Three-Phase Power Calculator
Three-phase power has three faces: real power (kW, the work done), reactive power (kVAR, the magnetising energy sloshing back and forth) and apparent power (kVA, what the supply must actually deliver). Enter line voltage, line current and power factor to get all three — or enter kW and power factor to find the line current a load will draw.
Formulas: S (kVA) = √3 × V × I ÷ 1000 · P (kW) = S × pf · Q (kVAR) = √(S² − P²) · I = P × 1000 ÷ (√3 × V × pf)
Worked example: a 400 V motor drawing 100 A at 0.85 power factor: S = 1.732 × 400 × 100 ÷ 1000 ≈ 69.3 kVA, P ≈ 58.9 kW, Q ≈ 36.5 kVAR. The supply, cables and transformer must be sized for the 69.3 kVA, not just the 58.9 kW — that gap is why power factor correction saves real money on large installations.
For the transformer side of the same numbers — full-load current from a kVA rating — use the transformer calculators.
Frequently asked questions
What is the difference between kW and kVA?
kW is real power — the energy actually turned into work or heat. kVA is apparent power — the total current the supply must carry, including reactive current. They are equal only at unity power factor.
What is a typical power factor?
Resistive heaters are near 1.0. Induction motors typically run 0.8–0.9 at full load and much lower at light load. Poor power factor means bigger cables and transformers for the same useful work.
Does this assume a balanced load?
Yes — the √3 formulas assume all three phases carry equal current. For unbalanced loads, calculate each phase separately and add the powers.