Distribution Transformer Sizing for Buildings

Undersize the transformer and it runs hot and limits expansion; oversize it and you pay for iron losses and a larger protective scheme forever. Here is the sizing method.

From demand load to kVA

Take the total demand load from the load schedule (after PEC demand factors), not the connected load. Convert to apparent power:

Use the measured or specified power factor — 0.85 is a common design assumption for mixed commercial load.

Add a load-growth allowance

Transformers last 25–30 years. Add 20–25% for future load unless the client specifies otherwise:

Select a standard rating

Pick the next standard size at or above S_required: 25, 50, 75, 100, 112.5, 150, 167, 200, 250, 300, 400, 500, 750, 1000 kVA. Example: a 315 kW demand at 0.85 PF is 371 kVA; with growth, 463 kVA → select 500 kVA.

Loading check

Target 60–80% at initial occupancy. Below 40% is wasteful; above 90% leaves no margin for imbalance or harmonics.

Specify for the BOQ

• Primary / secondary voltage (e.g. 13.8 kV / 400–230 V)
• Vector group (Dyn11 is typical for distribution)
• Impedance (~4–6%; lower impedance means higher fault current downstream)
• Cooling / type — oil-immersed (ONAN) or cast-resin dry type for indoor vaults
• Taps — off-circuit ±2 × 2.5%

These affect the price materially and feed the downstream fault-current and protection design.