How this calculator works
Motor current depends on electrical input power, while horsepower ratings usually describe mechanical output. Divide shaft power by efficiency, then divide by the supply voltage, power factor and phase multiplier. The horsepower unit provided is mechanical horsepower, exactly 550 foot-pounds-force per second, about 745.699872 W. Electrical and metric horsepower use different definitions. This estimate represents steady load rather than starting or stall current.
Formula
I = Pshaft/(eta k V PF). k = 1 for DC or single phase, sqrt(3) for three-phase line-to-line, and 3 for line-to-neutral. PF = 1 for DC.
Worked example
A 1000 W DC motor at 100 V and 50% efficiency draws 20 A. A 960 W shaft load at 120 V single-phase, unity efficiency and power factor 0.8 draws 10 A.
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Frequently asked questions
Why is efficiency separate from power factor?
Efficiency compares shaft output with real electrical input. Power factor compares real input with apparent input.
Can I estimate starting current?
No. Starting and locked-rotor current depend on motor design and controls.
What kind of horsepower is available?
The selector uses mechanical horsepower. Convert metric or electrical horsepower to watts before entering them.