How do I calculate three-phase motor current?
Convert shaft power to electrical input using efficiency, then divide by √3 × line voltage × power factor.
MAXScanner electrical calculator
Motor Current Calculator — kW/HP to Full-Load Amps
Estimate single-phase or three-phase AC motor full-load current from shaft power, voltage, power factor and efficiency, with input kW, apparent kVA and adjustable starting-current context.
Calculated running current (theoretical)
14.151 A
- Mechanical shaft output
- 7.5 kW
- Electrical input power
- 8.333 kW
- Apparent power
- 9.804 kVA
Substituted calculation
I = (7.5 kW × 1000 W/kW ÷ 0.9) ÷ (√3 × 400 V × 0.85) = 14.151 A
Calculated current is not a code-table FLC or a nameplate value. Use the motor nameplate and applicable electrical standard for final conductor, overload, starter and protective-device selection.
Formula and worked example
3φ: I = Pshaft ÷ (√3 × V × PF × η); 1φ: I = Pshaft ÷ (V × PF × η).
A 7.5 kW three-phase motor at 400 V, 0.85 PF and 90% efficiency draws about 14.15 A at rated shaft output.
What this calculation assumes
- Motor kW or horsepower is treated as mechanical shaft output; electrical input power is higher by the inverse of efficiency.
- The result is an engineering estimate. Motor nameplate current and the applicable electrical standard govern final cable, overload, starter and protective-device selection.
- Starting current is shown only from the multiplier you enter; actual inrush depends on the motor and starting method.
Last updated: 2026-09-16
Frequently asked questions
Why is calculated motor current higher when efficiency is lower?
A less-efficient motor needs more electrical input power to deliver the same shaft output, so line current increases.
Should I size a breaker from this calculated current?
Do not treat this estimate as a final protection setting. Use the motor nameplate and the rules in the electrical standard that applies to the installation.