What formula is used?

Power in watts = HP × 746. Current = P_watts ÷ (Voltage × Efficiency). Resistance = Voltage ÷ Current. This gives the effective resistance the motor presents to the supply.

Why does efficiency matter?

A motor with lower efficiency draws more current to deliver the same mechanical output, which reduces the apparent resistance. A 100% efficient motor draws the theoretical minimum current.

Is this the winding resistance or impedance?

This is the effective AC impedance at the operating point, not the DC winding resistance. DC winding resistance is much lower and is measured with a multimeter at standstill.

What is a typical motor efficiency?

Premium-efficiency motors (NEMA IE3/IE4) typically range from 88% to 96%. Standard motors run 80%–92%. Use the nameplate or datasheet value for accurate results.

Can I use this for three-phase motors?

Enter the phase voltage (line-to-neutral) and per-phase HP for three-phase calculations, or use total HP with line voltage for a rough total-circuit approximation.

Horsepower to Ohms Calculator

Name: Horsepower to Ohms Calculator
Availability: InStock
Author: Nham Vu

Enter horsepower, voltage, and efficiency to calculate the equivalent electrical resistance in ohms.

Motor Parameters

Horsepower (HP)

Mechanical output power of the motor

Supply Voltage (V)

RMS voltage supplied to the motor

Efficiency (%)

Motor efficiency — typically 80–96% for AC motors

Equivalent Resistance

—

ohms (Ω)

Calculation Breakdown

Input Power (watts): —
Electrical Power drawn (watts): —
Current (amps): —
Resistance (ohms): —

Formula Used

P_W = HP × 746

P_elec = P_W ÷ η

I = P_elec ÷ V

R = V ÷ I

where η = efficiency as a decimal (e.g. 0.90)

Summary

Enter horsepower, voltage, and efficiency to calculate the equivalent electrical resistance in ohms.

How it works

Enter the motor power in horsepower (HP).
Enter the supply voltage in volts (V).
Enter the motor efficiency as a percentage (e.g. 90 for 90%).
The tool converts HP to watts: P_watts = HP × 746.
It then calculates current: I = P_watts / (V × efficiency).
Finally it derives resistance via Ohm's law: R = V / I.

Use cases

Estimate winding resistance for motor protection relay settings.
Size circuit breakers and fuses for motor installations.
Validate conductor sizing against expected impedance.
Cross-check motor nameplate data during commissioning.
Teach the relationship between mechanical and electrical power.
Quick field calculations without a full motor datasheet.

Frequently Asked Questions

Last updated: 2026-06-14 · Reviewed by Nham Vu