Transformer kVA & Full-Load Amps Calculator
Work out full-load current from a kVA rating, or the transformer size a load needs after growth margin and derating.
1. Full-load amps from kVA
| Primary full-load current | – |
| Secondary full-load current | – |
Three phase: I = kVA × 1000 ÷ (√3 × VLL). Single phase: I = kVA × 1000 ÷ V.
2. Size a transformer for a load
Measured or calculated maximum demand.
Self-cooled dry-type: 0.3% per 100 m above 1,000 m (IEEE C57.96).
e.g. a manufacturer’s high-ambient factor.
| Load kVA | – |
| With growth margin | – |
| Combined derating factor | – |
| Minimum nameplate kVA | – |
| Next standard size | – |
| Full-load current of that size | – |
Notes on using these results
- Harmonic loads. Drives, rectifiers and IT equipment add heating that a growth margin does not cover reliably. Specify a K-factor or drive isolation rating instead of guessing a derate — see dry-type transformers.
- Ambient temperature. Ratings normally assume a 40 °C maximum ambient. For hotter sites, use the manufacturer’s derating factor in “Other derating”.
- Standard sizes shown are common ratings. Availability varies by manufacturer and voltage.
- Conductor and overcurrent sizing follow the NEC and the actual installation; this tool does not size them.
For preliminary sizing only. Results are estimates from the inputs you enter; the final rating should be confirmed by a qualified engineer and the manufacturer.