kVA is apparent power, the rating on transformers, generators and UPS units. Converting it to amps gives the full-load current at the rated voltage. Unlike the kW conversion, no power factor is involved, because apparent power already includes it.
Formulas
Three-phase: I = kVA × 1000 / (√3 × VLL)
V is the voltage on the side you are calculating. A transformer has a different full-load current on its primary and secondary, so run the calculation once for each side using that side’s line-to-line voltage.
Worked example
A 100 kVA three-phase transformer with a 400 V secondary:
I = 100,000 / (1.732 × 400) = 144.3 A.
On an 11 kV primary the same transformer draws 100,000 / (1.732 × 11,000) = 5.25 A. The ratio of the two currents equals the inverse of the voltage ratio, which is a quick sanity check on any transformer calculation.
Single-phase example: a 75 kVA single-phase supply at 240 V gives 75,000 / 240 = 312.5 A.
Using the result
Full-load current is the starting point, not the end. Protection on a transformer primary is normally set well above full-load current to ride through magnetizing inrush, and the cable and breaker sizes depend on the installation rules in force. For a US branch circuit or feeder, see the breaker size calculator. Transformer-specific protection and short-circuit calculations are covered in the protection section as it is published.
How to use the calculator
Enter the kVA rating from the nameplate, the voltage on the side you care about, and the phase. The result is the full-load current on that winding. Run it twice for a transformer, once with the primary voltage and once with the secondary voltage.
Common transformer sizes
Full-load current at 400 V three-phase:
- 100 kVA: 144.3 A
- 250 kVA: 360.8 A
- 500 kVA: 721.7 A
- 1,000 kVA: 1,443.4 A
A 1,000 kVA transformer on an 11 kV primary draws 52.5 A, and the ratio of 1,443.4 to 52.5 matches the 11,000 / 400 voltage ratio.
From full-load current to fault current
Full-load current is also the base for a quick estimate of the short-circuit current a transformer can deliver. Divide it by the transformer’s per-unit impedance. A 100 kVA, 400 V transformer with 4% impedance has a full-load current of 144.3 A, so the secondary fault current on an infinite source is about 144.3 / 0.04 = 3,608 A. It is an upper bound, because source and cable impedance reduce it. Breakers on the secondary must have a breaking capacity above it.
Generators and UPS units
A generator’s kVA rating is the apparent power at its rated voltage, but the real power it can supply is the kVA times its rated power factor, typically 0.8. A 100 kVA set therefore supplies 80 kW. Loads with a worse power factor, or with high starting current, can require a larger set than the kW figure alone suggests.
Common mistakes
- Dividing by power factor. kVA already contains it. Dividing again overstates the current.
- Using phase voltage in the three-phase formula. Use the line-to-line voltage with √3.
- Mixing up sides. Quote which winding the current refers to.
Questions
What is the difference between kVA and kW?
kVA is voltage times current. kW is the part that does useful work: kW = kVA × power factor.
How do I convert kVA to amps at 480 V?
For three-phase, divide kVA × 1000 by 831.4. For example 500 kVA gives about 601 A.
Does a generator’s kVA rating tell me how many amps it can supply?
At its rated voltage, yes, that is the rated current. A generator’s usable kW is lower, because it is rated at a stated power factor, commonly 0.8.
Is kVA to amps the same for 50 Hz and 60 Hz?
Yes. Frequency does not appear in the formula.