Motors, Controls, and Special Equipment
This domain tests Article 430 motor logic and the special-equipment articles that modify it: 440 for air conditioning, 422 for appliances, 424 for space heating, 680 for pools, 690 for solar, and 700–702 for emergency and standby systems. The central skill is knowing which current value feeds which calculation.
Article 430 logic
- 430.6: use Table 430.250 full-load current for conductors and short-circuit protection, not nameplate FLA; overload protection uses nameplate current.
- 430.22: branch-circuit conductors at 125% of FLC.
- 430.24: feeder conductors at 125% of the largest motor plus the sum of the others.
- 430.52: branch short-circuit and ground-fault protection, generally not more than 250% for inverse-time breakers, with the next standard size up permitted.
- 430.32: overload protection typically 115–125% of nameplate FLA.
- 430.102: disconnecting means within sight; 430.109: horsepower-rated disconnect.
Worked example
A 10 HP, 480 V, three-phase motor has a Table 430.250 FLC of 14 A. Branch conductors: 14 × 1.25 = 17.5 A, so 12 AWG copper at 75°C (25 A) is acceptable for the motor branch. Branch breaker: 14 × 2.50 = 35 A, a standard rating under 240.6. Overload: use nameplate FLA, say 13 A; 13 × 1.15 = 15 A to 13 × 1.25 = 16.25 A, so select a heater or overload setting in that range. Disconnect: 115% × 14 = 16.1 A, so a 30 A horsepower-rated switch is suitable. Each function uses a different rule and often a different current.
Traps that cost marks
- Using nameplate FLA for conductor sizing instead of table FLC.
- Using table FLC for overload sizing instead of nameplate FLA.
- Forgetting the 125% multiplier on motor branch conductors.
- Treating 250% as a required breaker size rather than a maximum.
- Ignoring the next-standard-size-up allowance in 240.6.
- Locating the disconnect out of sight of the motor.
- Confusing MCA and FLA on an air-conditioning nameplate.