Motors, HVAC, and Motor Controls
Article 430 fundamentals
Motor branch-circuit conductors are sized at 125% of the full-load current from Tables 430.248 (single-phase) or 430.250 (three-phase) - not the nameplate amperes, which is the most common exam trap. Overload protection (430.32) protects the motor from sustained overload and is sized from nameplate current at 115%–125%. Short-circuit and ground-fault protection (430.52, Table 430.52) protects the circuit: typically 250% of FLC for an inverse-time breaker, 175% for dual-element fuses, and 800% for non-time-delay fuses, with permitted increases when the motor will not start. Feeder sizing (430.24) takes 125% of the largest motor plus 100% of the others. A disconnecting means must be within sight of the motor and controller (430.102), sized at least 115% of FLC.
HVAC
Article 440 governs hermetic refrigerant motor-compressors. Use the nameplate minimum circuit ampacity for conductors and the maximum overcurrent protection value for the branch device; the branch-circuit selection current may differ from rated-load current. Air-conditioning equipment disconnects follow 440.14.
Traps
Confusing overload with branch-circuit protection, applying 125% twice, sizing the disconnect from the breaker rather than motor current, and using nameplate amps where the table value is required.
Worked example
A 10 hp, 480 V, three-phase motor: Table 430.250 gives 14 A. Conductors: 14 × 1.25 = 17.5 A, so 12 AWG copper. Inverse-time breaker: 14 × 2.50 = 35 A. Overloads are set from the nameplate current, not 14 A.