Transformer kVA Sizing Calculator
Picking a transformer starts from the load it must feed. Divide the load in kW by the power factor to get kVA, add a margin for future growth, then round up to the next standard transformer size — transformers are only sold in standard ratings, so the calculated figure is never the answer on its own.
Formulas: Required kVA = kW ÷ pf × (1 + growth) · then the next standard size at or above the requirement
Worked example: an 80 kW load at 0.85 pf with 25% growth: 80 ÷ 0.85 = 94.12 kVA, × 1.25 = 117.65 kVA required. The next standard size above that is 150 kVA (112.5 kVA is too small). Oversizing further wastes money and worsens efficiency at light load, so the next size up — not two sizes up — is the usual choice.
For the current that transformer will carry, use the three-phase power calculator; for its fault current, use the short-circuit current calculator.
Frequently asked questions
Why divide by power factor?
Because kW is only the useful power. A load at 0.85 pf draws 1 ÷ 0.85 = 1.18 times more apparent power (kVA) than its kW figure — and the transformer must be sized for kVA, not kW.
How much growth allowance is normal?
20–25% is typical for commercial and industrial installations. More than that usually means the load forecast itself needs revisiting, not a bigger transformer.
Does this cover motor starting?
No. Large motors draw 6–7 times full-load current at start, which can dip voltage even on a correctly sized transformer. Check starting current separately for big motors.