TradesHow this works
A motor circuit has three separate parts, and the code sizes each one differently. The branch conductor carries 125% of the table full-load current. The running overload is set from the motor nameplate amps, at 115% or 125% depending on the service factor. The branch breaker or fuse only guards against short circuits and ground faults, so it is allowed to run much larger so the motor can start without nuisance tripping.
This tool runs Article 430 for those three parts at once and shows the maximum breaker and fuse options side by side, so you can choose the protective device that fits the install. Every size is a starting point for a licensed electrician, not a permit or a certification.
Common questions
Why size everything from the table FLC, not the nameplate?
NEC 430.6(A)(1) says the conductor and branch protection are sized from the full-load current in Tables 430.248 and 430.250, not the nameplate amps. The nameplate current is used only to set the running overload under 430.32.
Why can the breaker be bigger than the wire ampacity?
A motor branch breaker only handles short circuits and ground faults, and it has to let the motor start without tripping on inrush. The separate overload device protects the wire and motor from running overload, so the breaker is allowed to exceed the conductor ampacity.
Breaker or fuse for a motor?
An inverse-time breaker is the common default and is resettable. A dual-element time-delay fuse allows a much smaller device for closer short-circuit protection but must be replaced after it blows. Both are maximums, so a smaller standard size that still starts the motor is fine.
Sources: NEC Table 430.248 (single-phase) and 430.250 (three-phase) full-load currents (2023) · NEC 430.22 conductor, 430.52 branch protection, 430.32 overload (2023) · NEC Table 310.16 ampacity (2023). Engine version 1.0.0. Risk tier 2.