What this answers
The label on an appliance says watts; a fuse or a cable is spoken of in amps. They are linked by the voltage and, for anything with a motor or a transformer, the power factor. The calculator converts in either direction, for a single-phase or a three-phase supply, and can solve for the voltage or the power factor instead.
Formula
Single phase: P = U × I × PF
Three phase: P = √3 × U × I × PF
Apparent power: S = U × I (single phase), √3 × U × I (three phase)
- P — real power in watts; S — apparent power in volt-amperes
- U — voltage; on three phase, the voltage between two lines
- I — current in each line; PF — power factor, cos φ
Worked examples
A 2,300 W heater at 230 V. The power factor of a heater is 1: 2,300 / 230 = 10 A.
The same power with a power factor of 0.8. 2,300 / (230 × 0.8) = 12.5 A.
11 kW at 400 V, three phase. 11,000 / (√3 × 400) = 15.88 A in each line.
10 kVA at 230 V. Apparent power ignores the power factor: 10,000 / 230 = 43.5 A.
What this doesn’t cover
- Which cable, fuse or installation is allowed. That is for a qualified electrician.
- Unbalanced three-phase loads, or loads with harmonics.
Common questions
Why is the three-phase formula different?
Three lines share the load, and the voltage between two of them is √3 times the voltage from a line to
neutral. That factor is the difference.