Equipment for General Use, Motors, and Controls
Scope
This domain covers Chapter 4 equipment - switches (404), receptacles (406), panelboards (408), luminaires (410), appliances (422), fixed electric space heating (424), transformers (450) - plus the motor and control material in Articles 430 and 440. Motor questions are the most computational items on the exam, so know the sequence: conductors, short-circuit and ground-fault protection, overload protection, disconnecting means, and controller.
Motor rules to know
- Use the horsepower tables (430.247, 430.248, 430.250) for conductors and short-circuit protection, not the nameplate current (430.6).
- Branch-circuit conductors: at least 125% of the table full-load current (430.22).
- Overload protection (430.32) is based on nameplate current: 125% when the service factor is 1.15 or higher, 115% when it is lower.
- Short-circuit and ground-fault protection (430.52): non-time-delay fuse 300%, dual-element fuse 175%, inverse-time breaker 250% of full-load current, with a limited next-size-up allowance.
- A disconnecting means must be within sight of the motor and controller (430.102).
Traps
Applying the 125% factor twice (once for conductors, again for the overcurrent device where the Code does not require it), using nameplate amps in a table-based calculation, and confusing overload protection (protects the motor from overheating) with branch-circuit protection (protects the circuit from short circuits).
Worked example
A 5 hp, 230 V, three-phase motor. Table 430.250 gives 15.2 A full-load current. Conductors: 1.25 x 15.2 = 19 A, so 12 AWG copper at 75 degrees C (25 A) is adequate; 240.4(G) permits the larger overcurrent protection that motor circuits require. Dual-element fuse: 1.75 x 15.2 = 26.6 A, so the next standard size, 30 A, is permitted under 430.52(C)(1) Exception 1. Overloads would be sized from the nameplate instead.
Rule of thumb: conductors and short-circuit protection come from the table amps; overloads come from the nameplate amps.