FIELD GUIDE • UPDATED SEPTEMBER 2026
Motor Circuit Sizing Overview
Separate conductor, short-circuit protection, overload, and disconnect requirements.
Motor circuits are not sized like ordinary resistive loads. Branch-circuit conductors, short-circuit and ground-fault protection, overload protection, controllers and disconnects each have their own purpose and sizing method.
Field method
For many branch-circuit calculations, NEC motor-table full-load current is used rather than the motor nameplate current. Overload protection commonly relies on nameplate current and service-factor or temperature-rise information as directed.
The protective device may be larger than conductor ampacity would suggest because it must allow motor starting current. That does not eliminate overload protection; the two devices protect against different conditions.
Common mistakes
A field checklist should capture horsepower, voltage, phase, frequency, nameplate amperes, service factor, duty, enclosure, controller type, overload class, conductor insulation, ambient conditions and terminal ratings.
Common mistakes include using only the breaker rating to size conductors, sizing overloads from the wrong current, ignoring multiple-motor feeder rules, and overlooking control-circuit or disconnect requirements.
Start with NEC Article 430 and the applicable motor full-load-current table. Use the WireWise motor planner only for a physics-based current estimate, then complete the code calculation with table and nameplate data.