Transformer Sizing Calculator

The 80% loading rule means the transformer is selected so that the expected maximum demand does not exceed 80% of its rated capacity. This provides headroom for: load growth (future additions), diversity variations, transformer losses (which increase with loading), voltage regulation, and cooling capacity. Continuous operation at 100% loading shortens transformer life — the 80% rule ensures a reasonable service life of 20-30 years. For critical loads requiring backup power, see our UPS Sizing Calculator.

Standard distribution transformer ratings (kVA) per IEC 60076 are: 50, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500. For UK DNO substations, common sizes are 315kVA and 500kVA for residential and 800-1000kVA for commercial developments. The primary voltage is typically 11kV or 33kV, with secondary at 400V three-phase.

Impedance voltage (expressed as a percentage, typically 4-6%) represents the voltage needed to circulate full-load current through the transformer with the secondary short-circuited. It affects two things: voltage regulation (higher impedance = more voltage drop under load) and fault level (higher impedance = lower fault current on the secondary). A 5% impedance on a 1000kVA transformer gives approximately 28.9kA fault level on the 400V secondary.

The prospective fault level on the secondary = (kVA × 1000 × 100) / (secondary voltage × √3 × impedance%). For a 500kVA transformer with 5% impedance at 400V: (500000 × 100) / (400 × 1.732 × 5) = 14,434A (14.4kA). All switchgear on the secondary side must have a breaking capacity exceeding this fault level. The actual fault level may be lower if source impedance is significant.

Oil-filled (ONAN/ONAF) transformers are cheaper, more efficient, and better cooled — preferred for outdoor substations. Dry-type (cast resin) transformers are used where fire risk is a concern: inside buildings, car parks, shopping centres, hospitals. Dry-type transformers are typically more expensive (30-50% premium) and physically larger, but they eliminate oil leak and fire risks. For indoor installations in the UK, dry-type is generally required by insurers.