Motors, Controls, and Utilization Equipment
Motors and controls
Article 430 governs. Table 430.250 gives full-load current by horsepower and voltage; 430.6(A)(1) requires table values (not nameplate current) for sizing conductors and short-circuit protection, while nameplate current is used for overload protection (430.32). Branch-circuit conductors are at least 125% of full-load current (430.22); feeder conductors are 125% of the largest motor plus 100% of the others (430.24). Short-circuit protection follows 430.52 - up to 250% of full-load current for an inverse-time breaker, 175% for a dual-element fuse, with increases allowed where the motor will not start. Overload devices are sized to nameplate current at the percentages in 430.32. A disconnecting means must be within sight of the motor (430.102) and must open all ungrounded conductors (430.103). Article 440 covers hermetic refrigerant motor-compressors, with 440.22 capping branch protection at 175% of rated-load current (225% maximum). Articles 422 and 424 cover appliances and space heating.
Worked example: 10 hp, 208 V, three-phase motor. Table 430.250 full-load current is 30.8 A. Conductors: 1.25 x 30.8 = 38.5 A, so 8 AWG copper at 75 degrees C (50 A). Inverse-time breaker: 250% x 30.8 = 77 A, so the next standard size, 80 A. Overload protection uses the nameplate amps, not 30.8 A.
Traps: sizing motor conductors from the nameplate; applying the 250% multiplier to the overload device; forgetting that the 125% factor never applies to overloads; assuming the controller and disconnect must be in the same place.