kVA to amps calculator
Enter a transformer's kVA rating and voltages. You get the full-load current on each side, the turns ratio and the worst-case fault current at the secondary.
- Single-phase or three-phase
- Primary and secondary full-load amps
- Fault current from the nameplate %Z
Not a transformer? Put the load's voltage in either box and read the amps under it.
The kVA to amps formula
Full-load current is the rating divided by the voltage, with √3 for three-phase.
I = kVA × 1000 ÷ (√3 × V)
Three-phase. √3 is about 1.732.
I = kVA × 1000 ÷ V
Single-phase.
Fault = I ÷ (%Z ÷ 100)
Maximum fault current at the secondary, using the secondary full-load current and the impedance from the nameplate.
Worked example
A 45 kVA three-phase transformer, 600 V to 208 V, with 3.5 %Z:
45,000 ÷ (1.732 × 208) = 124.9 A
That's the secondary full-load current. The primary carries 45,000 ÷ (1.732 × 600) = 43.3 A, the turns ratio is 600 ÷ 208 = 2.88 : 1, and the maximum fault current at the secondary is 124.9 ÷ 0.035, about 3,570 A or 3.57 kA.
Three-phase full-load amps
Full-load current for common three-phase transformer sizes, in amps.
| kVA | 208 V | 480 V | 600 V |
|---|---|---|---|
| 15 | 41.6 | 18.0 | 14.4 |
| 30 | 83.3 | 36.1 | 28.9 |
| 45 | 124.9 | 54.1 | 43.3 |
| 75 | 208.2 | 90.2 | 72.2 |
| 112.5 | 312.3 | 135.3 | 108.3 |
| 150 | 416.4 | 180.4 | 144.3 |
| 225 | 624.5 | 270.6 | 216.5 |
| 300 | 832.7 | 360.8 | 288.7 |
| 500 | 1,388 | 601.4 | 481.1 |
| 750 | 2,082 | 902.1 | 721.7 |
| 1000 | 2,776 | 1,203 | 962.3 |
Single-phase full-load amps
Full-load current for common single-phase transformer sizes, in amps.
| kVA | 120 V | 240 V | 480 V |
|---|---|---|---|
| 10 | 83.3 | 41.7 | 20.8 |
| 15 | 125.0 | 62.5 | 31.3 |
| 25 | 208.3 | 104.2 | 52.1 |
| 37.5 | 312.5 | 156.3 | 78.1 |
| 50 | 416.7 | 208.3 | 104.2 |
| 75 | 625.0 | 312.5 | 156.3 |
| 100 | 833.3 | 416.7 | 208.3 |
| 167 | 1,392 | 695.8 | 347.9 |
- Full-load current is what the transformer carries at its full rating. Real loads are often lower.
- These figures don't size conductors or overcurrent protection. Your local code adds its own rules for those.
Questions
How many amps is a 45 kVA transformer?
On a 208 V three-phase secondary it's 124.9 A at full load. On the primary it's 54.1 A at 480 V, or 43.3 A at 600 V.
How do you convert kVA to amps?
For three-phase, multiply the kVA by 1000 and divide by 1.732 times the voltage. For single-phase, multiply by 1000 and divide by the voltage.
What's the difference between kVA and kW?
kVA is apparent power and kW is the real power that does the work. kW = kVA × power factor. Transformers are rated in kVA because the current they carry depends on kVA, not kW.
What does the impedance (%Z) do?
It limits how much current flows in a short circuit at the secondary. The lower the %Z, the higher the fault current. You'll find it on the nameplate.
Why is the fault current a worst case?
It assumes the supply feeding the transformer can deliver unlimited current. The utility's own impedance and the conductors bring the real figure down.
Can I use this to size breakers or wire?
No. It gives full-load current from the physics. Sizing conductors and overcurrent protection follows your local code, which adds its own rules.
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