐷𝑒𝑚𝑎𝑛𝑑 𝑇𝑜𝑡𝑎𝑙 𝐶𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝐿𝑜𝑎𝑑 ∗100%)
Diversity Factor: summation of every branched circuits’ maximum demand to the system’s maximum demand (always greater than or equal to 1.0)
Diversity Factor: (Dormitory Maximum Demand + Classroom Maximum Demand)/Maximum Demand

6. Characteristics of Loads Definition
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” (𝐿.𝐹. = 𝐸 𝑃∗𝑇 ∗100%). 𝐸 is total energy for period of time 𝑇
Diversity Factor: (Dormitory Maximum Demand + Classroom Maximum Demand)/Maximum Demand

7. Characteristics of Loads Common Load
Lighting: about 20-50% of total load or VA/m2
Receptacle: if no specific information, VA/set (1, 2 or 3 outlets)
HVAC: 1TR (12,000 BTU) is about kVA
Motor
Escalator: manufacturer’s specification

8. Grounding Common Load - Lighting

9. Grounding Common Load - Escalator

10. Circuit Design
Branch Circuit
Feeder Circuit
Main Circuit

11. Branch Circuit 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

12. Branch Circuit Calculation
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

13. Branch Circuit Design Procedure
Find “Load”
Calculate “Circuit Breaker” (% of BC)
Calculate “Cable”
Verify “Voltage Drop”: 1-2% of rated voltage
𝑉 𝑑 =2∗𝐼∗𝑙∗ 𝑅 cos 𝜃 +𝑋 sin 𝜃
?? safety factor ??

14. Branch Circuit Design Recommendations
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

15. Branch Circuit Design Recommendations
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 ??

16. Branch Circuit Design Recommendations
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

17. Branch Circuit Drawing
# 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

18. Branch Circuit Panelboard
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)

19. Branch Circuit Panelboard
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%)

20. Branch Circuit Feeder
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

21. Branch Circuit Feeder – Demand Factor
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

22. Branch Circuit Feeder – Neutral
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, 𝐼 𝑛 = ∗( 𝐼 𝑝 −200)
With Harmonics, use the phase current

23. Branch Circuit Feeder - Design Procedure
Sum up total BCs’ load
Calculate “Circuit Breaker”
Calculate “Cable”: Phase then Neutral
Verify “Voltage Drop”: 1-2% of rated voltage
𝑉 𝑑 = 3 ∗𝐼∗𝑙∗ 𝑅 cos 𝜃 +𝑋 sin 𝜃

24. Branch Circuit Load Schedule
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)

25. Branch Circuit Load Schedule – Design Recommendation
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

26. Branch Circuit & Feeder Design Procedure
Make the Load Schedule
Design Branch Circuit
Design Feeder (from the load schedule)

27. Branch Circuit & Feeder Design Examples

28. Branch Circuit & Feeder Design Examples

29. Reference Electrical Systems Design: ประสิทธิ์ พิทยพัฒน์
Depend on the area, end-user can connect to the “meter” of the authority in three ways.