Three-phase power explained
The three power quantities
Apparent power S is measured in kVA, active power P in kW, and reactive power magnitude |Q| in kvar. For a balanced sinusoidal three-phase circuit, S = √3 × U × I / 1000, using line-to-line voltage U and line current I.
Formulas
P = S × PF; |Q| = S × √(1 − PF²). Power factor must be between zero and one. The sign of reactive power depends on whether the load is inductive or capacitive.
Worked example
At 400 V, 16 A and power factor 0.85: S ≈ 11.09 kVA, P ≈ 9.42 kW and |Q| ≈ 5.84 kvar.
Important limitations
The equations assume a balanced system and sinusoidal conditions. Harmonics, unbalanced loads and non-sinusoidal waveforms require more detailed analysis.