Transformer Sizing Calculator (kVA)
Sizes a transformer in kVA from the connected load and power factor, adds headroom, and gives the full-load current on both windings.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
kVA = kW ÷ power factor × (1 + headroom) I = kVA × 1000 ÷ (√3 × V) for three-phase
A transformer is rated in kVA because its limit is heat, and heat comes from current — which depends on apparent power, not on how much of that power does useful work. A 100 kVA transformer will deliver 100 kW at unity power factor and only 85 kW at 0.85.
What to watch out for
Three things drive the choice:
- Rate in kVA, not kW. Dividing by the power factor is not optional — a 50 kW load at 0.85 is 59 kVA, and a 50 kVA transformer would be overloaded by nearly 20%.
- Leave headroom. 20–25% is normal. Continuous operation near the nameplate shortens insulation life, and load almost always grows.
- Do not overdo it either. A heavily oversized transformer still has its full no-load core loss, running 24 hours a day whether anything is connected or not.
Frequently Asked Questions
What size transformer do I need for a 50 kW load?
At 0.85 power factor that is 59 kVA, and with 25% headroom about 74 kVA — so a 75 kVA unit, the next standard size up.
Why are transformers rated in kVA and not kW?
Because the winding heats according to current, and current follows apparent power. The manufacturer cannot know your power factor, so rating in kVA covers every case.
How much spare capacity should a transformer have?
20–25% is the usual allowance. That keeps continuous loading around 80% of nameplate, which balances insulation life against the no-load losses of an oversized unit.