hp to amps Calculator

Convert a motor's horsepower rating into amp draw for DC, single-phase AC, or three-phase AC circuits, using I = (HP × 746) / (V × η × PF), adjusted for efficiency and power factor.

Quick Facts

Conversion constant
1 HP = 746 W
The standard electrical horsepower-to-watts conversion used in NEMA and NEC motor calculations.
DC & single-phase formula
I = (HP×746)/(V×η×PF)
PF = 1 for DC circuits, since there is no reactive power.
Three-phase formula
I = (HP×746)/(V×η×PF×√3)
√3 ≈ 1.732 accounts for the 120° phase offset between the three lines.
Typical ratings
η ≈ 80-95%, PF ≈ 0.80-0.90
Check the motor nameplate for its exact efficiency and power factor.

Your Results

Calculated
Current Draw
-
Full-load current in amps (A)
Mechanical Output Power
-
HP × 746 W/HP
Electrical Input Power
-
Output power ÷ motor efficiency
Apparent Power
-
Input power ÷ power factor (VA)

Ready

Enter horsepower, voltage, supply type, efficiency, and power factor, then press Calculate.

About the HP to Amps Calculator

This tool converts a motor's horsepower rating into the electrical current (amperage) it draws from its power supply. It uses the standard electrical engineering relationship between mechanical power, electrical power, voltage, and current, and covers the three common supply types: DC, single-phase AC, and three-phase AC.

Understanding the formula

Every version of the formula starts by converting horsepower to watts using 1 HP = 746 W (the standard NEMA/NEC electrical conversion). From there, the current formula depends on supply type: for DC, I = (HP × 746) / (V × η); for single-phase AC, I = (HP × 746) / (V × η × PF); and for three-phase AC, I = (HP × 746) / (V × η × PF × √3). Here V is the supply voltage, η (eta) is motor efficiency written as a decimal, and PF is the power factor. For example, a 5 HP single-phase motor running at 230V, 85% efficient with a 0.85 power factor, draws about 22.4 A.

Efficiency, power factor, and phase type

Efficiency (η) accounts for the energy a motor loses to heat and friction — it is always less than 100%, typically 80-95% for common induction motors, and is printed on the nameplate. Power factor (PF) applies only to AC circuits: it measures how much of the current actually does mechanical work versus how much is reactive current that the source still has to supply, typically 0.80-0.90 for induction motors near full load. DC circuits have no reactive component, so PF is effectively 1. The extra √3 (≈ 1.732) in the three-phase formula exists because three-phase power is delivered across three lines that are 120° apart, so per-line current is lower than the single-phase case for the same total power.

Knowing the limits

This calculator reports the theoretical full-load current (FLA) at rated output — a steady-state estimate, not the starting (locked-rotor) current, which for AC induction motors can be 5 to 8 times higher for a brief moment at startup. Actual current also shifts with voltage fluctuations, motor loading below rated capacity, and manufacturing tolerances. Never use the raw result here to size wiring or breakers directly; the National Electrical Code (NEC 430.22 and related tables) generally requires a safety margin, commonly 125% of FLA for a single continuous-duty motor, and you should always check the motor's nameplate FLA when it is available.

Frequently Asked Questions

What is the formula to convert horsepower to amps?
It depends on the circuit type. For DC: I = (HP × 746) / (V × η). For single-phase AC: I = (HP × 746) / (V × η × PF). For three-phase AC: I = (HP × 746) / (V × η × PF × √3), where V is voltage, η is motor efficiency as a decimal, and PF is power factor. For example, a 5 HP single-phase motor at 230V, 85% efficient with a 0.85 power factor draws about 22.4 A.
Why is 746 used as the conversion factor from horsepower to watts?
One mechanical (electrical) horsepower equals 746 watts by definition (745.7 W more precisely), the standard conversion used throughout NEMA and NEC motor calculations. Multiplying horsepower by 746 converts the motor's rated shaft output into watts before working out how much current is needed to supply it.
How do motor efficiency and power factor change the current draw?
Both sit in the denominator of the formula, so lower values mean more current for the same horsepower output. No motor is 100% efficient — some input energy is lost as heat and friction — and AC motors also draw reactive current that does no mechanical work, measured by the power factor. A less efficient motor with a lower power factor draws noticeably more amps than an ideal one at the same horsepower rating.
Can I use the calculated current to size wiring or a circuit breaker?
Not directly. This calculator gives the estimated full-load running current (FLA), not the locked-rotor (starting) current, which can be 5 to 8 times higher for AC induction motors. For conductor and breaker sizing, the National Electrical Code (NEC 430.22 and related tables) generally requires a safety margin — commonly 125% of FLA for a single continuous-duty motor — and you should always check the motor's nameplate FLA when available.

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