Motor Current Calculator
Estimate full-load current, electrical input power, apparent power and motor losses for DC, single-phase AC or balanced three-phase AC motors.
Motor inputs
How to use the motor current calculator
- Select DC, single-phase AC or balanced three-phase AC.
- Enter the motor’s rated mechanical output power and choose kW, hp or W.
- Enter the supply voltage, expected efficiency and, for AC motors, the running power factor.
- Select Calculate to estimate current and supporting electrical values.
Use rated operating values rather than guessing whenever a motor nameplate or manufacturer data sheet is available. The result is a planning estimate; actual current changes with load, voltage, motor design, temperature and operating condition.
Motor current formulas
Single-phase AC: I = Pout ÷ (V × η × PF)
Three-phase AC: I = Pout ÷ (√3 × VLL × η × PF)
In these formulas, I is current in amperes, Pout is mechanical output power in watts, V is voltage, η is efficiency as a decimal and PF is power factor. For a balanced three-phase system, VLL is the line-to-line voltage. One mechanical horsepower is treated as 745.699872 watts.
Electrical input power equals output power divided by efficiency. For AC systems, apparent power in kVA equals input kW divided by power factor. Estimated motor losses equal electrical input power minus mechanical output power.
Worked example
This Motor Current Calculator example considers a balanced three-phase motor rated at 15 kW output on a 400 V supply. If its efficiency is 90% and its power factor is 0.85, its electrical input is 15 ÷ 0.90 = 16.67 kW. Apparent power is 16.67 ÷ 0.85 = 19.61 kVA. Dividing 15,000 W by √3 × 400 V × 0.90 × 0.85 gives an estimated full-load current of 28.30 A. A simple 125% planning value is 35.38 A, but that value is not a final conductor or protective-device selection.
Reading the results
The Motor Current Calculator reports five values for quick comparison.
| Result | What it means |
|---|---|
| Full-load current | Estimated line current at the entered mechanical load and performance values. |
| Electrical input | Real electrical power required after accounting for efficiency. |
| Apparent power | AC supply capacity associated with real power and power factor. |
| Motor losses | Estimated difference between electrical input and mechanical output. |
| 125% planning current | A preliminary comparison value only, not a code-compliant design by itself. |
Important planning guidance
Use every Motor Current Calculator result as an estimate, not as a final electrical design.
Starting or locked-rotor current can be several times higher than normal running current, so this result must not be used as a starting-current prediction. Cable size, overload protection, short-circuit protection, disconnects and starters depend on local electrical codes, installation method, ambient temperature, conductor material, duty cycle and manufacturer instructions. Consult a qualified electrician or engineer for an installation.
Efficiency and power factor are not interchangeable. Efficiency compares mechanical output with real electrical input. Power factor compares real power with apparent power in an AC circuit. Entering 100% for either value when the real motor operates below that value will understate current.
For general motor-system efficiency resources, see the U.S. Department of Energy motor systems guidance.
Common mistakes to avoid
Check the Motor Current Calculator inputs carefully before relying on the estimate.
- Using phase voltage instead of line-to-line voltage for a three-phase calculation.
- Entering input kW when the selected value is meant to be rated mechanical output.
- Using a percentage such as 85 instead of 0.85 in the power-factor field.
- Treating estimated running current as starting current.
- Selecting a breaker or cable from this estimate without applying the applicable electrical code.
Frequently asked questions
These answers explain the scope of the Motor Current Calculator.
Does this calculate motor starting current?
No. The Motor Current Calculator estimates running current from output power, voltage, efficiency and power factor. Starting current requires motor-specific information and the starting method.
Why is DC power factor hidden?
The basic DC current formula does not use AC power factor. The calculator therefore applies only voltage and efficiency in DC mode.
Should I use kW or horsepower?
Use the unit printed on the motor data. The calculator converts horsepower to watts internally, so equivalent ratings produce equivalent results.
Can I size a breaker from the 125% value?
No. The displayed value is only a quick planning comparison. Final equipment and conductor sizing must follow the applicable code and motor manufacturer instructions.
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Disclaimer: This calculator provides educational planning estimates and does not replace motor nameplate data, manufacturer instructions, electrical codes or professional engineering judgment.