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Motor Full-Load Amps Calculator

Calculate an electric motor’s full-load amps (FLA) from horsepower, voltage, efficiency and power factor for single-phase or three-phase, plus the 125% conductor size.

Motor power

Voltage

System

Efficiency

Power factor

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How it works

watts = hp × 746 · single-phase FLA = watts ÷ (V × eff × PF) · three-phase FLA = watts ÷ (√3 × V × eff × PF) · conductor amps = FLA × 1.25

A motor’s horsepower is its mechanical output; the electrical input is larger because of efficiency losses, and the current also depends on power factor. Converting hp to watts (746 W per hp) and dividing by the voltage, efficiency and power factor — with √3 for three-phase — gives the full-load amps the motor draws at rated load. The NEC then requires branch-circuit conductors at 125% of the motor FLA, because motors run continuously and start with a heavy inrush.

Worked example

A 5 hp three-phase motor at 230 V, 90% efficient, 0.85 PF: watts = 5 × 746 = 3,730 W. FLA = 3,730 ÷ (1.732 × 230 × 0.90 × 0.85) = 3,730 ÷ 304.8 ≈ 12.2 A. Conductors are sized at 125%, about 15.3 A, so 14 AWG copper (or larger per code tables) applies.

Sources: NEC Article 430 motors (Wikipedia: Electric motor)

Frequently asked questions

Is calculated FLA the same as the nameplate amps?

Close, but use the nameplate value for real sizing. This calculation estimates FLA from typical efficiency and power factor; the NEC actually requires you to size overload protection from the nameplate and conductors from its Table 430.250 values.

Why size conductors at 125%?

Motors are continuous loads with high starting current. The 125% factor gives the conductors and controls thermal margin so they are not run at their limit and can survive repeated starts.

What about starting (inrush) current?

Locked-rotor current can be 5–7× the FLA for a moment at start. Conductors are sized on FLA, but the overcurrent device and starter must be chosen to ride through that brief inrush without tripping.

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Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.