Motor Feeder Sizing at a Machine Shop and a Rooftop AC That "Runs Fine on 30"
A machine shop adds a 25 hp, 460 V, 3-phase TEFC motor (nameplate 33.5 A, service factor 1.15, 40 °C rise) fed from a new combination starter, and a 10 hp, 460 V, 3-phase motor on the same feeder (Table 430.250 FLC = 14 A). The feeder also carries 30 A of continuous miscellaneous load. Starter and motor terminations are marked 75 °C, and the motor branch circuit is run in THWN in rigid metal conduit. Separately, a 208/230 V single-phase rooftop AC unit is installed with nameplate RLA 18.0 A, minimum circuit ampacity 24.5 A, and maximum overcurrent protective device 40 A. The installer proposes 10 AWG conductors on a 40 A breaker, saying "it runs fine on 30, but the label says 40."
1. Determine the 25 hp branch-circuit conductors, the branch-circuit short-circuit and ground-fault protection, and the overload protection.
Read the worked answer
Use the table FLC, not the nameplate, for conductors and short-circuit protection. 430.6(A)(1) requires Table 430.250 values. 25 hp at 460 V = 34 A.
Conductors (430.22): 125% of FLC = 34 x 1.25 = 42.5 A. At 75 °C terminations, 8 AWG Cu = 50 A, so 8 AWG in the RMC is acceptable - provided the ambient and conduit fill adjustments do not drop it below 42.5 A. If any termination were 60 °C-only, 8 AWG would be limited to 40 A and 6 AWG would be required. That is the trap: the answer hinges on the termination marking, not on the motor.
Branch-circuit short-circuit and ground-fault protection (430.52): an inverse-time breaker may be set at 250% of FLC = 34 x 2.50 = 85 A. Since 85 A is not a standard rating, 240.6 permits the next higher standard size, 90 A, when the 250% value does not correspond to a standard rating. Do not confuse this with the 30 A small-conductor rule - 240.4(G) permits motor branch circuits to be protected per Article 430.
Overload protection (430.32): this is the only place the nameplate amps are used. With a service factor of 1.15, the overload device is selected at 125% of nameplate FLA = 33.5 x 1.25 = 41.9 A. Select the heater or electronic overload setting at or below that value; 430.32(C) allows a higher setting only if the motor will not start or run without tripping, and then not more than the next size up.
Summary: 8 AWG Cu THWN (75 °C), 90 A inverse-time breaker, overload set near 42 A, and the running overload device is what protects the motor from overload - the 90 A breaker protects only against short circuit and ground fault.
2. Size the feeder conductors and the feeder overcurrent device for the combination starter panel.
Read the worked answer
Feeder conductors (430.24): 125% of the largest motor FLC plus the sum of the other loads.
- 1.25 x 34 A = 42.5 A (25 hp)
- 10 hp = 14 A
- Miscellaneous = 30 A
- Total = 86.5 A
Now check the copper at 75 °C: 4 AWG = 85 A, which is less than 86.5 A and fails. 3 AWG Cu = 100 A is required. This is the step most candidates skip - they stop at 85 A and assume 4 AWG "is close enough."
Feeder overcurrent device (430.62): not greater than the largest branch-circuit short-circuit and ground-fault protective device for any motor on the feeder plus the sum of the FLCs of all other motors and other loads.
- 90 A (25 hp branch device) + 14 A + 30 A = 134 A
- Use the standard size not exceeding that: 125 A (240.6). Verify the 125 A device also carries the calculated load of 86.5 A - it does.
Judgment points:
- If the 125 A feeder breaker trips on starting, do not simply go up a size; 430.63 and 430.62 give limited relief, and a soft starter or reduced-voltage starting is usually the right answer.
- The feeder EGC is sized from Table 250.122 on the feeder OCPD (125 A = 6 AWG Cu), not from the motor circuit ampacity.
- Verify the disconnect at the controller is within sight of the motor and readily accessible; the motor itself needs a separate disconnecting means only where the controller is not within sight (430.102).
3. Evaluate the AC unit's branch circuit: conductor size, overcurrent device, and disconnect. Is the installer's 10 AWG on a 40 A breaker acceptable?
Read the worked answer
Yes, it is permitted, and that is the point of the question. Article 440 circuits are exempt from the small-conductor limits of 240.4(D) through 240.4(G); the circuit is governed by the nameplate and 440.22/440.33.
- Conductors: 440.33 sets the minimum at the nameplate minimum circuit ampacity, 24.5 A. 10 AWG Cu at 75 °C = 35 A, which exceeds 24.5 A. 10 AWG is acceptable.
- Overcurrent device: 440.22 allows up to 175% of RLA (18.0 x 1.75 = 31.5 A) or 225% where the unit will not start, but 440.22(C) and the nameplate cap the maximum. The 40 A nameplate value is a maximum, not a required setting. A 30 A breaker and a 40 A breaker are both compliant; the 40 A gives more starting margin.
- What is not allowed: exceeding 40 A, or using conductors smaller than the 24.5 A minimum circuit ampacity. Both are satisfied here.
Disconnecting means (440.14): a disconnecting means must be within sight of and readily accessible from the air-conditioning equipment, and it must be capable of being used to disconnect the equipment from all ungrounded conductors. A lockable non-fused pullout at the unit is typical. For a unit on a roof, the local disconnect must be at the unit, not at the panel.
Other checks: the nameplate MCA already includes the 125% factor - do not apply another 1.25 multiplier. Verify the equipment grounding conductor and the circuit's GFCI requirements per the adopted edition for the specific location, and confirm the branch-circuit conductors' ampacity is not reduced below MCA by ambient temperature on a rooftop; a rooftop ambient of 40–50 °C can force a size increase from 10 to 8 AWG even though the conductor's own rating looks adequate at 30 °C.