Home › Guides

How to size a distribution transformer (kVA)

A transformer that is too small overheats and ages fast; one that is too large costs more and wastes no-load losses every hour. Here is the standard approach.

1. Maximum demand, not connected load

Add the rated loads, then apply a diversity (demand) factor, because not everything runs at once. Typical factors: 0.6–0.8 for industrial plants, 0.4–0.6 for commercial buildings. Better still, use measured peak demand if the site already exists.

2. Convert to kVA

kVA = kW ÷ power factor. Use 0.85–0.9 for most industrial loads unless you know better.

3. Allow for growth and loading

Add 20–30% for future load. Oil-immersed transformers can run at full rating continuously, but running at 60–80% gives the best balance of losses and life.

4. Pick the next standard rating

Standard ratings: 100, 160, 200, 250, 315, 400, 500, 630, 800, 1,000, 1,250, 1,600, 2,000, 2,500 kVA.

Other things to specify

• Primary voltage (for example 11 kV or 33 kV) and secondary (0.4 or 0.415 kV).
• Vector group, usually Dyn11.
• Copper or aluminium windings; copper is smaller and lighter, aluminium is cheaper.
• Tap changer range, usually ±2 × 2.5% off-load.
• Standard: IEC 60076 or your utility specification.
• Losses or efficiency class, if your utility sets limits.

See also 11 kV vs 33 kV transformers.