Motors, Air Conditioning, and Industrial Controls (NEC Articles 430, 440)
Articles 430 and 440 reward careful table work
Motor branch circuits are built from Table 430.250 full-load currents, not from the motor nameplate amperes. 430.22 sizes branch-circuit conductors at 125 percent of FLC. 430.52 caps the branch short-circuit and ground-fault protective device at 175 percent of FLC for a dual-element fuse and 250 percent for an inverse-time breaker, with the next standard size up permitted if the device will not carry starting current. Overload protection is separate: 430.32 sets it at 125 percent of the nameplate full-load amperes. A disconnecting means must be within sight - visible and not more than 50 ft away - per 430.102, and 430.24 sizes a circuit serving several motors by adding 125 percent of the largest FLC to the sum of the others.
Article 440 changes the inputs for hermetic refrigerant motor-compressors: use the rated-load current and branch-circuit selection current from the nameplate. 440.22 sets protection at 175 percent of rated-load current (225 percent where starting current trips the device), and 440.32 sizes branch conductors at 125 percent of the largest compressor rated-load current plus other loads.
Worked example: a 30 hp, 480 V, three-phase motor. Table 430.250 gives 40 A FLC. Conductors = 125 percent = 50 A, so 8 AWG copper at 75°C. Dual-element fuse maximum = 175 percent of 40 A = 70 A. Overload protection = 125 percent of the nameplate amperes.
Traps: using nameplate current where the table governs (motors) or the table where the nameplate governs (A/C compressors), and confusing overload protection with short-circuit and ground-fault protection.