Size a motor circuit the code way.
Pick the horsepower and voltage to get the NEC Table 430.250 full-load current, the minimum conductor ampacity at 125 percent, and the maximum breaker size, all from the table value, not the nameplate.
Your motor
What this means
Full-load current by horsepower
| Motor | FLC (A) | Min conductor (125%) | Max breaker (250%) |
|---|
How motor sizing works
Why Use Table 430.250 and Not the Nameplate
NEC 430.6 requires that conductor and overcurrent sizing for a motor use the full-load current value from Table 430.250, not the amps printed on the motor nameplate. The nameplate current is used only for overload protection. This calculator returns the table value for the horsepower and voltage you select.
Sizing the Motor Circuit
Branch-circuit conductors for a single motor are sized at 125 percent of the full-load current under NEC 430.22. The branch-circuit short-circuit and ground-fault device, such as an inverse-time breaker, is generally allowed up to 250 percent of the full-load current under NEC 430.52, then adjusted to the next standard size if the motor will not start.
Full-Load Current vs Overload
Conductor and breaker sizing come from the table full-load current. Overload protection, which guards the motor itself, is sized from the actual nameplate amps and the service factor. Keep the two separate to size a motor circuit correctly.
Common questions
What is the full-load current of a 5 HP motor at 230V?
NEC Table 430.250 lists 15.2 amps for a 5 HP three-phase motor at 230 volts.
Why not just use the nameplate amps?
NEC 430.6 requires the table value for conductor and overcurrent sizing. The nameplate amps are used only for overload protection.
What size conductor for a motor?
Branch-circuit conductors are sized at 125 percent of the table full-load current under NEC 430.22, then checked against ampacity and derating.
How big can the motor breaker be?
An inverse-time breaker is generally allowed up to 250 percent of the full-load current under NEC 430.52, rounded to the next standard size if needed for starting.
Does this work for single-phase motors?
No. This uses the three-phase Table 430.250. Single-phase motors use Table 430.248, which has different values.
Reference values from NEC Table 430.250. Always confirm against the current adopted code and field conditions before installing.