Motor Full Load Current Calculator
Estimate the full load current of an induction motor from its rated output power, voltage, power factor and efficiency. Use it for cable, starter and overload relay selection when the nameplate is not available.
- Input power = output ÷ efficiency = 15 ÷ 0.91 = 16.48 kW
- I = Pin ÷ (√3 × V × pf) = 16,484 ÷ (1.732 × 415 × 0.86) = 26.67 A
Motor Current Formulas
Single phase: I = Pout ÷ (V × cos φ × η)
Pout is the rated shaft output in watts (1 HP = 745.7 W), V the line voltage, cos φ the power factor and η the efficiency at full load.
Worked example
A 15 kW, 415 V three phase motor with pf 0.86 and 91 % efficiency: input = 16.48 kW, I = 16,484 ÷ (1.732 × 415 × 0.86) = 26.7 A.
Frequently Asked Questions
How do I convert kW to amps for a 3 phase motor?
Divide the input power in watts by √3 × voltage × power factor. Input power is the rated output divided by efficiency.
What is the starting current of an induction motor?
With direct on line starting, locked rotor current is typically 6 to 8 times full load current. Star delta starting reduces it to about one third.
How is the overload relay set?
Normally at the motor nameplate full load current, adjusted for service factor if the standard and motor allow it.
Learn more on the blog
Motor starting methods compared
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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