Amps come from watts and volts — and, on AC, the power factor. Divide the power by the voltage (and by the power factor) and you have the current a load draws. The only wrinkle is that single-phase, three-phase and DC each use a slightly different version of that same division. Here are all three.
The three formulas
| Supply | Amps = |
|---|---|
| DC | watts ÷ volts |
| Single-phase AC | watts ÷ (volts × PF) |
| Three-phase AC | watts ÷ (√3 × volts × PF) |
Example: a 3.5 kW (3,500 W) single-phase load at 240 V (230 V) and a 0.9 power factor draws 3,500 ÷ (240 × 0.9) ≈ 16.2 A. The same 3.5 kW on DC at 24 V draws 3,500 ÷ 24 = 146 A — which is why battery-side cables are so fat.
kW to Amps Calculator
Enter kW, supply type and voltage for the current, apparent power in kVA and a breaker suggestion.
What power factor should I use?
Power factor is how “in step” the current is with the voltage — the ratio of real power (kW) to apparent power (kVA). Resistive loads like heaters and kettles are 1.0; motors, transformers and older lighting run 0.7–0.9. If you don’t know it, 0.9 is a reasonable general figure, and 0.8 for motor-heavy loads.
Why three-phase draws fewer amps
The load is spread across three conductors, and the √3 (about 1.73) in the formula reflects that. A 10 kW load at 0.9 PF draws about 46 A single-phase at 240 V but only about 13 A per phase at 480 V three-phase (230/400 V elsewhere). Less current means thinner cable and smaller breakers — which is exactly why big loads go on three-phase.
Is this the current I size cable and breaker for?
It is the starting point. The figure here is the full-load running current; wires and breakers are then chosen to carry at least that, with allowances for continuous running, grouping and installation method. Motors briefly draw 6–8× this on start-up, which matters for breaker and generator choice but not for wire sizing (the surge lasts under a second). Feed this current into the breaker and wire-size calculators to finish the job.
Working the other way
To go from amps back to kW, just multiply instead of divide: kW = amps × volts × PF ÷ 1,000 (single-phase). A 16 A single-phase load at 240 V and 0.9 PF is about 3.5 kW — the example above, run in reverse. The calculator converts in both directions.