15 HP Motor Branch Circuit and the MCC Feeder That Serves It
A 15 HP, 208 V, 3-phase, Design B motor drives a conveyor. Nameplate: 44 A, service factor 1.15, temperature rise 40 C. It is started across the line from a combination starter with thermal overloads, mounted next to the motor. The MCC is 120 ft away; conductors are THHN in EMT with 75 C terminations. A 5 HP, 208 V, 3-phase cooling fan motor (Table 430.250 value 16.7 A) is fed from the same MCC feeder. The conveyor trips its 100 A breaker on roughly one start in five, and the maintenance lead wants to change to a 150 A breaker because that is what the last plant used.
1. Size the branch-circuit conductors for the 15 HP motor and the motor disconnecting means.
Read the worked answer
Motor full-load current. Use Table 430.250 for a 15 HP, 208 V, 3-phase motor: 46.2 A. Do not use the 44 A nameplate value for conductors or short-circuit protection; 430.6(A)(1) requires the table value. The nameplate is used only for overload sizing.
Branch-circuit conductors, 430.22(A). 125% of FLC:
46.2 x 1.25 = 57.8 A
#6 Cu THHN at 75 C = 65 A, which covers 57.8 A. No adjustment applies (3 current-carrying conductors, normal ambient). #6 Cu.
Disconnecting means, 430.110(A). Ampacity at least 115% of FLC:
46.2 x 1.15 = 53.1 A
A 60 A rated disconnect is the minimum. A 60 A non-fused switch at the motor satisfies 430.102(A) and (B) for the controller and motor disconnects, and gives the maintenance crew a lockable point at the machine.
Give the crew the reasoning, not just the answer: 44 A nameplate would have produced a 55 A conductor and a marginal #6, and the next person to touch this will re-derive it from the nameplate unless the basis is written on the drawing.
2. Select the overload protection and the branch-circuit short-circuit and ground-fault protective device. Evaluate the maintenance lead's 150 A proposal.
Read the worked answer
Overload protection, 430.32(A)(1). Service factor of 1.15 means 125% of nameplate current:
44 x 1.25 = 55 A
The overload relay or heater must be selected so it will not carry more than 55 A and will carry the 44 A nameplate current. This protects the motor from sustained overload; the breaker does not.
Branch short-circuit and ground-fault protection, 430.52(C)(1) Table. Inverse-time breaker at 250% of FLC:
46.2 x 2.50 = 115.5 A
115.5 A is not a standard rating, so 240.6 permits the next higher standard: 125 A maximum. A 100 A device is also acceptable and is what the existing installation has.
Evaluating the 150 A proposal. 150 A is not permitted by the table. The code path when a motor will not start is not to guess a bigger breaker:
- Verify the overloads are actually set to the 55 A maximum and not tripping instead of the breaker.
- Check the actual starting current and acceleration time - a conveyor with a jammed or loaded start draws locked-rotor current far longer than a free start.
- Check the 100 A device for being a mismatched or aging breaker before replacing anything.
- Only then use the exception provisions of 430.52 that allow an increase where the calculated value will not carry the starting current, with a documented ceiling and AHJ acceptance. Design B motors have a lower ceiling than other designs in the exception language, so confirm the cap before submitting anything.
A 150 A breaker on a #6 conductor is a conductor protection problem as well as a motor problem.
3. Size the MCC feeder conductors and the feeder overcurrent device serving both motors.
Read the worked answer
Feeder conductors, 430.24. 125% of the largest motor FLC plus 100% of the other motors:
(46.2 x 1.25) + 16.7 = 57.8 + 16.7 = 74.5 A
#6 Cu at 65 A is short. #4 Cu THHN at 75 C = 85 A is required. Include an equipment grounding conductor sized from Table 250.122 on the feeder device rating.
Feeder overcurrent device, 430.62(A). Not greater than the largest branch-circuit short-circuit and ground-fault device for any motor on the feeder, plus the sum of the full-load currents of the other motors:
125 + 16.7 = 141.7 A
The device must not exceed 141.7 A, so the largest standard rating available is 125 A. It is not 150 A, and it is not derived by adding motor currents and applying 125%.
Why this matters to the crew. Two different numbers come out of the same two motors: the conductor is sized by 430.24 (74.5 A, #4 Cu) and the device is capped by 430.62 (141.7 A, 125 A). If the conveyor's branch device is later changed to 150 A, the feeder cap changes with it and the feeder device has to be re-evaluated. Write both numbers on the panel schedule so the next change does not silently break the feeder protection.