How to use the kVA calculator
- Choose what to calculate: kVA from amps, amps from kVA, or convert kW to kVA.
- Pick single-phase or three-phase and enter the voltage. For three-phase, use the line-to-line voltage.
- Enter the power factor to get real power in kW (0.8 is standard for generators).
The formulas
Single-phase: kVA = V × A ÷ 1,000 A = kVA × 1,000 ÷ V Three-phase: kVA = √3 × V × A ÷ 1,000 A = kVA × 1,000 ÷ (√3 × V) Real power: kW = kVA × PF Reactive: kVAR = √(kVA² − kW²)
Worked examples
- 100 A at 480 V three-phase: 1.732 × 480 × 100 ÷ 1,000 = 83.1 kVA → a 100 kVA transformer.
- 50 A at 240 V single-phase: 240 × 50 ÷ 1,000 = 12 kVA.
- 75 kVA transformer, 208 V three-phase: 75,000 ÷ (1.732 × 208) = 208 A.
- 80 kW load at PF 0.8: 80 ÷ 0.8 = 100 kVA, with 60 kVAR of reactive power.
Full-load amps for common transformer sizes
| kVA | 208 V 3-ph | 240 V 3-ph | 480 V 3-ph | 400 V 3-ph |
|---|---|---|---|---|
| 15 | 41.6 A | 36.1 A | 18.0 A | 21.7 A |
| 30 | 83.3 A | 72.2 A | 36.1 A | 43.3 A |
| 45 | 124.9 A | 108.3 A | 54.1 A | 65.0 A |
| 75 | 208.2 A | 180.4 A | 90.2 A | 108.3 A |
| 112.5 | 312.3 A | 270.6 A | 135.3 A | 162.4 A |
| 150 | 416.4 A | 360.8 A | 180.4 A | 216.5 A |
| 300 | 832.7 A | 721.7 A | 360.8 A | 433.0 A |
Understanding power factor
Power factor (PF) is the ratio of real power to apparent power. Purely resistive loads such as heaters and incandescent lamps have a PF of 1.0, so kVA equals kW. Motors, transformers and many electronic power supplies draw extra current that does no useful work, lowering the PF to around 0.8–0.9. A low power factor means larger cables, transformers and generators for the same useful power, and many utilities charge commercial customers a penalty when PF drops below about 0.9. Capacitor banks or drives with active front ends can correct it.
Sizing tips
- Leave headroom: size for 125% of continuous loads, matching NEC practice for conductors and overcurrent protection.
- Motor starting: direct-on-line motors draw 6–8 times their running current briefly; generators and small transformers must handle that inrush.
- Single-phase loads on three-phase supplies: balance them across the phases, otherwise one phase reaches its limit first.
- Check the nameplate: always use the transformer or generator manufacturer’s ratings for final design and have a licensed electrician verify it.
Single-phase vs three-phase
Homes in North America usually have a 120/240 V single-phase service, while commercial buildings use 208 V or 480 V three-phase. In Europe, Asia and Australia, 230 V single-phase and 400 V three-phase are standard. Three-phase power delivers √3 (about 1.73) times as much power for the same current per conductor, which is why larger motors, chillers and industrial equipment use it.