Transformer Sizing Calculator

How to use this calculator

1. Choose single-phase or three-phase.
2. Enter the load as kW, kVA or amps (with power factor for kW/amps).
3. Add a spare-capacity % for future growth (20–25% is typical).
4. Enter the primary and secondary voltages and click Size transformer.

How transformer sizing works

A transformer is rated in kVA, so the first step is converting your load to kVA: from kW, kVA = kW ÷ PF; from single-phase amps, kVA = V × A ÷ 1000; from three-phase amps, kVA = √3 × V × A ÷ 1000. It is good practice to add spare capacity (often 20–25%) for future growth, then round up to the next standard rating.

Full-load current

For the chosen size, the full-load current on each winding is I = kVA × 1000 ÷ V (single-phase) or I = kVA × 1000 ÷ (√3 × V) (three-phase). Use these currents to size cables, breakers and protection on the primary and secondary. Standard ratings used: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA.

Quick reference: transformer kVA to full-load amps

Frequently asked questions

How do I size a transformer for my load?

Convert the load to kVA, add 20–25% spare capacity, then choose the next standard rating above that figure.

What size transformer for a 50 kW load?

At PF 0.9 that is about 55.6 kVA; with 25% spare ≈ 69 kVA, so you would pick a standard 75 kVA transformer.

Why add spare capacity?

Running a transformer near 100% continuously shortens its life and leaves no room for growth; 20–25% headroom is common practice.