THREE-PHASE / THEORY LAB
Three-phase power.
Know which voltage to use.
For a balanced, sinusoidal three-phase load, use line-to-line voltage and line current in P = √3 × V × I × PF. PF is cos φ in this model. P is real power, S apparent power, and Q reactive-power magnitude.
Three worked examples
400 V, 10 A, PF 0.8
P = √3 × 400 × 10 × 0.8 ≈ 5.543 kW. S ≈ 6.928 kVA. Q ≈ 4.157 kvar.
400 V, 10 A, PF 1
P = S ≈ 6.928 kW / kVA; Q = 0 kvar.
P = 12 kW, V = 400 V, PF 0.8
I = 12000 / (√3 × 400 × 0.8) ≈ 21.651 A.
Line quantities and phase quantities
In a star connection, line voltage is √3 times phase voltage, while line current equals phase current. The alternative formula is P = 3 × Vphase × Iphase × PF. Do not substitute line quantities into this second formula. An unbalanced or distorted load needs another model; this calculator does not select cables, protection or installation ratings.
Schneider Electric / Electrical Installation Guide: power definitions ↗
Try the five foundation questions