Transformer Sizing Calculator
Determine the required kVA rating of a transformer based on connected loads, demand factor, and efficiency. Includes primary and secondary current calculation.
Results
What is it?
Transformer kVA rating is determined by the actual load demand divided by the power factor, then padded with a safety margin for future growth and fault headroom: kVA = (Total Load × Demand Factor) / PF × (1 + Safety Margin). Primary and secondary full-load currents: I = kVA × 1000 / (√3 × V).
How to use
Enter the sum of all connected loads in kW. Set the demand factor (the ratio of simultaneous use — not all loads run at once) and the expected average power factor. Add a 25% safety margin for growth and harmonic derating.
Example scenario
Factory with 150 kW connected load, DF = 0.75, PF = 0.85, 25% margin. Demand = 112.5 kW. kVA = 112.5/0.85 = 132.4 kVA. With margin = 165.5 kVA → select standard 200 kVA transformer. Secondary (415 V) current = 200×1000/(1.732×415) = 278 A.
Pro tip
Always round up to the next standard transformer size: 25, 50, 75, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000 kVA. Budget 5–10% derating for harmonic-heavy loads (VFDs, UPS, rectifiers). For transformers with >15% harmonic current, use a K-rated transformer.