LumaQuote Electrical calculators kVA calculator

kVA Calculator

Work out kVA from volts and amps, or the full-load current of a transformer from its kVA rating. Single-phase and three-phase, with the next standard transformer size.

How to calculate kVA

kVA is volts times amps, divided by 1,000. Three-phase adds the square root of 3 (1.732), with the line-to-line voltage.

Single-phase: kVA = volts × amps ÷ 1,000

Three-phase: kVA = 1.732 × volts × amps ÷ 1,000

Turn it around to get the current a transformer can deliver at full load:

Single-phase: amps = kVA × 1,000 ÷ volts

Three-phase: amps = kVA × 1,000 ÷ (1.732 × volts)

Examples

75 kVA three-phase at 208 V: 75,000 ÷ (1.732 × 208) = 208.2 A on the secondary.
The same 75 kVA at 480 V: 75,000 ÷ (1.732 × 480) = 90.21 A on the primary.
50 kVA single-phase at 240 V: 50,000 ÷ 240 = 208.3 A.

Transformer full-load amps chart

Three-phase transformers

Transformer	208 V	240 V	480 V	600 V
15 kVA	41.64 A	36.08 A	18.04 A	14.43 A
30 kVA	83.27 A	72.17 A	36.08 A	28.87 A
45 kVA	124.9 A	108.3 A	54.13 A	43.30 A
75 kVA	208.2 A	180.4 A	90.21 A	72.17 A
112.5 kVA	312.3 A	270.6 A	135.3 A	108.3 A
150 kVA	416.4 A	360.8 A	180.4 A	144.3 A
225 kVA	624.5 A	541.3 A	270.6 A	216.5 A
300 kVA	832.7 A	721.7 A	360.8 A	288.7 A
500 kVA	1,387.9 A	1,202.8 A	601.4 A	481.1 A
750 kVA	2,081.8 A	1,804.2 A	902.1 A	721.7 A
1,000 kVA	2,775.7 A	2,405.6 A	1,202.8 A	962.3 A

Single-phase transformers

Transformer	120 V	240 V
10 kVA	83.33 A	41.67 A
15 kVA	125.0 A	62.50 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	625.0 A	312.5 A
100 kVA	833.3 A	416.7 A
167 kVA	1,391.7 A	695.8 A

Full-load current at the rated voltage. Overcurrent protection for transformers follows its own rules (NEC Article 450, CEC Section 26), so use those tables to size the breakers.

kVA vs kW

kVA is apparent power, what the transformer and wiring have to carry. kW is the real power the loads use. The link between them is the power factor: kW = kVA × power factor. A 75 kVA transformer feeding loads at power factor 0.9 delivers about 67.5 kW. That is why transformers, generators and UPS units are rated in kVA: they have to carry the current whatever the power factor is.

How to size a transformer

Add up the load in kVA. Use the panel schedule or a load calculation, not just the connected nameplates.
Allow for continuous load and growth. Many designers add 25% so the transformer runs cooler and leaves room for added circuits. The calculator can add it for you.
Pick the next standard size up. Three-phase dry types usually come in 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 kVA.
Check both sides. Size the primary and secondary protection and conductors from the full-load amps on each side.

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KVA calculator FAQ

What is kVA?

Kilovolt-amps: volts times amps, divided by 1,000. It is the apparent power a transformer, generator or UPS has to carry, whatever the power factor of the load.

How many amps is a 75 kVA transformer?

208.2 A at 208 V three-phase and 90.21 A at 480 V three-phase.

How many amps is a 50 kVA single-phase transformer at 240 V?

208.3 A.

How do I convert kVA to kW?

Multiply by the power factor. At power factor 0.9, 100 kVA is 90 kW. At 1.0 they are equal.

What size transformer do I need?

Add up the load in kVA, add about 25% for continuous load and growth, then pick the next standard size. Confirm the overcurrent protection with NEC Article 450 (CEC Section 26 in Canada).

Is three-phase kVA calculated with line-to-line voltage?

Yes. Use the line-to-line voltage, such as 208 or 480 V, with 1.732 in the formula. If you only have line-to-neutral voltage, multiply it by 1.732 first.