How to size a transformer
Transformers are rated in kVA (apparent power). Calculate the load's kVA from its voltage and current, add a margin for future loads and continuous operation, and pick the next standard size.
Single-phase kVA = V × I ÷ 1,000
Three-phase kVA = 1.732 × V × I ÷ 1,000
kVA from kW = kW ÷ Power factor
Full-load amps = kVA × 1,000 ÷ (V or 1.732 × V)
Example: 150 A at 208 V three-phase
1.732 × 208 × 150 ÷ 1,000 = 54.0 kVA. With 25% margin: 67.5 kVA, so a 75 kVA transformer. Its full-load current is 90.2 A at 480 V primary and 208 A at 208 V secondary.
Standard sizes
| Type | Common kVA ratings |
|---|---|
| Single-phase | 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167 |
| Three-phase | 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 |
Frequently asked questions
How many amps is a 75 kVA transformer?
At 208 V three-phase, 208 A. At 480 V three-phase, 90 A.
Should a transformer be loaded to 100%?
It can carry its nameplate load, but sizing for 75–80% leaves room for growth and reduces heat and losses.
What is the difference between kVA and kW?
kVA is apparent power; kW is real power. kW = kVA × power factor.