Motors, Controls, and HVAC Equipment
The Motors, Controls, and HVAC Equipment domain tests Articles 430 and 440: motor full-load current from tables, branch-circuit conductors, short-circuit protection, overload protection, disconnects, and air-conditioning nameplate calculations. The exam expects table values, not nameplate amps, for motor branch circuits.
Core rules
- Motor full-load current comes from Table 430.248 for single-phase motors and Table 430.250 for three-phase motors. Branch-circuit conductors are sized at 125% of full-load current per 430.22. Overload protection per 430.32 uses nameplate full-load amps and service factor: 125% for a 1.15 service factor or 40°C rise, and 115% otherwise.
- Short-circuit and ground-fault protection per 430.52: inverse-time breaker at 250% of full-load current, dual-element time-delay fuse at 175%, and non-time-delay fuse at 300%. If the device will not start the motor, 430.52(C)(1) Exception permits the next standard size.
- Disconnect sizing per 430.110 requires at least 115% of full-load current. Controller sizing is in 430.83.
- HVAC equipment per 440 uses nameplate rated-load current and branch-circuit selection current. Conductors are sized at 125% of rated-load current per 440.32. Protection is generally 175% of rated-load current per 440.22, with 225% allowed where needed.
Worked example
A 5 hp, 230 V single-phase motor has a Table 430.248 full-load current of 28 A. Branch conductors: 28 × 1.25 = 35 A; 10 AWG copper at 75°C is 35 A and is acceptable with 75°C terminations. Inverse-time breaker: 28 × 2.50 = 70 A; use the 70 A standard rating. Overload: a nameplate current of 26 A with a 1.15 service factor gives 26 × 1.25 = 32.5 A; select an overload that does not exceed 32.5 A. Disconnect: 28 × 1.15 = 32.2 A, so a 60 A switch is adequate.
Traps
- Using nameplate full-load amps for branch conductors instead of the table value.
- Applying 125% to overload sizing instead of the service-factor rule.
- Using the next-size-up allowance without the 430.52 exception conditions.
- Applying 240.4(D) to a motor branch circuit.
- Using Article 430 rules for a hermetic refrigerant motor-compressor covered by Article 440.