Motors, Motor Controls and HVAC Equipment
What the exam targets
Articles 430 (motors) and 440 (air conditioning and refrigeration). Expect full-load current tables, conductor sizing, overload versus short-circuit protection, disconnects, and nameplate data for HVAC equipment.
Concepts to own
- Table 430.250 full-load current is used for conductor ampacity and short-circuit protection, not the motor nameplate (430.6). Overload protection uses nameplate current.
- Branch-circuit conductors: 125% of full-load current (430.22).
- Overload (running) protection per 430.32: typically 125% of nameplate FLA where the service factor is 1.15 or the temperature rise is 40 °C.
- Short-circuit and ground-fault protection per 430.52: 175% for a dual-element fuse, 250% for an inverse-time breaker, 300% for a non-time-delay fuse; one step up is allowed if the motor won't start.
- Disconnecting means within sight (visible, no more than 50 ft) and rated at least 115% of FLC.
- HVAC: the nameplate minimum circuit ampacity sizes the conductors and the maximum overcurrent protective device sizes the OCPD; Article 440 overrides the motor tables.
Traps
Using nameplate FLA for conductor sizing; applying 125% twice; sizing the disconnect at 100% of FLC; mixing the 430.52 percentages between fuse types; using the maximum overcurrent device rating as the conductor ampacity.
Worked example
A 10 hp, 460 V, three-phase motor. Table 430.250 gives 14 A. Conductors: 14 × 1.25 = 17.5 A, so 12 AWG at 75 °C (25 A) works. Overload relays: 14 × 1.25 = 17.5 A. Inverse-time breaker: 250% × 14 = 35 A.