Electrical Engineering

Motor Current Calculator

Estimate AC motor full-load current from mechanical rated output, operating voltage, efficiency and power factor. Compare single-phase and three-phase calculations and see the associated electrical input power.

Your estimate

Current (A): 10.377 A
Active input power: 6.111 kW
Apparent power: 7.19 kVA
Mechanical output power: 5.5 kW

Estimated full-load current, not starting current. Use motor nameplate current and manufacturer data for protection and equipment selection. This is not a breaker, overload, cable-sizing or compliance tool. Consult a qualified professional and applicable local codes.

How to calculate motor current

First convert the mechanical rating to watts: kW × 1,000 or mechanical HP × 745.7. Electrical input power is mechanical output divided by efficiency expressed as a fraction. Apparent power is electrical input divided by power factor.

For single-phase operation, I = Pout / (V × PF × η). For balanced three-phase operation, I = Pout / (√3 × V × PF × η). Pout is watts and η is efficiency / 100. Use the motor’s line-to-line supply voltage for the three-phase formula.

Efficiency and power factor affect different quantities

Efficiency compares mechanical shaft output to real electrical input. A 5.5 kW motor at 90% efficiency requires approximately 6.111 kW at its electrical terminals. The remaining input is converted into losses such as heat.

Power factor relates real input power to apparent power. With lower PF, the same real input requires more current. Do not use efficiency in place of power factor or apply either correction twice. Both inputs vary with load and motor design.

kW to amps and HP to amps

There is no single fixed conversion from motor kW or HP to amperes. Supply voltage, phase arrangement, power factor and efficiency must all be specified. This tool uses mechanical horsepower at approximately 745.7 W per HP, not metric horsepower.

At equal mechanical output, voltage, efficiency and PF, the single-phase formula produces √3 times the three-phase line current. Real single-phase and three-phase motors can have different efficiencies and power factors, so use ratings for the specific machine.

Why nameplate current can differ

A motor nameplate describes a particular rated voltage, frequency, load and operating condition. Manufacturing tolerances, winding connection, temperature and the actual supply can change measured current. Part-load PF and efficiency often differ from full-load values.

These are engineering estimates of steady full-load current, not locked-rotor or startup current. Use nameplate current and manufacturer data for protection, overload settings, contactors and cable selection. Local codes and standards vary; consult a qualified professional for safety-critical decisions.

Related power planning

Use the three-phase power calculator when electrical input is known and the voltage drop calculator to examine cable resistance effects. The related power consumption tool estimates energy from operating hours. UPS runtime estimates address battery energy; they do not establish whether an inverter can start a motor or tolerate its inrush current.

Examples

0.75 kW motor

Input: Three-phase, 230 V, efficiency 90%, PF 0.85
Result: 2.461 A; 0.833 kW electrical input; 0.980 kVA.

2.2 kW motor

Input: Three-phase, 400 V, efficiency 90%, PF 0.85
Result: 4.151 A; 2.444 kW electrical input; 2.876 kVA.

5.5 kW motor

Input: Three-phase, 400 V, efficiency 90%, PF 0.85
Result: 10.377 A; 6.111 kW electrical input; 7.190 kVA.

11 kW motor

Input: Three-phase, 480 V, efficiency 90%, PF 0.85
Result: 17.295 A; 12.222 kW electrical input; 14.379 kVA.

22 kW motor

Input: Three-phase, 415 V, efficiency 90%, PF 0.85
Result: 40.009 A; 24.444 kW electrical input; 28.758 kVA.

Frequently asked questions

How do I convert motor kW to amps?

Convert shaft output kW to watts, then divide by voltage, power factor and efficiency. Also divide by √3 for balanced three-phase line current.

How do I convert HP to amps?

Multiply mechanical HP by approximately 745.7 to obtain watts, then use the appropriate phase formula. This tool does not use metric horsepower.

Can I use the result to choose a circuit breaker?

No. Breaker selection depends on applicable codes, motor starting behavior and manufacturer data. This estimated current is not an authoritative protection setting.

What happens if efficiency falls?

For fixed shaft output, lower efficiency increases the electrical input required and therefore increases the estimated current.

Does this include motor starting current?

No. It estimates steady full-load current. Starting current depends on the motor, starter, load and supply.

Why should I prefer nameplate current?

It reflects the manufacturer’s rated operating conditions for that motor. A generic calculation cannot capture all design and operating details.

Last reviewed: September 23, 2026