Selecting a Distribution Transformer

Quick pick (TL;DR)

kVA = peak load ÷ ~0.8 (load & future spare).
Fix primary/secondary voltage and the vector group (Dyn11 is the usual distribution default).
Oil-immersed (ONAN) outdoors / high rating; cast-resin dry indoors / fire-sensitive areas.
Impedance ~6% balances fault level vs voltage drop.
Add Buchholz + winding/oil temperature protection (and 87T differential on large units).

Step 1 — kVA sizing

1100 kVA peak load

Given measured peak load = 1100 kVA spare / loading factor = 0.80 Step 1 — Required rating kVA = 1100 / 0.80 = 1375 kVA Step 2 — Pick size next standard rating = 1600 kVA Step 3 — FLC primary = 1600k / (√3 · 22000) = 42 A secondary= 1600k / (√3 · 400) = 2309 A Step 4 — Fault level I_fault ≈ FLC / Uk = 2309 / 0.06 ≈ 38.5 kA (sets LV switchgear rating)

Load to ~80% for headroom; Uk sets fault level — 6% is the common compromise.

Step 2 — Oil vs dry & vector group

	Oil-immersed	Cast-resin dry
Use for	Outdoor, high rating	Indoor, fire-sensitive
Fire risk	Oil — needs bund	Low
Cost	Lower	Higher

Dyn11 is the usual distribution choice — gives an LV neutral and contains triplen harmonics.

Step 3 — Protection

Oil: Buchholz, WTI/OTI, pressure relief.
Large units: 87T + REF.
Off-load taps fixed; OLTC if voltage varies.

Transformer sizing calculator

kVA · full-load current · fault level

kVA = load ÷ factor → std; FLC = kVA/(√3·V); fault ≈ FLC/Uk.

Peak load (kVA)

Loading factor

Secondary V (V)

Impedance Uk (%)

Fault is approximate (ignores source Z). Confirm switchgear from the fault study.

Common mistakes

No spare capacity.
Oil transformer indoors without bund.
Wrong vector group for paralleling.

Requisition & datasheet

Rating, voltages, vector group, Uk.
Type, cooling, taps, protection.

Frequently asked questions

What should I know about Selecting a Distribution Transformer?

kVA = peak load ÷ ~0.8 (load & future spare). Fix primary/secondary voltage and the vector group (Dyn11 is the usual distribution default). Oil-immersed (ONAN) outdoors / high rating; cast-resin dry indoors / fire-sensitive areas. Impedance ~6% balances fault level vs voltage drop. Add Buchholz + winding/oil temperature protection (and 87T differential on large units).

Which standards or codes apply?

IEC 60076 (power transformers), IEC 60076-11 (dry-type), IEEE C57, IEC 60214 (tap changers).

Standards — IEC 60076; IEC 60076-11; IEEE C57; IEC 60214.