Motor Circuit Sizing Calculator — NEC 430 Conductors, Breaker, Overload & Disconnect

Two FLA Values, Two Different Uses

This calculator uses two different FLA values. The NEC Table FLA (from Tables 430.248 and 430.250) is the design basis for conductor sizing, OCPD sizing, and disconnect rating — it's a standardized value that applies regardless of which specific motor is installed. The Nameplate FLA you enter is used only for the overload relay setting, because the overload device must be calibrated to the actual motor's thermal response, not a generic table value. Enter the nameplate FLA from the motor's actual data plate for accurate overload sizing.

Why the OCPD Is Sized So Much Larger Than the Conductor

The branch circuit OCPD for a motor circuit is permitted at 250% of FLA for an inverse time breaker — far above the 125% used for general circuits. This is not an error. The OCPD's job here is to protect against short circuits and ground faults only, not sustained overloads. Motor starting inrush (typically 600% of FLA) would trip a 125%-sized breaker on every start. The overload relay in the motor controller handles sustained overload protection at 115-125% of nameplate FLA. Both devices are required — removing either leaves part of the protection spectrum uncovered.

Conductor Derating Is Not Included Here

This calculator shows the minimum conductor ampacity per NEC 430.22 and the smallest standard conductor that meets it at 75°C in conduit. It does not apply derating factors for conduit fill (4+ current-carrying conductors), elevated ambient temperature, or conduit in a raceway with other heated conductors. Always apply NEC 310.15 correction and adjustment factors to the 75°C ampacity before confirming the conductor is adequate. If derating reduces the conductor's effective ampacity below the NEC 430.22 minimum, you must increase the conductor size until the derated ampacity meets or exceeds the minimum.

The Disconnect Must Be Motor-Rated, Not General-Use

The minimum disconnect ampere rating from this calculator (115% of Table FLA per NEC 430.110) is only one of two requirements. The disconnect must also carry an HP rating at the motor's operating voltage equal to or greater than the motor's HP. A general-use safety switch or toggle switch is not motor-rated and cannot interrupt locked-rotor current safely — it lacks the listed interrupting capacity for motor circuits. Use a safety switch or enclosed breaker listed for use as a motor circuit disconnecting means, with both an adequate ampere rating and a matching HP rating at the system voltage.

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