HP to Amps Calculator (motor current from horsepower, DC, 1-phase & 3-phase)

Understanding the HP to amps calculator — concept and principle

This calculator estimates the current a motor draws at full rated load from its horsepower rating, efficiency, power factor and supply voltage, for DC, single-phase and three-phase supplies. It also shows the input power, apparent power and losses. Electricians and engineers use it for a first estimate before checking nameplate data.

Shaft power is divided by efficiency to get electrical power, then by voltage and power factor to get current.

How it is calculated

Mechanical output P_out = hp × 745.7 W (mechanical hp)

Electrical input P_in = P_out ÷ η

Current: DC I = P_in ÷ V; single-phase I = P_in ÷ (V × PF); three-phase I = P_in ÷ (√3 × V × PF)

Apparent power (AC) = P_in ÷ PF; losses = P_in − P_out

Metric hp (PS) = 735.5 W; electric hp = 746 W. V is line-to-line for three-phase.

The principle

A motor's horsepower rating is its mechanical output at the shaft. To deliver it, the motor must draw more electrical power, because some is lost as heat in windings, iron and bearings; efficiency accounts for that. In AC circuits current that is out of phase with the voltage does no useful work but still flows in the wires, so the power factor raises the current for a given real power. Three-phase supplies share the load across three conductors, giving the √3 factor.

Worked example

Default inputs: 5 hp (mechanical), single-phase 230 V, efficiency 85%, power factor 0.85.

P_out = 5 × 745.7 = 3,728 W = 3.728 kW.
P_in = 3,728 ÷ 0.85 = 4.386 kW; losses = 0.658 kW.
Apparent power = 4.386 ÷ 0.85 = 5.161 kVA.
I = 4,386 ÷ (230 × 0.85) = 22.4 A.

Each point is the calculator re-run with only “Voltage (V)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Voltage (V)	Current drawn (A)
57.5	89.7
115	44.9
172	29.9
230	22.4
288	17.9
345	15
460	11.2

How to read the result

This is the running current at full rated load. At start-up a motor draws several times more for a short time, and at light load it draws less. The same motor on a three-phase supply would draw much less current per conductor. Wiring and protection must be sized from the nameplate current or the tables in the applicable electrical code, not from this estimate.

Common mistakes

Using output horsepower as electrical input without dividing by efficiency.
Omitting the power factor for AC motors.
Using phase voltage instead of line-to-line voltage for three-phase.
Sizing cables and breakers from this estimate.

Keep in mind: Cable, breaker and overload sizing must follow the nameplate and local electrical codes, and electrical work should be done by a qualified electrician.

Frequently asked questions

Why is the nameplate current often higher than this estimate?

Small motors often have lower efficiency and power factor than the values entered here, and code tables include margins. Always use the nameplate or code value.

Why does three-phase draw less current?

Power is shared across three conductors, and the √3 factor reduces the current per line.

What is the difference between hp and PS?

Mechanical hp is 745.7 W; metric hp (PS) is 735.5 W.

Sources: US Department of Energy, "Determining Electric Motor Load and Efficiency" (DOE/GO-10097-517) · NIST Special Publication 811 (2008), Appendix B.8

Checked 9 Nov 2026. US Department of Energy, Motor Challenge fact sheet “Determining Electric Motor Load and Efficiency” (DOE/GO-10097-517): P_i = V × I × PF × √3 ÷ 1000 and P_ir = hp × 0.7457 ÷ η. Horsepower factors: NIST Special Publication 811 (2008), Appendix B.8.

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