Calculate motor load percentage from measured amps or full-load current from hp or kW, with single-phase, three-phase and DC formulas, efficiency, power factor, startup, and energy cost.

Estimate shaft output from measured current, then compare it with rated shaft output.

Efficiency 90% and AC power factor 0.85 are assumed defaults. Use values at the operating load; for full-load current use full-load values. Balanced three-phase voltage is line-to-line.

Motor group, startup and energy (optional)

Blank uses one motor. Group estimates assume identical simultaneous operation.

Blank omits starting current. Applies to calculated full-load current, not measured part-load current.

Blank omits daily energy and cost. Entered hours apply while this section is closed.

Blank omits cost. Requires operating hours. Flat energy charge only.

Motor Load Formula

This calculator estimates mechanical shaft output from electrical input using the entered operating efficiency, then compares it with rated shaft output.

L = Pshaft / Prated × 100
  • Where L is the motor load (%)
  • Pshaft is estimated mechanical shaft output (kW)
  • Prated is nameplate rated mechanical shaft output (kW)

Select motor load or full-load current and the supply type. For load, electrical input = phase factor × V × I × PF; shaft output = electrical input × efficiency. Use consistent operating-point values. Balanced three-phase uses √3 and RMS line-to-line voltage; single-phase uses 1; DC also uses PF = 1. Divide shaft output by rated shaft output and multiply by 100.

Motor Load Definition

Motor shaft load is mechanical output divided by rated mechanical output. Operating power factor and efficiency can vary with load. Current alone is a poor AC load indicator at light load; use suitable measured or manufacturer data. Optional energy estimates use electrical input kW × identical motor count × daily hours; cost uses the entered flat USD/kWh rate. Blank hours or rate omit their respective outputs.

Motor Load Example Problem

How to calculate motor shaft load?

  1. Choose the motor and supply.

    For this example use DC, 100 V, rated output 1 kW and efficiency 100%.

  2. Enter measured running current.

    Use 5 A. The electrical input is 100 × 5 = 500 W and shaft output is 500 W.

  3. Calculate shaft load.

    Load = 0.5 kW ÷ 1 kW × 100 = 50%. The 100% efficiency is an idealized arithmetic example, not a typical motor assumption.

FAQ

What is motor load?

Motor load compares estimated mechanical shaft output with rated output under the motor’s specified operating conditions. Rated power is not a universal instantaneous maximum.

Can the motor load be greater than 100%?

A calculated load above 100% indicates output above the entered rating. It does not establish that operation is safe. Follow nameplate, manufacturer and applicable protection requirements; service factor and thermal limits are not calculated.

What happens when motor load increases?

An increase in motor load will cause a motor to run at a higher power and likely a higher temperature as more energy is going into the motor.

Motor Load Calculator screenshot