Formula
DC: I = 1000 × P (kW) ÷ V
AC single phase: I = 1000 × P ÷ (V × PF)
AC three phase: I = 1000 × P ÷ (√3 × VL-L × PF)
Worked example
A 5 kW three-phase load at 400 V with PF 0.8: I = 5000 ÷ (1.732 × 400 × 0.8) = 9.02 A per line.
Reference table
Quick reference (PF = 0.8)
| kW | Amps @ 230 V (1φ) | Amps @ 400 V (3φ) |
|---|---|---|
| 1 | 5.43 | 1.8 |
| 3 | 16.3 | 5.41 |
| 5 | 27.17 | 9.02 |
| 10 | 54.35 | 18.04 |
| 25 | 135.87 | 45.11 |
| 50 | 271.74 | 90.21 |
Where this shows up in the real world
Spec sheets for EV chargers, heat-pump units and industrial machines lead with kW, while every installation decision — wire gauge, breaker, disconnect — needs amps. A 7.2 kW Level 2 EV charger on a US 240 V circuit draws 30 A, which is why those installs land on 40 A breakers under the 80% continuous rule.
Common mistakes to avoid
Don't carry a single-phase habit into three-phase work or vice versa — the √3 changes everything, and the most expensive wrong answers come from using the wrong system's formula. And cable sizing isn't finished at the calculated amps: ambient temperature, conductor bundling and run length all derate capacity under NEC 310 or BS 7671.
Frequently asked questions
Why does a lower power factor mean more amps?
Watts measure useful power, but conductors carry the full apparent current. At PF 0.8, the circuit draws 25% more current than the same kW at PF 1.0.
Can I size a cable straight from this?
Use it for the design current, then apply your local code's derating factors (ambient temperature, grouping, installation method) before choosing cable size.
kW or kVA for motor circuits?
Motor nameplates usually give kW output; the input current also depends on efficiency and PF — see our HP to amps tool for the motor-specific version.