DG Set Sizing
DG set sizing selects generator capacity from the loads that must actually run on backup — their running kVA plus the starting demand of the largest motors — inside a loading band the engine stays healthy at, rather than copying the transformer rating.
Why it matters on an MEPF project
Two opposite failures dominate: sets sized to the incomer (running chronically under-loaded — wet stacking, glazing, and with CPCB IV+ machines, after-treatment grief) and sets sized to a wishful list that stalls when the chiller starts. The honest method segregates essential loads, sequences big-motor starts (or softens them with star-delta/VFD), and lands the set where typical backup load sits in a healthy band of its rating.
Sizing also inherits context: CPCB IV+ pricing changed the oversizing penalty; acoustic and stack rules constrain the room; AMF changeover design decides whether the sizing ever gets exercised cleanly; and in NCR, run-hour policy pushes the whole strategy toward solar+storage with the DG as true standby.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Essential-load list | What genuinely must run on outage — walked, not assumed | load segregation |
| Motor starting | Largest starter's inrush accommodated (sequencing/soft start) | design practice |
| Healthy loading | Avoid chronic light-load running; size to sit mid-band | engine manufacturer guidance |
| Compliance envelope | CPCB IV+ (new sets), acoustic enclosure norms, NCR run rules | CPCB notifications / CAQM directions |
Common mistakes
- Copying transformer kVA as generator kVA.
- Forgetting starting kVA — the set that trips on the first compressor.
- Chronic 20% loading — the engine pays for the flattery.
- No load-shedding logic, so "essential" quietly becomes "everything".
Related on this site
One of 68 working definitions in the MEPF & solar engineering glossary. The buying side of the same decisions is in the buyer's guides; the numbers are in the calculators.
Frequently asked
What size DG do I need for my factory?
List the true essential loads, add the largest motor's starting demand, and select so typical backup sits in a healthy loading band. Our DG preliminary load calculator gives the steady-state kVA from the essential load and lists the motor starts for the generator maker to check.
Is bigger safer?
No — diesel engines dislike light running, and post-CPCB IV+ capacity is expensive. Right-sized with sequenced starts beats oversized every time.
Standards referenced: CPCB genset notifications · engine manufacturer application guidance