Motor Sizing Calculator
Calculate full load amps, wire size, breaker size, and starter size for electric motors based on HP, voltage, and phase.
Results
Visualization
How It Works
Electric motor sizing involves calculating the full load amps (FLA) from the horsepower rating, then using NEC tables to determine the correct wire gauge, overcurrent protection, and starter size. Motors draw high inrush current at startup (6-8x FLA), which is why breakers are sized at 250% of FLA rather than the usual 125%.
The Formula
Single Phase: FLA = (HP x 746) / (V x PF x Eff)
Three Phase: FLA = (HP x 746) / (V x 1.732 x PF x Eff)
Breaker = FLA x 2.5 (NEC 430.52)
Conductor = FLA x 1.25 (NEC 430.22)
Three Phase: FLA = (HP x 746) / (V x 1.732 x PF x Eff)
Breaker = FLA x 2.5 (NEC 430.52)
Conductor = FLA x 1.25 (NEC 430.22)
Variables
- HP — Horsepower -- 1 HP = 746 watts of mechanical output
- FLA — Full Load Amps -- the current drawn at rated horsepower and voltage
- PF — Power factor -- typically 0.85 for induction motors
- Eff — Motor efficiency -- typically 0.88 for standard motors
- SF — Service factor -- allows temporary overload (1.15 = 15% over nameplate HP)
Example
A 5 HP, 240V, three-phase motor: FLA = (5 x 746) / (240 x 1.732 x 0.85 x 0.88) = 11.9A. Wire = 14 AWG (but 10 AWG minimum for motor circuits). Breaker = 11.9 x 2.5 = 30A. Starter = NEMA Size 1.
Tips
- Always use the NEC FLA tables (430.248/250) rather than nameplate amps for sizing conductors and breakers.
- Motor branch circuit conductors must be rated for at least 125% of motor FLA for continuous duty (NEC 430.22).
- Inverse-time breakers can be sized up to 250% of FLA, while instantaneous-trip breakers follow different rules.
- Service factor of 1.15 means the motor can handle 15% overload continuously -- but wire and breaker sizing is based on nameplate FLA, not SF.
- Variable frequency drives (VFDs) change the sizing requirements -- consult the VFD manual for conductor and protection sizing.