Electrical

Motor HP → Amps

Estimate motor current from horsepower, voltage, phase, power factor, and efficiency.

Select the motor electrical configuration.
Rated mechanical output horsepower.
Enter the motor line voltage.
Typical motor values are less than 1.0.
Motor efficiency expressed as a percentage.

Estimated Motor Current

Field Note: This calculator provides an estimated running current. For actual equipment selection, overload settings, conductor sizing, and protection, use the motor nameplate data and applicable electrical requirements.

Motor Current Formula

Motor horsepower represents mechanical output power. The electrical input power must account for motor efficiency and power factor.

Three-Phase Motor

Current = (HP × 746) ÷ (√3 × Voltage × Power Factor × Efficiency)

Single-Phase Motor

Current = (HP × 746) ÷ (Voltage × Power Factor × Efficiency)

Efficiency is entered as a decimal in the calculation. For example, 90% efficiency is entered as 0.90.

Example

Consider a 10 HP, 480 V, three-phase motor with a power factor of 0.85 and efficiency of 90%.

I = (10 × 746) ÷ (√3 × 480 × 0.85 × 0.90)

Estimated current: ≈ 11.8 A

Motor Current Troubleshooting

Actual current is higher than the estimate

Motor current can differ from a theoretical calculation because actual motor efficiency, power factor, loading, voltage, temperature, and motor design vary.

Current is high under normal operating conditions

Check the motor load, supply voltage, phase balance, mechanical condition, motor connections, and nameplate information. Compare measured current with the motor's rated current.

One phase has significantly different current

On a three-phase motor, investigate voltage imbalance, connections, conductors, contactors, overload devices, winding condition, and unequal loading.

Common Mistakes

  • Using horsepower without accounting for motor efficiency.
  • Forgetting the √3 factor for a three-phase motor.
  • Assuming power factor is exactly 1.0.
  • Treating calculated current as the motor nameplate FLA.
  • Using the calculator for protection or conductor sizing without checking the applicable electrical requirements.
  • Ignoring actual motor loading and supply voltage.

Related Calculators

Engineering Note

Motor current depends on more than horsepower and voltage. Actual current is affected by motor efficiency, power factor, operating load, supply voltage, motor design, and operating conditions.

Use this calculator for estimation and engineering reference. For actual equipment selection, protection, overload settings, and conductor sizing, verify the motor nameplate data and the requirements applicable to the installation.