Motor FLA Calculator
Motor full-load current is the starting point for sizing conductors, overloads and short-circuit protection. Enter the motor's horsepower, voltage, phase, efficiency and power factor, and this calculator returns the full-load amps plus the 125% conductor ampacity used for sizing. Nameplate FLA, where available, always wins over a calculated value.
Formulas: 3-phase FLA = HP × 746 ÷ (√3 × V × efficiency × pf) · 1-phase FLA = HP × 746 ÷ (V × efficiency × pf) · Conductor ampacity = 125% × FLA
Worked example: a 10 HP, 400 V three-phase motor at 90% efficiency and 0.85 power factor: FLA = 10 × 746 ÷ (1.732 × 400 × 0.90 × 0.85) ≈ 14.1 A, so conductors are sized for at least 17.6 A. Starting current is typically 6–7 times FLA for a few seconds — that is normal and is what the protection settings must ride through.
Once you have the FLA, check the cable run with the voltage drop calculator and the conductor size with the wire size calculator.
Frequently asked questions
Why is nameplate FLA better than calculated FLA?
Nameplate FLA is measured for that exact motor. The calculation estimates from rated horsepower, efficiency and power factor, which are nominal values — close, but never exact.
Why size conductors at 125% of FLA?
Motors draw more than full-load current during starting and can run slightly overloaded. The 125% rule (NEC 430.22) gives the conductor thermal margin for those conditions.
What about the breaker size?
Short-circuit and ground-fault protection for motors is set higher than FLA — typically up to 250% for an inverse-time breaker (NEC 430.52) — so it does not trip on starting current. Overload protection is a separate, much tighter setting.