Distribution Transformer Sizing for Pakistani Industry 2026 — kVA, Losses, Vector Group
18 Jun 2026
How do I size a distribution transformer for a Pakistani factory?
Four steps: (1) sum the connected load in kVA (Σ kW ÷ PF); (2) apply a diversity factor (0.6–0.85 — not everything runs at once); (3) add 20–30% growth headroom and motor-starting allowance; (4) round up to a standard rating (50/100/200/315/400/630/1000/1250 kVA). Rule of thumb: a factory with ~500 kW connected load at PF 0.9 and 0.75 diversity needs roughly 500÷0.9×0.75 ≈ 417 kVA → a 630 kVA transformer with growth headroom. Spec it as Dyn11, ONAN, 11kV/0.4kV to IEC 60076, and load it to ~70–80% of rating for best efficiency and life. Don't oversize wildly — a lightly loaded transformer still pays fixed no-load (iron) losses 24/7.
The sizing calculation
| Step | What | Example (500 kW factory) |
|---|---|---|
| 1. Connected load | Σ kW ÷ average PF | 500 / 0.90 = 556 kVA |
| 2. Diversity factor | × 0.6–0.85 | 556 × 0.75 = 417 kVA peak |
| 3. Growth + starting | + 20–30% | 417 × 1.25 = 521 kVA |
| 4. Round to standard | next std rating | 630 kVA |
Standard ratings + typical Pakistani use
| kVA | Typical site |
|---|---|
| 100–200 | Small workshop, commercial plaza, tube-well cluster |
| 315–400 | Mid factory, cold storage, small textile unit |
| 630 | Marble/steel/medium textile factory |
| 1000–1250 | Large industrial, multi-line plant |
Losses — why oversizing costs money 24/7
- No-load (iron/core) losses — fixed, present whenever the transformer is energised, even at zero load. Oversize and you pay these constantly.
- Load (copper) losses — rise with the square of load. Undersize and you cook the windings.
- Sweet spot is ~70–80% loading: load losses reasonable, iron losses spread over real output.
- Pakistani 45°C ambient derates capacity — check the temperature-rise class; ONAN units derate above 40°C reference.
Vector group + spec essentials
- Dyn11 — delta primary / star secondary, 30° phase shift; the standard for 11kV/0.4kV distribution. Star secondary gives the neutral for 400/230V LT loads.
- ONAN cooling (oil natural, air natural) for most distribution sizes.
- Impedance ~4–6% — affects fault level + parallel operation.
- Off-load tap changer ±2.5%/±5% to trim for Pakistani feeder voltage variation.
- Standard: IEC 60076; oil to IEC 60296.
Common Pakistani transformer mistakes
- Oversizing 'to be safe' — pays iron losses 24/7 and runs at poor efficiency. Size to real diversified load + growth.
- Ignoring diversity — summing nameplate of every motor gives a transformer twice the size you need.
- No ambient derate — 45°C summer cuts usable kVA below nameplate.
- Wrong vector group for parallel — transformers in parallel must share vector group + impedance, or they circulate current.
- No LT-side protection coordination — main ACB/MCCB must be rated for the transformer's fault level (impedance-driven).
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