The formula — and the efficiency trap
Motor nameplate kW (or HP) is shaft output, not electrical input. The motor draws more from the line than it delivers to the shaft — divide by efficiency to get input power. Skipping η is the most common reason a calculated FLC comes out ~10% under the nameplate value.
Worked example
15 kW, three-phase, 415 V, PF 0.85, η 0.90:
- Electrical input = 15 ÷ 0.90 = 16.67 kW
- I = 16,667 ÷ (1.732 × 415 × 0.85) = 27.3 A
- DOL starting inrush ≈ 6× FLC ≈ 164 A for several seconds
Field notes
- The nameplate wins. This calculation estimates; the manufacturer's tested FLC on the nameplate is the number for protection settings and cable sizing.
- Starting current matters more than FLC for voltage-dip studies: DOL inrush is typically 5–7× FLC. Star-delta and soft starters reduce it. Check the run yields acceptable drop with the cable voltage drop calculator.
- PF and η vary with load and size: small motors ~0.8/0.85, large modern IE3 motors 0.88/0.95. Use datasheet values when you have them, and the three-phase power calculator for the full kW/kVA/kVAR picture.
Frequently asked questions
What is the formula for motor full load current?
Three-phase: I = P ÷ (√3 × V × PF × η), single-phase: I = P ÷ (V × PF × η), where P is shaft power in watts, V voltage, PF power factor and η efficiency. Nameplate kW is shaft output, so efficiency must be included.
How many amps does a 15 kW three-phase motor draw?
At 415 V, PF 0.85 and 90% efficiency: I = 15,000 ÷ (1.732 × 415 × 0.85 × 0.9) ≈ 27.3 A. Always confirm against the nameplate FLC.
How do I convert HP to kW for this calculation?
1 HP = 0.746 kW. A 20 HP motor is 14.92 kW of shaft power.
Why is my measured current lower than the calculated FLC?
FLC applies at full mechanical load. Motors running below rated load draw proportionally less current — and their power factor drops too, so lightly loaded motors look surprisingly reactive.
Provided for reference and education; use nameplate data and the applicable wiring code for installations. See our disclaimer.