NEC 430 — Motor Circuits: The 125% Rule, Locked-Rotor Currents, and the Largest-Motor Adder

NEC 430 covers motor branch circuits and feeders — well pumps, pool pumps, sump pumps, garage door openers, HVAC compressors, etc. The rules differ from regular branch-circuit sizing because motors have inrush currents at startup that briefly exceed normal full-load current.

Conductor sizing (430.22): branch-circuit conductors for a single motor must have an ampacity not less than 125% of the motor's full-load current (FLA). Use the FLA from the motor nameplate, NOT the horsepower rating's table value (different rules apply, but nameplate FLA is what controls in residential).

Example: a 1.5 HP well pump with a nameplate FLA of 10 A on 240 V needs:

• Conductors: 10 × 1.25 = 12.5 A → #14 CU minimum (15 A capacity)
• Branch breaker: typically 20 A to allow inrush startup

Branch-circuit overcurrent protection (430.52): motor breakers can be sized HIGHER than the conductor ampacity to allow startup inrush. NEC Table 430.52 sets the maximum:

• Inverse-time circuit breaker (standard residential): 250% of motor FLA maximum
• A 10 A FLA motor can have a breaker up to 25 A (next standard size up = 25 A or 30 A) even if the conductors are sized for 12.5 A

This is THE rule that surprises new electricians — the breaker can be much larger than the conductor ampacity for motor circuits, because motors trip fast on overload.

Largest motor adder for the service load calc (430.24 / 220.50): when multiple motors are served by the same feeder, the FEEDER must carry 125% of the largest motor's FLA + 100% of all other motors. For service-entrance load calculation, this typically translates to "add 25% of the largest motor to the calculated load."

Common residential motors and their FLAs:

• Sump pump (1/3 HP, 120 V): 7-10 A
• Well pump (1.5 HP, 240 V): 8-11 A
• Pool pump (1 HP, 240 V): 8-9 A
• Central A/C compressor (3 ton): 18-22 A
• Garage door opener (½ HP, 120 V): 5-6 A

For HVAC specifically, see NEC 440 — air-conditioning equipment has slightly different rules (440.32 motor-compressor sizing).

For load-calc impact, the free Malfettone Load Calculator at /tools/load-calculator has a "Largest motor (optional)" field that applies the 25% adder to the service load per NEC 220.50.