Look at any distribution transformer nameplate and you will see a kVA rating, never a kW rating. That is not an oversight — it reflects what actually limits a transformer.
Why kVA and not kW
Transformer losses and heating come from two sources: core losses (set by voltage) and copper losses (set by current). Neither depends on the load’s power factor. A transformer carrying its rated current at rated voltage is fully loaded whether that current is doing useful work or not — so its capacity is naturally expressed in volt-amperes.
Real power a transformer can deliver
To find the usable kW, multiply the kVA rating by the load’s power factor:
A 1,000 kVA transformer feeding a 0.85 PF load can supply 850 kW. The same transformer feeding a 0.95 PF load can supply 950 kW — which is why power factor correction frees transformer capacity.
Full-load current
Full-load current is often the more practical number, because it sets cable and protection sizes:
- Single-phase: I = (kVA × 1,000) ÷ V
- Three-phase: I = (kVA × 1,000) ÷ (√3 × V)
| Rating | Voltage | Phases | Full-load current |
|---|---|---|---|
| 50 kVA | 240 V | Single | 208 A |
| 500 kVA | 480 V | Three | 601 A |
| 1,000 kVA | 400 V | Three | 1,443 A |
You can check these with the kVA to amps calculator.
Choosing a loading margin
Also consider:
- Ambient temperature and altitude, which may require derating.
- Harmonic loads (drives, UPS, LED drivers), which increase heating — K-rated transformers may be required.
- Inrush and motor starting, which affect voltage dip rather than steady-state rating.
Quick worked example
A facility has a measured peak demand of 420 kW at 0.88 PF.
- Convert to kVA: 420 ÷ 0.88 = 477 kVA.
- Apply an 80% loading target: 477 ÷ 0.80 = 596 kVA.
- Select the next standard size: 750 kVA (or 630 kVA in IEC markets, if growth is limited).
Run your own figures with the kW to kVA converter.