Three-phase motor current calculator
Estimate the full-load line current of a three-phase motor from its rated power, supply voltage, power factor and efficiency.
How it is calculated
I = P × 1000 ÷ (√3 × V × PF × η), with P in kW, V line-to-line in volts, PF the power factor and η the efficiency as a fraction.
Worked example
15 kW, 460 V, PF 0.85, η 90 %: I = 15,000 ÷ (1.732 × 460 × 0.85 × 0.90) = 24.61 A.
Steps
- Enter the motor output power from the nameplate.
- Enter the supply voltage and the power factor and efficiency at full load.
- Read the line current and use it to size cable, breaker and overload.
Frequently asked questions
Is this the nameplate FLA?
It is a calculated estimate. Use the nameplate full-load amps for final sizing, since they include the manufacturer test data.
Why does power factor matter?
A lower power factor draws more current for the same kW. Motors at light load have a low PF.
Which current do I use for the breaker?
Size motor branch-circuit conductors for at least 125 % of the full-load current (NEC 430.22), and the protective device per NEC 430.52, or the equivalent rule in your country.
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References
Three-phase power relation P = √3 V I PF η; NEC NFPA 70 Article 430 for motor branch-circuit sizing. Confirm against the local electrical code.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.