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Motor Amps and Load Calculator

Estimate a motor’s input power and current from its nameplate, or find how heavily it is loaded from measured volts, amps and power factor.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system
Supply

Result

Estimated full-load current
29.6A
Input power at full load
20.05kW

About 29.6 A at full load (20.05 kW input).

  • This is an estimate from efficiency and power factor. For wire, breaker and overload sizing, use the nameplate full-load amps and the electrical code that applies.
Show the working
  1. Input power = hp × 0.7457 ÷ efficiency
    20.046 kW
  2. Current = kW × 1,000 ÷ (V × PF × √3)
    29.6 A

Two calculations

  • From the nameplate: input power at full load = horsepower × 0.7457 ÷ efficiency (kW), and current = input power ÷ (voltage × power factor), × 1/√3 for three-phase.
  • From measurements: input power = voltage × current × power factor × √3 ÷ 1,000 for three-phase, as in the DOE guide to motor load. Dividing by the full-load input power gives the load fraction.

DOE notes that most motors are designed for 50–100 % of rated load, reach maximum efficiency near 75 %, and lose efficiency sharply below about 50 % — so a lightly loaded motor wastes energy.

Safety and sizing

This is an engineering estimate. Conductors, overcurrent protection and overloads are sized from the nameplate full-load amps and the electrical code that applies, not from this calculation. Measurements on live equipment should only be made by qualified people using appropriate protective equipment.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Current at full load from the nameplate
Supply
Three-phase
Nameplate horsepower
25 hp
Full-load efficiency
93 %
Voltage (mean line-to-line for three-phase)
460 V
Power factor
0.85

Result

Estimated full-load current
29.6 A
Input power at full load
20.05 kW

About 29.6 A at full load (20.05 kW input).

Assumptions

  • 1 hp = 0.7457 kW (NIST). Measured values are averages of the three phases.

Limitations

  • Efficiency and power factor vary with load; nameplate full-load values are used for the estimate.
  • Not a substitute for nameplate full-load amps or the electrical code when sizing circuits.

Frequently asked questions

Why is my estimate different from the nameplate amps?

Nameplate full-load amps are measured; the estimate depends on the efficiency and power factor you enter, which vary between motors. Use the nameplate value for anything safety-related.

How do I measure power factor?

With a power quality meter or clamp meter that measures it. The DOE guide also describes estimating load from current or from speed (slip).

What load is ideal?

DOE notes that most motors are designed to run at 50–100 % of rated load, with maximum efficiency usually near 75 %, and that efficiency tends to fall sharply below about 50 % load.

Sources

  1. Determining Electric Motor Load and Efficiency — U.S. DOE. Equation 1: three-phase input kW = V × I × PF × √3 ÷ 1,000 (worked example 469.7 V, 37 A, PF 0.763 → 22.9 kW); Equation 2: full-load input kW = hp × 0.7457 ÷ efficiency.

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