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 calculates theoretical equivalent load resistance at full operating power assuming unity power factor. It is not physical DC winding resistance (which is measured at standstill with an ohmmeter and is much lower), nor does it account for reactive impedance in AC motors.

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?

No. This calculator is strictly for single-phase DC or unity-power-factor equivalent models. Balanced three-phase AC systems require dividing total power by (√3 × line voltage × power factor), so omitting √3 and power factor produces major calculation errors.

Horsepower to Ohms Calculator

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

Mechanical Output 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). Assumes unity power factor (PF = 1.0). For educational and theoretical modeling only; do not use for sizing NEC/IEC motor circuit protection.

Summary

This calculator converts mechanical horsepower into equivalent electrical resistance (ohms) using supply voltage and motor efficiency (1 HP = 746 W). It models an ideal DC or unity-power-factor equivalent load at steady state. It does not measure physical DC winding resistance or replace full AC power-factor calculations for motor branch circuit protection.

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

Model equivalent steady-state load resistance of a motor circuit.
Compare theoretical operating resistance across different supply voltages.
Understand equivalent impedance concepts in electrical engineering coursework.
Cross-check mechanical-to-electrical power conversion at unity power factor.
Teach the relationship between mechanical horsepower and electrical watts.
Quick theoretical benchmark calculations without full motor datasheets.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu