Selection & Sizing

Turn your load into the right kVA rating and a real budget number: the sizing formula, standard ratings, oil vs dry decision, 2026 FOB Qingdao price tables, and two worked examples.

Distribution transformers come in 6, 10, 11, 20 and 33 kV primary classes. A practical guide to matching voltage class to your grid, load size, distance and equipment — with the tap-range factor that buyers often miss.

S11 is 8% cheaper, but over 20 years its extra losses cost more than the saving. Real no-load/load loss numbers for S11, S13 and S20 at 1,000 kVA, price multipliers, and the payback math that tells you which grade to buy.

The same 1,000 kVA transformer costs about $8,534 oil-immersed and $12,490 dry-type — a 46% gap that is only the first difference. Fire safety, maintenance, losses, overload and lifespan decide which type your project really needs. Complete 2026 FOB price table, loss comparison, application matrix and a 5-question decision tree.

Sizing the transformer is only one third of the job. The transformer's impedance sets the short-circuit current; the short-circuit current sets the breaker ratings and cable sizes; those set the protection settings. Follow one 800 kVA project through the full chain — load list, transformer, short-circuit current, switchgear, feeders and protection coordination — with real 2026 FOB prices and ready-to-use formulas.

An autotransformer (auto transformer) uses one shared winding, so for a small voltage change it needs roughly (1 − 1/ratio) less copper and iron than a two-winding transformer — about 50% less at 2:1, ~20% at 5:1, ~10% at 10:1 — but it gives no galvanic isolation. Here is the trade-off, the sizing rule and what to put on the order (IEC 60076-1 / IEEE C57.12.10 / GB 1094).

Most distribution transformers ship with a simple off-load (de-energized) tap changer at ±2×2.5% — you can only change taps with the transformer dead. An on-load tap changer (OLTC) adjusts voltage while carrying full load, but adds a 10–20% price premium (roughly +$850 to +$1,700 on a 1,000 kVA unit at 2026 FOB Qingdao prices) plus its own diverter-switch maintenance. Here is the five-point rule for deciding which one your project actually needs.

An amorphous-core (S15/SH15) distribution transformer cuts no-load loss by roughly 60–70% versus a CRGO silicon-steel core because its ~0.025 mm iron-based metallic-glass ribbon has far lower core loss per kilogram; the trade-off is a 20–30% higher price and a larger tank, so it pays off only where the unit runs lightly loaded around the clock (as of 2026).

For distribution transformers up to about 2,500–3,150 kVA, a hermetically sealed (corrugated tank) unit keeps insulating oil sealed from air and moisture — no breather, no silica gel, longer oil life — while conservator (free-breathing) designs are for larger units that need a visible oil level and Buchholz relay protection. This guide compares both systems and gives the cut-over point.

For fire-sensitive or environmentally constrained sites, a natural ester fluid raises the transformer's fire point from ~170 °C (mineral oil) to above 300 °C and is fully biodegradable, but costs roughly 3–5× more; silicone oil offers the same high fire safety with different moisture behaviour at a similar premium. This guide compares all three dielectric fluids and when each one pays off.