Motors, Controls, and Special Equipment
Motor full-load current is not nameplate current
Article 430 requires motor branch-circuit conductors and protection to be based on the table full-load current (430.247 through 430.250), not the nameplate amperes. Nameplate values are used for overload protection. This single distinction generates more motor questions than any other.
The four sizing rules
- Branch-circuit conductors: 125% of table FLC (430.22).
- Overload protection: typically 125% of nameplate current for a marked motor above 1 hp (430.32).
- Short-circuit and ground-fault protection: from Table 430.52, commonly 250% of FLC for an inverse-time breaker or 175% for a dual-element fuse, with permitted increases if the device will not start the motor.
- Disconnecting means: within sight of the motor and controller, sized at least 115% of FLC (430.102, 430.110).
Controls and other equipment
Controllers must be capable of starting and stopping the motor and are located within sight unless the disconnect is lockable. Special-equipment articles frequently tested include 440 (air conditioning and refrigeration), 680 (pools and spas), 690 (solar photovoltaic), 700 (emergency systems), and 500/501 (hazardous locations, where the division and class drive every requirement).
Worked example
A 10 hp, 230 V, three-phase motor. Table 430.250 gives 28 A FLC. Branch-circuit conductors must carry 125% = 35 A, so 8 AWG copper is selected. Overload protection is set from the nameplate current times 125%. Branch-circuit protection on an inverse-time breaker may be 250% of 28 A = 70 A, the next standard size. Note the three different currents from one motor.