What formula is used for DC circuits?

V = (HP × 746) / (A × Eff). One horsepower equals 746 watts; dividing output watts by current and efficiency gives the required voltage.

What formula is used for single-phase AC circuits?

V = (HP × 746) / (A × PF × Eff). The power factor reduces the effective power drawn, so a higher voltage is required compared to DC for the same current and HP.

What formula is used for three-phase AC circuits?

V = (HP × 746) / (√3 × A × PF × Eff). The √3 factor (≈ 1.7321) reflects the three-phase line-to-line voltage relationship. The result is the line-to-line voltage.

Why is 746 used in these formulas?

One mechanical horsepower is defined as approximately 745.7 watts, conventionally rounded to 746 W. This conversion factor links mechanical and electrical power units.

What is a typical power factor for electric motors?

Most induction motors operate at a power factor between 0.75 and 0.95 under full load. Lightly loaded motors can have lower power factors. The nameplate or datasheet will specify the rated value.

What efficiency value should I use?

Standard motors typically achieve 85–95% efficiency. Premium-efficiency motors can exceed 95%. Use the value from the motor nameplate or datasheet for accurate results.

Does this tool account for starting current or inrush?

No. This calculator uses steady-state (running) values. Starting current can be 6–8 times the running current and must be handled separately when selecting circuit breakers or contactors.

Can I use the result to choose wiring or a breaker?

No. The rearranged power equation does not establish a motor nameplate voltage, full-load current, conductor ampacity, voltage-drop allowance, branch-circuit protection, controller rating or starting behavior. Use applicable codes, manufacturer data and a qualified professional.

Horsepower to Volts Calculator

Calculate theoretical running voltage from HP, current, efficiency and power factor. This does not determine nameplate voltage, conductor size, protection or starting current.

Circuit Parameters

Circuit Type

Horsepower (HP)

Current (A)

Motor Efficiency (%) (1 – 100)

Steady-state equation only. Do not use this result alone to select nominal supply voltage, conductors, breakers, controllers or transformers.

Result

Enter values and press Calculate

Formula Reference

DC Circuits

V = (HP × 746) / (A × Eff)

Single-Phase AC

V = (HP × 746) / (A × PF × Eff)

Three-Phase AC

V = (HP × 746) / (√3 × A × PF × Eff)

Result copied!

Summary

Calculate theoretical running voltage from HP, current, efficiency and power factor. This does not determine nameplate voltage, conductor size, protection or starting current.

How it works

Select your circuit type: DC, single-phase AC, or three-phase AC.
Enter the mechanical power in horsepower (HP).
Enter the circuit current in amperes (A).
Enter the motor efficiency as a percentage (typically 85–95%).
For AC circuits, enter the power factor (a value between 0 and 1).
Read the theoretical steady-state voltage and step-by-step breakdown; verify against nameplate and qualified electrical design data.

Use cases

Rearrange the steady-state power equation for classroom or engineering checks.
Compare a calculated value with complete measured/nameplate data.
Check DC, single-phase and balanced three-phase formula behavior.
Convert motor specifications from HP to electrical parameters for procurement.
Cross-check wiring diagrams when the voltage rating is missing from documentation.
Design HVAC, pump, or conveyor systems by working backward from mechanical output.
Compare DC and AC supply options for the same mechanical load.
Document assumptions used in a theoretical motor-power calculation.

Frequently Asked Questions

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