Electrical Equipment, Motors, and Controls
Motor circuits, Article 430
The governing principle is that motor conductors and protection are sized from table values, not nameplate current. Use Tables 430.247 through 430.250 for full-load current (FLC), then apply: branch-circuit conductors at 125 percent of FLC (430.22); overload protection sized to the motor nameplate and service factor (430.32); short-circuit and ground-fault protection per 430.52, typically up to 250 percent of FLC for an inverse-time breaker, 175 percent for a dual-element fuse, and 300 percent for a non-time-delay fuse. A disconnecting means must be within sight of the motor and controller (430.102).
Controls and equipment
Understand starters and contactors, overload relays, two-wire versus three-wire control, and lockout/tagout. For hermetic refrigerant motor-compressors, Article 440 governs: size conductors from the nameplate minimum circuit ampacity and protect with the nameplate maximum overcurrent protective device. Also know working clearances (110.26): 30 in. wide, 36 in. deep for 0-150 V to ground, and 6-1/2 ft headroom. Terminations follow 110.14(C) ampacity columns.
Traps
- Sizing branch conductors from the nameplate current instead of the article tables.
- Applying 125 percent twice, or applying it to the overload device.
- Confusing running overload protection with short-circuit protection; the overload does not protect the branch circuit.
- Placing the disconnect out of sight of the motor.
- Using the wrong degree column when conductors or terminals are not rated 75 C.
Worked example
A 10 HP, 460 V, three-phase motor. Table 430.250 gives FLC = 14 A. Branch conductors must carry at least 125 percent x 14 = 17.5 A, so 12 AWG copper at 75 C (25 A) works. The inverse-time breaker may be sized up to 250 percent x 14 = 35 A. The running overload is set from the motor nameplate current, not from the 14 A table value.