| Primary value | 1.25 ÷ 0.8 × connected kW (≈ 1.56 × connected kW (in kVA)) |
| Applies to | Distribution transformer sizing for residential / commercial buildings · Service connection from utility 11 kV / 33 kV network · Pre-tender substation footprint estimation |
| Exceptions | Diversity factor (residential) → 0.5 – 0.6 |
| Diversity factor (commercial) → 0.7 – 0.8 | |
| Power factor → 0.8 lag (assumed for sizing) | |
| Future-load buffer → +25% (industry standard) | |
| Pole-mount limit → ≤ 25 kVA | |
| H-frame / plinth limit → 25 – 250 kVA | |
| Plinth / room only → > 250 kVA | |
| Measured as | Transformer rating in kVA = (Connected load × Diversity factor × 1.25 future-buffer) ÷ Power factor. Round up to the next standard rating (63, 100, 160, 200, 250, 315, 400, 500, 630, 1000, 1250, 1600 kVA). |
| Source | CEA Regulations — Distribution Standards 📚 Cross-referenced |
4 related items across IS codes, knowledge articles, design rules, maps and tools
Transformer sizing is where the substation footprint, OPEX and DG sizing all converge. Undersized = utility refuses sanction, lights dim under load. Oversized = paying for kVA you don't draw, transformer runs cold and inefficient. The rule lands you within one rating class of optimum — final size is approved by the utility at sanction stage.
A 200-flat residential block with 5 kW per flat (1000 kW connected) at diversity 0.55 → demand 550 kW × 1.25 ÷ 0.8 ≈ 860 kVA → 1000 kVA transformer rounded up. Most utilities also impose a ceiling of around 4.5 MVA per HT (11 kV) circuit, so larger projects need multiple feeders.