How to size motor branch-circuit protection

Why a motor circuit breaks the normal wire-and-breaker rules

Look up Article 430 in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use Table 430.52(C)(1) and the related sections; it does not reproduce the table.

On a lighting or receptacle circuit, one breaker protects the wire from everything. A motor circuit splits that job. The branch-circuit short-circuit and ground-fault device (breaker or fuses) only has to clear faults, so it is sized big enough to ride through inrush. The overload relay protects the motor and the wire from running hot. And the conductors are sized at 125% of the table full-load current. Three numbers, three sections — and it is normal for the breaker to be larger than the wire’s ampacity.

Table FLC vs nameplate. 430.6(A) says to use the NEC table full-load current — not the nameplate — for conductors, the SC/GF device, and the disconnect. The nameplate FLA is only used for the overloads. Pull the table value from the motor full-load current chart guide.

Worked example. A 7.5 hp, 460 V three-phase motor has a table FLC of 11 A. Conductors: 11 × 125% = 13.75 A, so 14 AWG copper is the minimum. Inverse-time breaker: 11 × 250% = 27.5 A — not a standard size, so Exception No. 1 lets you go to 30 A (see the standard breaker sizes guide). With a nameplate of 10.5 A and a 1.15 service factor, the overload tops out at about 13.1 A. If the motor will not start on that breaker, Exception No. 2 allows a higher ceiling — check your edition.

For conductor ampacity and derating on long or crowded motor feeds, see the NEC 310.16 ampacity chart and the ampacity correction factors guide. For starting current, see motor locked-rotor code letters; for the starter itself, NEMA starter sizes. Field PM keeps motor schedules, submittals, and inspection records attached to the job.