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kVA calculator
kVA is volts times amps divided by 1,000 for a single-phase load, and volts times amps times 1.732 divided by 1,000 for a three-phase load. A three-phase load drawing 100 A at 480 V is 83.1 kVA. Enter any two of voltage, current and kVA to get the third, the real power in kW, and the transformer size that carries it.
Updated October 12, 2026
The formulas
| You want | Single-phase | Three-phase |
|---|---|---|
| kVA | V x A / 1,000 | V x A x 1.732 / 1,000 |
| Amps | kVA x 1,000 / V | kVA x 1,000 / (V x 1.732) |
| kW | kVA x power factor | kVA x power factor |
1.732 is the square root of 3. Three-phase voltage is line to line (480 V, 208 V), and current is per line. Same formulas as Maddox, Alfa Transformer and Hammond publish.
Two worked examples. A 208 V three-phase load drawing 50 A: 208 x 50 x 1.732 / 1,000 = 18 kVA. A 240 V single-phase load drawing 50 A: 240 x 50 / 1,000 = 12 kVA.
kVA or kW?
kVA is apparent power: what the wires and the transformer have to carry. kW is real power: what does work. They differ by the power factor. A resistive heater has a power factor of 1, so its kW and kVA match. A motor runs around 0.8 to 0.9 at full load (from TECO and ABB motor data sheets), so 100 kW of motor load needs about 111 to 125 kVA of supply. Transformers and generators are rated in kVA because the current, not the real power, heats them.
Full-load current of standard transformers
| Three-phase | 208 V | 240 V | 480 V | 600 V |
|---|---|---|---|---|
| 15 kVA | 41.6 | 36.1 | 18 | 14.4 |
| 30 kVA | 83.3 | 72.2 | 36.1 | 28.9 |
| 45 kVA | 125 | 108 | 54.1 | 43.3 |
| 75 kVA | 208 | 180 | 90.2 | 72.2 |
| 112.5 kVA | 312 | 271 | 135 | 108 |
| 150 kVA | 416 | 361 | 180 | 144 |
| 225 kVA | 625 | 541 | 271 | 217 |
| 300 kVA | 833 | 722 | 361 | 289 |
| 500 kVA | 1,388 | 1,203 | 601 | 481 |
| 750 kVA | 2,082 | 1,804 | 902 | 722 |
| 1,000 kVA | 2,776 | 2,406 | 1,203 | 962 |
Amps per line at full rated kVA, computed with the formula above. They match Hammond Power Solutions’ full-load current cards. Ratings are the DOE list for low-voltage dry-type transformers (10 CFR 431.196).
| Single-phase | 120 V | 240 V | 277 V | 480 V |
|---|---|---|---|---|
| 1 kVA | 8.33 | 4.17 | 3.61 | 2.08 |
| 1.5 kVA | 12.5 | 6.25 | 5.42 | 3.13 |
| 2 kVA | 16.7 | 8.33 | 7.22 | 4.17 |
| 3 kVA | 25 | 12.5 | 10.8 | 6.25 |
| 5 kVA | 41.7 | 20.8 | 18.1 | 10.4 |
| 7.5 kVA | 62.5 | 31.3 | 27.1 | 15.6 |
| 10 kVA | 83.3 | 41.7 | 36.1 | 20.8 |
| 15 kVA | 125 | 62.5 | 54.2 | 31.3 |
| 25 kVA | 208 | 104 | 90.3 | 52.1 |
| 37.5 kVA | 313 | 156 | 135 | 78.1 |
| 50 kVA | 417 | 208 | 181 | 104 |
| 75 kVA | 625 | 313 | 271 | 156 |
| 100 kVA | 833 | 417 | 361 | 208 |
| 167 kVA | 1,392 | 696 | 603 | 348 |
| 250 kVA | 2,083 | 1,042 | 903 | 521 |
| 333 kVA | 2,775 | 1,388 | 1,202 | 694 |
Single-phase full-load amps, computed and matching Hammond’s card where it lists the voltage.
From kVA to a transformer
Work out the load in kVA with the calculator.
Divide by 0.8 if you want the transformer to run at no more than 80% of its rating. Maddox and Alfa Transformer both recommend that margin.
Round up to the next standard size. An 18 kVA load is 22.5 kVA after the margin, so a 30 kVA unit.
For several loads, motors, growth and harmonics, use the transformer sizing calculator. For motors rated in horsepower, the hp to amps calculator gives the current first.
Common mistakes
Using phase-to-neutral voltage in the three-phase formula. On a 208Y/120 system the formula takes 208, not 120.
Multiplying by 3 instead of 1.732. The factor is the square root of 3 when voltage is line to line.
Treating kW as kVA. Divide kW by the power factor before choosing a transformer.
Sizing to the load with no margin. A transformer at 100% of its rating all day has no room for a new machine.
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