What is the NEC full load current for a 40 hp 460 volt motor?

Table 430.250 lists 52 amps for a 40 hp three phase motor at 460 volts. Conductor sizing uses this table value, not the nameplate, per NEC 430.6(A).

What size breaker protects a 40 hp 460 volt motor circuit?

An inverse time breaker may be sized up to 250 percent of the 52 amp table current per Table 430.52, which allows up to a 125 amp breaker, rounded up to 150 amps if needed for starting per 430.52(C)(1) Exception 2 limits.

What Size Wire for a 40 HP Three Phase Motor?

A user ran this exact motor calc through the app: 40 hp, three phase, 460 volts. Motor circuits follow their own logic in NEC Article 430, and the sequence matters, so here is the walk through.

Step one: table current, not nameplate

Per 430.6(A)(1), conductor sizing uses the full load current from the tables, not the number stamped on the motor. Table 430.250 lists a 40 hp motor at 460 volts three phase at 52 amps. The nameplate amps get used later, for the overload protection only.

Step two: conductors at 125 percent

430.22 requires branch circuit conductors for a single continuous duty motor to be sized at 125 percent of the table current. That gives 52 times 1.25, which is 65 amps. From the 75 degree column of Table 310.16, 6 AWG copper THHN at 65 amps is the exact fit. Aluminum would need 4 AWG at 75 amps. If the run is long, check voltage drop; motor starting current is roughly six times running current, and a soft start on a marginal feeder still appreciates fatter wire.

Step three: short circuit and ground fault protection

Table 430.52 permits an inverse time circuit breaker at up to 250 percent of the table current, which is 130 amps, and 430.52(C)(1) lets you round to the next standard size, 150 amps, where 130 will not hold the starting inrush. A 125 amp breaker is the common first pick. Yes, a 125 amp breaker on 6 AWG wire looks wrong to anyone who has not read Article 430; the breaker only handles faults, while the overloads protect the wire and motor from overcurrent.

Step four: overloads from the nameplate

430.32(A)(1) sizes separate overload protection at 115 to 125 percent of the actual nameplate current depending on service factor and temperature rise. A 40 hp motor with a 1.15 service factor and a nameplate of 48 amps gets overloads set near 60 amps. The starter or VFD handles this in practice, and a VFD changes several of these answers, including conductor sizing on the output side per 430.122.

Also remember the disconnect within sight of the motor per 430.102(B), rated at least 115 percent of the table current per 430.110.

Motor calcs are the most missed questions on journeyman exams. Ask BONBON walks the full 430 sequence for any horsepower and voltage, with every table cited.