| Primary value | 50 – 60 % of connected load (100% only for hospitals and data centres) |
| Applies to | Standby DG for residential apartment blocks (covers lifts, common-area lighting, water pumps) · Commercial buildings backing up critical loads (servers, AC, fire pumps) · Pre-tender DG capacity sizing for project budget |
| Exceptions | Residential — common areas only → 30 – 50% of connected |
| Residential — full per-flat backup → 70 – 80% of connected | |
| Office — critical-load backup → 50 – 60% | |
| Hospital — full redundancy required → 100% of connected (N+1) | |
| Data centre — Tier-III/IV → 100% with N+1 redundancy | |
| Diesel consumption → ≈ 0.26 L/kWh at full load | |
| Measured as | DG rating in kVA = Connected load × Backup percentage × 1.25 ÷ 0.8 PF (same form as transformer). Round up to standard ratings (62.5, 82.5, 125, 160, 200, 250, 320, 400, 500 kVA). |
| Source | NBC 2016 — Part 8, Section 2 📚 Cross-referenced |
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Most Indian projects budget DG at 50–60% of total connected load — covers lifts, lights, pumps, fire and security systems, plus a few sockets per flat. Going to 100% backup is reserved for hospitals and data centres where load shedding is unacceptable; even there, an N+1 DG configuration adds another ~30% to the diesel-room footprint.
A 1000 kW residential connected load typically gets a 500 kVA DG (50%) sized for common areas + 30% per-flat backup. The DG room is 25% larger than the transformer room because of the radiator, exhaust stack and acoustic enclosure. Diesel consumption budget: ~30 L/h at full 500 kVA load.