Engineering question
How can an engineer estimate three-phase motor current without confusing shaft output, electrical input, power factor and efficiency?
A motor's kW rating is usually mechanical shaft output, while the cable and switchgear carry electrical input. Dividing shaft kW by √3 times voltage and power factor therefore understates current unless efficiency is included. The calculated value remains an estimate because actual nameplate current reflects the manufacturer's design, rating basis and applicable standard.
Use the estimate for an early load list or plausibility check. Use the motor nameplate, manufacturer data, starting method, duty, ambient conditions and protection-coordination study for final equipment selection.
Calculation basis
Formulas and units
Electrical input from shaft output
Pin(kW) = Pout(kW) / η
η is motor efficiency as a decimal.
Three-phase line current
I = Pin × 1000 / (√3 × VLL × PF)
VLL is line-to-line RMS voltage and PF is the operating power factor.
Combined shaft-output form
I = Pout × 1000 / (√3 × VLL × PF × η)
Use only when the entered kW is shaft output.
Worked example
Apply the formula
15 kW shaft-output motor, 415 V, power factor 0.85 and efficiency 90%.
- 1Pin = 15/0.90 = 16.667 kW.
- 2Denominator = √3 × 415 × 0.85 ≈ 610.9.
- 3I = 16,667/610.9 ≈ 27.28 A.
Result: Estimated full-load current is 27.3 A. The nameplate value remains the selection authority.
Open Motor Current CalculatorWhat to do with the estimate
- Check the project load list and transformer demand assumptions.
- Compare with the nameplate and OEM data.
- Select cables using installation method, ambient/grouping derating and voltage drop.
- Coordinate overload, short-circuit and earth-fault protection.
- Check VFD or starter input/output ratings using manufacturer guidance.
Starting and variable-speed operation
Direct-on-line starting current can be several times full-load current, but a universal multiplier should not be used for final studies. Star-delta, soft starters and VFDs produce different line-current, motor-current and torque behaviour. VFD input current also depends on drive topology, harmonics and efficiency; use drive documentation rather than treating it as a simple motor-current multiplier.
Common mistakes
- Treating rated shaft kW as electrical input
- Using phase voltage instead of line-line voltage
- Entering 85 instead of 0.85 for PF
- Replacing the nameplate current with a calculated estimate
Troubleshooting checks
- Estimate is low: verify shaft/input basis and efficiency.
- Measured currents are unbalanced: inspect supply voltage, connections, winding and load.
- Current is high on all phases: check load, voltage, frequency, mechanical condition and motor connection.
- VFD display differs from incomer meter: distinguish motor-side current from drive input current.
Assumptions
- Balanced three-phase supply
- Sinusoidal fundamental calculation
- Power factor and efficiency correspond to the operating point
Limitations
- Not a cable, breaker or overload selection result
- Does not calculate starting transients or harmonics
- Part-load efficiency and PF may differ materially from rated values
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