Formula
Single phase: I (A) = 1000 × S (kVA) ÷ V (V)
Three phase (line-to-line voltage): I (A) = 1000 × S (kVA) ÷ (√3 × VL-L)
Worked example
A 25 kVA three-phase generator at 400 V line-to-line: I = 1000 × 25 ÷ (1.732 × 400) = 36.08 A per line.
Reference table
Quick reference
| kVA | Amps @ 230 V (1φ) | Amps @ 400 V (3φ) |
|---|---|---|
| 1 | 4.35 | 1.44 |
| 5 | 21.74 | 7.22 |
| 10 | 43.48 | 14.43 |
| 25 | 108.7 | 36.08 |
| 50 | 217.39 | 72.17 |
| 100 | 434.78 | 144.34 |
Where this shows up in the real world
Transformer and generator nameplates list kVA, but breakers, conductors and panel schedules are sized in amps — so this conversion sits at the start of nearly every service-sizing job. A US electrician checking whether a 75 kVA transformer can feed a 208Y/120V panel, or a UK contractor sizing the supply for a 50 kVA three-phase machine at 400V, is doing exactly this math before opening a code book.
Common mistakes to avoid
The classic error is mixing up line-to-line and line-to-neutral voltage in three-phase work: use the line-to-line figure (208V, 400V, 480V) with the √3 formula, never the phase voltage. The second trap is forgetting that the kVA rating is the transformer's ceiling, not the load — size conductors from the actual load current, and protection per NEC Article 450 or your local code.
Frequently asked questions
Why does three phase divide by √3?
In a balanced three-phase system, power is shared across three conductors. Using line-to-line voltage, the relationship S = √3 × V × I applies, so solving for current introduces √3 (about 1.732).
Is kVA the same as kW?
No. kVA is apparent power; kW is real power. They are related by the power factor: kW = kVA × PF. Generators and transformers are rated in kVA because they must carry the full apparent current.
Which voltage should I use?
For single phase, use the line-to-neutral supply voltage (e.g. 230 V in Pakistan/EU, 120 V in North America). For three phase, use line-to-line voltage (e.g. 400 V or 480 V).