Insights
How to Size a Distribution Transformer (kVA)
A practical method to estimate transformer capacity from load power and power factor, with a worked example for a 12kV/0.4kV substation.
Transformer capacity is selected from the actual load, not a round number. Undersizing causes overheating; oversizing wastes capital and raises no-load losses.
The basic formula
For a three-phase load, apparent power S (kVA) ≈ P (kW) / (√3 × cosφ × η), where cosφ is the power factor and η the system efficiency. A more practical rule: S ≈ total kW / 0.8 when the power factor is around 0.8.
Worked example
A workshop load of 800 kW at cosφ = 0.8 needs roughly 1000 kVA. Common XSDQ ratings are 800 / 1000 / 1250 / 1600 / 2000 kVA. Choose 1000 kVA, or 1250 kVA if future expansion is expected.
Dry-type vs oil-immersed
- SCB18 (cast-resin dry-type): fire-safe, indoor, meets GB 20052-2024 Grade 1 efficiency — preferred for buildings, hospitals, data centers.
- S22-M (oil-immersed, sealed): lower cost, outdoor, also Grade 1 — preferred for grids and industrial outdoor sites.
See SCB18 and S22-M for rated parameters.
XSDQ Electric — Lu'an Xinsheng Electric Co., Ltd., brand XSDQ Electric.