Motors, HVAC, and Motor Control Circuits
Motors, HVAC, and Motor Control Circuits
This domain covers Articles 430 and 440: motor FLC tables, conductor sizing, overload protection, branch short-circuit protection, disconnects, and HVAC nameplate values.
Core rules
- Use Tables 430.248 and 430.250 for motor full-load current, not the motor nameplate FLA, for branch conductors and overloads.
- Branch-circuit conductors: 125% of motor FLC (430.22).
- Overload protection is sized to motor nameplate current, typically 115–125% (430.32).
- Short-circuit and ground-fault protection use Table 430.52 percentages, often 250% for inverse-time breakers.
- HVAC equipment uses nameplate MCA (minimum circuit ampacity) and MOP (maximum overcurrent protection); do not substitute the compressor RLA alone.
Worked example
A 10 hp, 480 V, 3-phase motor has a Table 430.250 FLC of 14 A. Branch conductors = 14 × 1.25 = 17.5 A; a 12 AWG copper conductor at 75°C (25 A) is acceptable after adjustment. Overload = 14 × 1.25 = 17.5 A, using the nameplate current for the actual setting. Branch OCPD may be up to 250% of 14 A = 35 A for an inverse-time breaker; check 430.52 and conductor protection.
Traps
- Using nameplate FLA instead of the Code FLC table for conductors.
- Confusing overload protection with branch short-circuit protection.
- Setting a breaker at the MOP when the nameplate says MCA.
- Ignoring the motor disconnect required at the controller location.