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

2. Topics
Motor Branch Circuit
Protection & Controller
Feeder

3. Motor Branch Circuit

4. Motor Branch Circuit Motor Branch Circuit Conductors
Motor Branch Circuit Short Circuit Protection
Overload Protection
Motor Controller
Motor Disconnect
Motor Control Circuits

5. Motor Branch Circuit Conductor Disconnect Short Circuit Protection
Overload
Protection
Controller &
Control Circuit

6. Motor Branch Circuit Type of Motor
Induction Motor
1 phase and 3 phase
Synchronous Motor (only 3 phase)
Armature & Field (DC)
DC Motor
Armature & Field

7. Motor Branch Circuit Ratings of Motor
Power: HP or kW
# of phase
Voltage
Current ( 𝑃𝑜𝑤𝑒𝑟 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 /𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦)
Check Manufacturer for Data

8. Motor Branch Circuit Ratings of Motor

9. Motor Branch Circuit Current (cable) Calculation
Normal Operation: 𝐼 𝑐 ≥1.25 𝐼 𝑛
Multi-speed Motor: maximum current
On-Off Operation: 𝐼 𝑐 ≥ 𝐾 1 𝐼 𝑛 when 𝐾 1 is the multiplier

10. Motor Branch Circuit Current (CB) Calculation
Motor Type
Fuse
(Fast Blown)
(Slow Blown) CB
(instantaneous)
(invert time)
Induction (DOL)
3.0
1.75
7.0
2.5
Induction (Auto Transformer)
2.0
Wound Rotor
1.5 DC

11. Motor Branch Circuit Current Calculation
Ground Cable
115% of the rated current
𝐼 𝑝𝑒 =1.15 𝐼 𝑛

12. Short Circuit Protection
Able to handle the starting current
Fuse
Non Time-Delay Fuse
Time-Delay Fuse
Circuit Breaker
Inverse Time Circuit Breaker (normal)
Instantaneous Circuit Breaker

13. Short Circuit Protection Circuit Breaker

14. Overload Protection To protect overheat of the motor
Circuit Breaker cannot function well for overload because of the starting current
Three methods of detecting temperature
Bimetal Thermal Overload Relay
Electronic Overload Relay
Thermistor Protective Relay

15. Motor Controller Starter Inverter DOL: usually less than 7.5kW
Reduced Voltage Starter
Inverter

16. Motor Controller Reduced Voltage Starter Star-Delta Starting
Autotransformer Starting
Resistant Starting
Part-Winding Starting
Elecronics Starting

17. Motor Controller Contactor
Turn on-off the motor
The rating of Contactor needs to be higher than the rating of Motor
Utilization Category
AC2: handle 2-5 times the nominal current
AC3: handle 5-7 times, but stop at nominal
AC4: handle high amount of on-off operation

18. Motor Controller Reduced Voltage Starter Star-Delta Starting
Autotransformer Starting
Resistant Starting
Part-Winding Starting
Elecronics Starting

19. Motor Controller components
Power Circuit
Usually 3 phase power circuit
Control Circuit
Single phase or DC circuit

20. Motor Controller Direct On Line Starting

21. Motor Controller Star-Delta Starting

22. Disconnecting Means Disconnect when circuit is live
Design for inductive load
Install “within sight” and less than 15m
Rating is higher than 115% of motor rating

23. Feeder Cable All motor: 𝐼 𝐹 ≥1.25 𝐼 𝑛,𝑚𝑎𝑥 + 𝐼 𝑛
Combination of Motor and other loads: 𝐼 𝐹 ≥1.25 𝐼 𝑛,𝑚𝑎𝑥 + 𝐼 𝑛 𝐼 𝐿
𝐼 𝐿 is continuous load
Neutral can be a lot smaller but not smaller than Ground

24. Feeder CB All motor: 𝐼 𝐹 ≥ 𝐼 𝐶𝐵,𝑚𝑎𝑥 + 𝐼 𝑛
Combination of Motor and other loads: 𝐼 𝐹 ≥ 𝐼 𝐶𝐵,𝑚𝑎𝑥 + 𝐼 𝑛 𝐼 𝐿
𝐼 𝐿 is continuous load
Neutral can be a lot smaller but not smaller than Ground

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