What formula is used for DC circuits?

For DC: Amps = (HP × 746) / (Voltage × Efficiency). The 746 factor converts horsepower to watts (1 HP = 746 W).

What formula is used for single-phase AC?

For single-phase AC: Amps = (HP × 746) / (Voltage × Efficiency × Power Factor). The power factor accounts for the phase difference between voltage and current.

What formula is used for three-phase AC?

For three-phase AC: Amps = (HP × 746) / (√3 × Voltage × Efficiency × Power Factor), where √3 ≈ 1.7321. The line voltage (e.g., 208 V, 480 V) should be entered.

What is a typical efficiency for electric motors?

Most standard induction motors have efficiencies between 85% and 95% at full load. Smaller motors (under 1 HP) may be 75–85%. Check the motor nameplate for the rated efficiency.

What power factor should I use?

Typical motor power factors range from 0.75 to 0.90 at full load. A value of 0.85 is a common starting estimate when the nameplate value is not available.

Why do the results differ from the nameplate amps?

Nameplate current is measured at full-load operating conditions. Calculated values may differ slightly due to manufacturing tolerances, actual vs. rated efficiency, and actual operating voltage.

Horsepower to Amps Calculator

Enter horsepower, voltage, efficiency, and circuit type to instantly calculate the required amperage.

Circuit Parameters

Circuit Type

Horsepower (HP)

Voltage (V)

Efficiency (%) — typically 80–95

Power Factor — 0 to 1, typically 0.85

Enter values and click Calculate

Common Motor Voltages

DC: 12, 24, 48, 120, 240 V

1-Phase AC: 115, 120, 208, 230, 240 V

3-Phase AC: 208, 230, 460, 480, 575, 600 V (line-to-line)

Summary

Enter horsepower, voltage, efficiency, and circuit type to instantly calculate the required amperage.

How it works

Select the circuit type: DC, single-phase AC, or three-phase AC.
Enter the motor rating in horsepower (HP).
Enter the supply voltage in volts (V).
Enter the efficiency as a percentage (typically 80–95% for motors).
For AC circuits, enter the power factor (typically 0.8–1.0).
The calculator applies the appropriate formula and displays the current in amps.

Use cases

Size circuit breakers and fuses for electric motors.
Select the correct wire gauge for motor wiring.
Estimate current draw when upgrading or replacing motors.
Verify nameplate amperage against rated horsepower and voltage.
Design electrical panels for industrial equipment.
Compare single-phase vs. three-phase current requirements.
Calculate generator or UPS sizing for motor loads.
Convert motor specs from imperial (HP) to SI equivalents.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu