Motor Full Load Current Calculator

Divide the rated output by √3, the line voltage, the efficiency and the power factor: I = P × 1000 / (√3 × V × η × cos φ). An 11 kW motor at 400 V with 89% efficiency and a power factor of 0.85 draws about 21 A.

A motor's kW rating is its mechanical output at the shaft. The supply has to provide that output plus the motor's losses, so the electrical input is the output divided by the efficiency. Leaving the efficiency out underestimates the current by 5 to 20%.

Use the nameplate full load current when you have the motor, because it reflects that motor's actual efficiency and power factor. The calculation and the typical table values are for design before the motor is chosen.

They are the rated operational currents IEC 60947-4-1 Annex G gives for standard three-phase cage induction motors, as reproduced in the Schneider Electric Electrical Installation Guide. They are guidance for selecting switchgear, not a guarantee for any particular motor.

A three-phase motor at 400 V draws roughly 2 A per kW at full load around 10 to 20 kW, nearer 1.7 A per kW for large motors, and 2.5 A per kW or more for small motors, which have lower efficiency and power factor.

Cables are sized for the full load current, with the voltage drop during starting checked separately. Direct-on-line starting draws about 6 to 8 times full load current for a few seconds, which the protection must ride through.