Estimates the electrical current a motor draws based on its rated horsepower, supply voltage, and efficiency.
How it works
Horsepower is converted to watts (×746), then divided by voltage and efficiency to give the estimated current draw.
What this does not include
This simplified single-phase/DC form does not include power factor, which further affects current draw in real AC installations — check a motor’s nameplate for exact figures.
How to use this calculator
- Enter the motor’s horsepower, supply voltage, and efficiency (from the nameplate if available).
A worked example
A 5 hp motor at 240V, 85% efficiency: amps = (5 × 746) ÷ (240 × 0.85) = 18.2843 A.
A 1 hp motor at 120V, 80% efficiency: amps = 7.7708 A.
What the variables mean
| Variable | Meaning |
|---|---|
| Horsepower | Motor’s rated mechanical output |
| Voltage | Supply voltage |
| Efficiency | How much input electrical power actually converts to mechanical output |
Edge cases worth knowing
Lower efficiency means higher current draw for the same mechanical output — energy lost to heat and friction still has to come from somewhere, which is why the less efficient motor in the second example draws more current per horsepower than a more efficient one would.
Zero voltage makes the current calculation undefined — there’s no supply to divide the power by, so the calculator declines to show a result.
Frequently asked questions
Why does efficiency matter for current calculation?
Because not all electrical power input becomes mechanical output — some is lost as heat, so a less efficient motor draws more current for the same horsepower.
Where do I find a motor’s efficiency rating?
On the motor’s nameplate, typically listed as a percentage — most modern motors run 80-95% efficient depending on size and design.
Does higher voltage always mean lower current for the same horsepower?
Yes — for the same power output, current and voltage are inversely related, which is why high-power equipment is often run at higher voltages.
