Calculate motor full-load current, output power, or voltage for DC, single-phase, and three-phase motors with hp/kW conversion, efficiency, power factor, and start-current context.
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Motor Current Formula
This calculator relates mechanical shaft output to supply voltage, line current, efficiency and power factor at the same operating load.
For balanced three-phase AC, k = √3, V is RMS line-to-line voltage and I is RMS line current. For single-phase AC, k = 1. For DC, k = 1 and PF = 1. Efficiency η is entered as a percentage and divided by 100.
Current: I = Pout / (k × V × PF × η). Voltage: V = Pout / (k × I × PF × η). Output power: Pout = k × V × I × PF × η. Use watts in these equations.
Select the value to find and the motor supply, then enter the remaining quantities. Replace the default 90% efficiency and 0.85 AC power factor with values for your motor and load. The starting-current field is optional; blank omits the estimate.
Motor Power and Unit Reference
| Quantity | Relationship |
|---|---|
| Mechanical horsepower | 1 hp ≈ 745.699872 W |
| Kilowatt | 1 kW = 1000 W |
| Kilovolt | 1 kV = 1000 V |
| Milliampere | 1000 mA = 1 A |
| Electrical input power | Pin = Pout / η |
| Motor losses | Pin − Pout |
| AC apparent power | S = k × V × I, in VA |
Example Calculations
Example 1: Three-phase motor current
For 10 mechanical hp at 400 V line-to-line, 90% efficiency and 0.85 power factor: I = (10 × 745.699872) / (√3 × 400 × 0.85 × 0.90) ≈ 14.069609 A.
Example 2: DC mechanical output
At 100 V, 10 A and 90% efficiency, Pout = 100 × 10 × 0.90 = 900 W. Input is 1000 W and estimated motor loss is 100 W.
FAQ
Is this the motor nameplate current?
It is a running-condition estimate, not a substitute for nameplate or measured current. Efficiency and power factor vary with load. The three-phase model assumes balanced conditions and does not model drive waveforms or phase imbalance.
Does it predict startup current?
No. An entered starting/running-current multiple gives a separate arithmetic estimate. Use manufacturer locked-rotor and starting data; the steady-power equation does not predict inrush.
Can I use this to size cables or breakers?
No. Cable, fuse, breaker and overload selection requires the applicable electrical rules, motor data and installation conditions. For DC, include all electrical input within the same efficiency boundary; separate field or drive power is otherwise omitted.
Formula references: U.S. Department of Energy motor guide; ABB motor nameplate guide; NIST conversion factors.
