DG Set Sizing for Indian Buildings — Practical Guide

DG Set Sizing for Indian Buildings — A Practical Guide

Despite the improvement in grid reliability across major Indian cities, a diesel generator set (DG) remains essential backup power infrastructure for all commercial, institutional, and hospitality buildings. Incorrect DG sizing is common — oversized DGs waste capital and run inefficiently at low load (wet-stacking, high fuel consumption), while undersized DGs trip under load and fail when most needed.

1. DG Set Rating — kVA vs kW

DG sets are rated in kVA (apparent power). The relationship to kW (real power) depends on power factor:

kW = kVA × Power Factor (typically 0.8 for DG output)

A 500 kVA DG can supply 500 × 0.8 = 400 kW of real power at 0.8 power factor. The building’s actual connected load power factor (typically 0.85–0.90 with capacitor banks) may be different — always calculate kVA demand.

2. Load Assessment Methodology

Step 1 — Critical vs Total Load

Step 2 — Calculate Maximum Demand on DG

1. List all loads that will run simultaneously on DG supply
2. Apply demand factor: fraction of connected load operating at any one time
3. Typical demand factors: HVAC 0.7–0.85, lighting 0.6–0.8, lifts 0.5–0.6, general power 0.5–0.7
4. Sum the demands: total kW demand
5. Convert to kVA: kVA = kW / power factor (use 0.85 for mixed load)
6. Add 20–25% margin for future load growth and efficient DG operation

Step 3 — Motor Starting Current

The largest motor starting load (typically the biggest HVAC motor — chiller, AHU, or pump) draws 5–7× running current for 2–8 seconds during starting. The DG must handle this without voltage dip exceeding 15% or frequency drop exceeding 2 Hz.

• Soft starters and VFDs dramatically reduce motor starting current — specify for all motors >7.5 kW on DG circuits
• If starting all HVAC together after DG start: sequence start delays (15–30 seconds between each large motor)

3. Sizing Example — 200-Room Hotel, Delhi

4. AMF Panel Requirements

• AMF (Automatic Mains Failure) panel detects mains failure and starts DG within 10 seconds
• Change-over time: NBC Part 9 Clause 8.3 — emergency lighting within 5 seconds, other loads 10–30 seconds
• ATS (Automatic Transfer Switch): rated for full DG kVA, 4-pole switching (neutral switching essential for floating neutral systems)
• Interlocking: mains and DG cannot be connected simultaneously — prevent back-feeding
• Load shedding relay: automatically disconnects non-essential loads when DG starts — prevent overload

5. Indian DG Brands and CPCB Norms

Note: CPCB II emission norms are mandatory for all DG sets above 800 kW in India. Below 800 kW, CPCB II is widely adopted. Ensure emission certificate accompanies supply.