Why use the 80% loading rule for transformers?

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.

What are the standard transformer sizes available?

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.

What is impedance voltage and why does it matter?

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.

How do I calculate the secondary fault level?

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.

Should I choose oil-filled or dry-type transformer?

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.

TradeCalculator

Transformer Sizing Calculator

Size a distribution transformer from load kW, apply the 80% loading rule, and determine primary/secondary currents, voltage regulation, and secondary fault level.

Total Load

Maximum demand after diversity

Power Factor

0.85 typical for mixed loads

Primary Voltage

HV supply voltage

Secondary Voltage

LV distribution voltage

How We Calculate This

This calculator determines the required transformer size from the connected load, applying the loading rule and calculating key electrical parameters.

Key Formulas

Load kVA = kW ÷ Power Factor

Required kVA = Load kVA ÷ Loading factor

FLC (secondary) = kVA × 1000 ÷ (V × √3)

Fault level = kVA × 100 ÷ (V × √3 × Z%)

Standard Sizes

Residential: 315-500kVA typical
Commercial: 500-1000kVA typical
Industrial: 1000-2500kVA typical
Impedance: 4-6% standard

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.