How it works
For a balanced three-phase load, apparent power is √3 × line voltage × line current. Real power, the part that does work, is that times the power factor. Reactive power is the rest, which circulates without doing useful work.
To size cables and breakers from a motor's output rating, divide by both power factor and efficiency to get the electrical input current.
S (VA) = √3 × V_L × I_L
P (W) = S × PF
Q (VAR) = S × sin(arccos PF)
I = P_out / (√3 × V_L × PF × η)Worked example
A load on a 415 V supply draws 20 A per line at power factor 0.85. Apparent power is √3 × 415 × 20 = 14.38 kVA, real power 12.22 kW and reactive power 7.57 kVAR.