The formulas
Use line-to-line voltage for three-phase systems (480 V, 208 V, 600 V), not line-to-neutral.
kW versus kVA
kVA (apparent power) is what wiring, breakers and transformers have to carry. kW (real power) is what does work and what the utility bills as energy. The ratio between them is the power factor. A load with a PF of 0.8 needs 25% more kVA, and therefore more current, than its kW suggests.
Worked example
A 50 hp motor on 460 V three-phase, 93% efficient, power factor 0.85:
For conductor and overcurrent sizing, the NEC requires motor full-load current from Table 430.250 (65 A for this motor) rather than a calculated value. Use this result for load studies and generator sizing.
Frequently asked questions
- How many amps is 100 kW at 480 V three-phase?
- At unity power factor, 100,000 ÷ (1.732 × 480) = 120 A. At PF 0.85 it's 141 A.
- What power factor should I use?
- Use measured or nameplate values when you have them. Otherwise 1.0 for resistive heat, about 0.85 for loaded induction motors, and 0.9–0.95 for modern electronic loads with PF correction.
- Is horsepower the input or output of the motor?
- Nameplate horsepower is mechanical output at the shaft. The calculator divides by efficiency to get electrical input.
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.