Work out the line current an apparent-power rating draws, on either a single-phase or three-phase supply.
If you have the supply rating rather than the current, start with the kVA to kW calculator.
Every diagram and table on this page updates with the calculator above.
Two formulas, one difference: three-phase divides by an extra √3.
√3 ≈ 1.732, and V is the line-to-line voltage.
Current follows apparent power. That is precisely why cables and breakers are sized from kVA and not from kW.
Three-phase supplies carry three voltage waves spaced 120° apart. Because the peaks do not coincide, the line-to-line voltage is √3 times the line-to-neutral voltage, and the system delivers √3 times more power for the same line current.
The practical consequence is that a three-phase feeder needs only 58% of the current a single-phase one would for the same kVA — which is why industry runs on three phases.
Use the formula above with the actual supply voltage and phase configuration.
Grouped cables, high ambient temperature, thermal insulation and buried runs all reduce capacity — sometimes by half.
Choose a cross-section whose derated capacity exceeds the design current.
On long runs the drop limit, not the current limit, usually decides the size.
Confirm earth fault loop impedance allows the protective device to operate in time.
| Design current | Typical breaker | Indicative copper CSA |
|---|---|---|
| Up to 16 A | 16 A | 1.5 – 2.5 mm² |
| 16 – 25 A | 25 A | 4 mm² |
| 25 – 40 A | 40 A | 6 – 10 mm² |
| 40 – 63 A | 63 A | 16 mm² |
| 63 – 100 A | 100 A | 25 – 35 mm² |
| 100 – 160 A | 160 A | 50 – 70 mm² |
Indicative only — always size from the installation method and the applicable wiring standard.
Standard ratings at your selected voltage and supply type. Switch between single and three phase to watch the current fall.
| kVA | Line current (A) |
|---|---|
| 5 kVA | 7.2 |
| 10 kVA | 14.4 |
| 15 kVA | 21.7 |
| 20 kVA | 28.9 |
| 25 kVA | 36.1 |
| 30 kVA | 43.3 |
| 50 kVA | 72.2 |
| 75 kVA | 108.3 |
| 100 kVA | 144.3 |
| 150 kVA | 216.5 |
| 200 kVA | 288.7 |
| 300 kVA | 433 |
| 500 kVA | 721.7 |
| 750 kVA | 1,082.5 |
| 1,000 kVA | 1,443.4 |
The grid recalculates for whichever supply type is selected in the calculator, so the same table answers both the single-phase and three-phase question.
| kVA | 208 V | 230 V | 400 V | 415 V | 480 V |
|---|---|---|---|---|---|
| 10 kVA | 28 | 25 | 14 | 14 | 12 |
| 25 kVA | 69 | 63 | 36 | 35 | 30 |
| 50 kVA | 139 | 126 | 72 | 70 | 60 |
| 100 kVA | 278 | 251 | 144 | 139 | 120 |
| 200 kVA | 555 | 502 | 289 | 278 | 241 |
| 500 kVA | 1,388 | 1,255 | 722 | 696 | 601 |
| 1,000 kVA | 2,776 | 2,510 | 1,443 | 1,391 | 1,203 |
At fixed voltage, current is a straight line through the origin. Doubling the connected apparent power doubles the current, and with it the heating in every conductor along the route.
Conductor loss is I²R. Doubling the current quadruples the heat — which is why upgrading a feeder often costs far more than the extra load appears to justify.
Using phase-to-neutral voltage with the √3 formula understates the current by a factor of three.
Power factor belongs in the kW calculation, not the current one. Including it here undersizes the cable.
A cable in a grouped, hot, insulated run can lose 40% or more of its tabulated capacity.
Select the next standard device above the design current, never the one below it.
29 calculators built on the same maths as the tool above. Each one has its own inputs, interactive diagrams and worked examples.
Common questions about converting kva to amps.