Motors, Controls, and Utilization Equipment
Motor circuit basics
Article 430 governs motor branch circuits, overload protection, short-circuit and ground-fault protection, controllers, and disconnects. Motor full-load current comes from the code tables, not the motor nameplate, for sizing branch-circuit conductors and short-circuit protection. The branch-circuit conductors are generally sized at 125% of the table full-load current. The overload device protects the motor from overload and is sized from the nameplate or the tables using the permitted percentage. The branch-circuit short-circuit and ground-fault device may be sized much larger to allow starting current.
Traps
A common mistake is using the nameplate full-load amps for conductor sizing. The table value controls. Another trap is confusing overload protection with short-circuit protection. The disconnect must be within sight of the motor and controller unless an exception applies. For HVAC and appliance circuits, check the nameplate minimum circuit ampacity and maximum overcurrent protection rather than calculating from general rules.
Worked example
A 10 hp, 460 V, three-phase motor has a table full-load current of 14 A. The minimum branch-circuit conductor ampacity is 125% of 14 A, or 17.5 A. A 12 AWG copper conductor with a 75°C terminal rating is acceptable if other correction factors do not reduce it below 17.5 A. An inverse-time breaker may be sized up to 250% of 14 A, or 35 A, subject to the code starting rules. Rule of thumb: conductors at 125% FLC; overload near nameplate; breaker sized for starting.