Motors, Transformers, HVAC, and Control Circuits
Core concepts
Motor calculations use tables, not nameplate current. Article 430 gives full-load current from Tables 430.248 (single-phase) and 430.250 (three-phase). Branch-circuit conductors are sized at 125 percent of FLC (430.22). The running overload device protects the motor from overload and is sized from the nameplate (430.32), typically 115–125 percent of nameplate current. The branch-circuit short-circuit and ground-fault protective device is sized from Table 430.52: 250 percent of FLC for an inverse-time breaker, 175 percent for a dual-element time-delay fuse, with permission to go to the next standard size.
Transformers are protected under 450.3. Primary-only protection for transformers over 1000 V differs from the 9 A rule for transformers 1000 V or less. Secondary conductors are covered by 240.21(C), which allows the 10 ft. and 25 ft. secondary tap rules when the conductors terminate in a single overcurrent device.
HVAC equipment under Article 440 uses the nameplate minimum circuit ampacity (MCA) for conductor sizing and the maximum overcurrent protective device (MOP) for the disconnect and breaker.
Traps
- Using nameplate current instead of table FLC for motor branch conductors.
- Using Table 430.52 instead of nameplate current for overload sizing.
- Confusing MCA with MOP on an HVAC nameplate.
Worked example
A 5 hp, 230 V, three-phase motor has an FLC of 28 A from Table 430.250. Minimum conductor ampacity = 1.25 × 28 = 35 A. Maximum inverse-time breaker = 2.50 × 28 = 70 A.