kVA Calculator
Convert between kVA, kW and amps for single-phase and three-phase power, and size a transformer for a load with headroom. Pick single or three phase and the voltage; the sizing mode gives the next standard size and its full-load amps.
Your numbers
Three phase uses the line-to-line voltage (208, 480).
From the nameplate or load data, between 0 and 1. 0.8 is only a starting value; resistive heaters are 1.
Amps
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- kW
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- Reactive power
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Standard sizes from the kVA ratings in the US Department of Energy efficiency tables for distribution transformers (10 CFR 431.196), plus the small dry-type sizes on Hammond Power Solutions' current cards. Makers also build sizes outside that list.
Size a real installation with the load calculation, the motor starting current, harmonics and the NEC; this page gives the arithmetic.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
kVA to amps and amps to kVA
Apparent power in kVA is volts times amps divided by 1,000. On single phase that is the whole formula: kVA = V × A ÷ 1,000, and amps = kVA × 1,000 ÷ V. On a balanced three-phase system multiply by √3 (about 1.732) and use the line-to-line voltage: kVA = 1.732 × V × A ÷ 1,000, and amps = kVA × 1,000 ÷ (1.732 × V).
Use the voltage the current flows at. A 480 V to 208Y/120 V transformer carries 90.2 A on the 480 V side at 75 kVA and 208.2 A on the 208 V side, the same kVA on both. 277 V and 120 V are line-to-neutral voltages: pick single phase for a load connected that way.
kW to kVA and the power factor
kW is the real power a load uses; kVA is what the supply has to deliver, including the reactive current that motors, transformers and many drives draw. The ratio between them is the power factor: kW = kVA × PF and kVA = kW ÷ PF. The reactive part is kvar = √(kVA² − kW²).
Transformers are rated in kVA because their windings heat up with the current, whatever the power factor. Take the power factor from the nameplate or the load data. The calculator starts at 0.8 as a placeholder only; a resistive heater is 1.
Sizing a transformer
Add up the load in kVA, add headroom for growth and for loads you are not sure of, then take the next standard rating up. The headroom is a planning margin you set; the page starts at 25%. The result shows how loaded that size will be and its full-load current at your voltage, which is what the conductors and the protection are sized from.
The standard sizes on the English page are the kVA ratings in the Department of Energy efficiency tables for distribution transformers (10 CFR 431.196): single phase 10 to 833 kVA, three phase 15 to 2,500 kVA, plus the smaller dry-type sizes on Hammond Power Solutions' current cards. A real installation also has to account for motor starting current, harmonics from drives and electronics, ambient temperature and the NEC rules for transformer protection.
Three-phase transformer full-load amps
Standard three-phase sizes and the current each carries at full load: kVA × 1,000 ÷ (1.732 × volts).
| Size | 208 V | 480 V |
|---|---|---|
| 15 kVA | 41.6 A | 18 A |
| 30 kVA | 83.3 A | 36.1 A |
| 45 kVA | 124.9 A | 54.1 A |
| 75 kVA | 208.2 A | 90.2 A |
| 112.5 kVA | 312.3 A | 135.3 A |
| 150 kVA | 416.4 A | 180.4 A |
| 225 kVA | 624.5 A | 270.6 A |
| 300 kVA | 832.7 A | 360.8 A |
| 500 kVA | 1,387.9 A | 601.4 A |
| 750 kVA | 2,081.8 A | 902.1 A |
| 1,000 kVA | 2,775.7 A | 1,202.8 A |
Single-phase transformer full-load amps
Standard single-phase sizes at 120 V and 240 V: kVA × 1,000 ÷ volts.
| Size | 120 V | 240 V |
|---|---|---|
| 5 kVA | 41.7 A | 20.8 A |
| 10 kVA | 83.3 A | 41.7 A |
| 15 kVA | 125 A | 62.5 A |
| 25 kVA | 208.3 A | 104.2 A |
| 37.5 kVA | 312.5 A | 156.3 A |
| 50 kVA | 416.7 A | 208.3 A |
| 75 kVA | 625 A | 312.5 A |
| 100 kVA | 833.3 A | 416.7 A |
| 167 kVA | 1,391.7 A | 695.8 A |
For electrical and industrial sales engineers
A transformer or generator RFQ usually arrives as a conversation: the customer reads volts, amps and nameplate kW off a panel, mentions a machine they are adding next year and asks what size you would quote. Write down every load with its voltage, phase and power factor, and the growth they expect, before you size anything.
Sources, each read on 2026-10-30. Results are arithmetic for planning; the final design follows the load calculation, the maker's data and the NEC:
- Hammond Power Solutions, Full load currents, single and three phase dry-type transformers (formulas and amps by kVA and voltage)
- eCFR, 10 CFR 431.196: energy conservation standards for distribution transformers (the kVA ratings in the tables)
Related: the Ohm's law calculator for volts, amps, ohms and watts, the generator wattage calculator to size a generator from the appliances, and the voltage drop calculator for the feeder from the transformer.
More for electrical and industrial sales: Sales engineer, Electrical distributor sales, Selling to engineers.
The load list comes from the customer
Volts, amps, nameplate kW and the machine they plan to add next year come up on the call or the site visit, often faster than you can type. Record it in ParrotNotes and you get a transcript, a summary and the action items, so the sizing and the quote use the numbers the customer gave.
Get Started FreeRelated tools
- Ohm's Law CalculatorEnter any two of volts, amps, ohms and watts to get the other two with the formula used, plus amps to watts and kVA to amps for single and three phase.Open the tool
- Generator Wattage CalculatorAdd up the running and starting watts of the appliances you want to power to size a portable or home standby generator, with a margin.Open the tool
- Voltage Drop CalculatorFind the voltage drop and percent drop on a copper or aluminum wire run by AWG size, length, load and phase, against the 3% and 5% guides.Open the tool