DG Set Sizing for a Factory: Starting Loads, Harmonics and the Solar Interaction

Size a factory DG set from the load it must carry, not the connected load: add the running kVA of the loads you will keep on during an outage, then check the largest motor start and the step-load sequence against the voltage dip you can accept. Add margin for harmonics, derating and solar. A set sized only on running kW usually runs but stalls on a big start.

Which loads should the DG set carry?

List every load, then decide which stay on in an outage. Separate the DG-backed critical panel from the rest. Compressors, process lines with batch loss, fire pumps, lifts, IT and the lighting needed for safe movement usually go on it; air-conditioning for offices and spare machines often do not. Every kVA you keep off the critical panel is a kVA you do not buy, so this decision comes before any sizing. The DG sizing calculator follows the steps below, and the page on transformers and DG sets shows what we supply.

How do you work out the running kVA?

1. For each motor, take rated kW and divide by efficiency and power factor to get input kVA. A 55 kW motor at 0.93 efficiency and 0.88 power factor draws about 67 kVA.
2. Add the other loads in kVA, using the demand you expect at the same moment, not the nameplate sum. A diversity factor from logged data is better than a guess.
3. Add the loads that start together after an outage, such as cooling towers, pumps and compressors.
4. Convert to the set's rating. Sets are rated in kVA at 0.8 power factor, so a 400 kW running load at 0.8 needs about 500 kVA before margin.

Running load alone is the minimum. The next two sections are where sets fail.

Why do starting loads and step loads decide the size?

An induction motor started direct on line draws several times its full-load current for a moment. The generator's voltage dips as the alternator and engine respond, and if the dip is too deep, contactors drop out and other motors stall. The starting method changes the burden sharply.

These multiples are typical values, not guarantees; use the motor data sheet. Ask the DG supplier to run its sizing software on your actual start sequence, with the voltage dip you can accept, because alternator reactance and engine response vary by make. Then sequence the restart. Start the largest motors first, in steps, with a delay between groups, so the set recovers between steps. Staggering is cheaper than a bigger set. Our guide to DG synchronisation and parallel redundancy covers when two sets in parallel are the better answer.

How do harmonics and site conditions change the rating?

• Harmonics. Variable frequency drives, UPS units and rectifiers draw non-sinusoidal current, which heats the alternator and distorts voltage more on a DG set than on the stiff grid. Where non-linear loads are a large share, ask the supplier for the permitted non-linear load, and expect a bigger alternator or filtering.
• Derating. Engines lose output at high ambient temperature and altitude, and a poorly ventilated room worsens it. Ask for output at your site's maximum ambient and altitude.
• Light load. Running a diesel engine at very low load for long periods can foul it. Many makers specify a minimum loading; check it, especially if solar will reduce the net load.
• Power factor. A capacitor bank can destabilise a DG's voltage regulator, so isolate it when running on the DG unless the supplier has approved it. See LT panels and APFC.

What happens when solar runs alongside the DG?

A grid-tied solar inverter needs a stable voltage and frequency reference. On a DG supply, solar can feed the set backwards (reverse power) if it generates more than the factory uses, which trips the set. Two controls are usual: the inverters shut down when the DG supplies the plant, or a DG-solar controller limits the solar output to a share of the load so the set always carries a minimum. The second keeps diesel use down but needs engineering and a DG that tolerates the loading pattern. Our article on solar with DG sets goes through the failures. The Rights of Consumers Rules, as the Ministry of Power's annual report describes them, also ask consumers using DG sets as essential backup to endeavour to shift to cleaner technology such as renewables with battery storage within five years of the rules or within the timeline of the State Commission. See battery energy storage and the DG running cost calculator.

Which rules govern a factory DG set?

• Emissions. Under G.S.R. 804(E) of 3 November 2022 (Environment (Protection) Third Amendment Rules, 2022), emission limits for new genset engines up to 800 kW took effect on 1 July 2023. G.S.R. 436(E) of 14 June 2023 let supplies against purchase orders received by 30 June 2023 continue until 30 June 2024. A set bought now should carry the certification for the current norms. See DG set emission norms.
• Noise. CPCB's noise rules for diesel sets up to 1000 kVA manufactured from 1 January 2005 set 75 dB(A) at 1 metre from the enclosure, with an integral acoustic enclosure.
• NCR. The Commission for Air Quality Management's Direction No. 76 of 29 September 2023, as PIB reports it, allows sets up to 800 kW built to G.S.R. 804(E) to run without restriction, even during GRAP. Confirm the latest direction before you buy.
• Electrical inspection. CEA 2023 regulation 34 sends generating units above a state-notified capacity to the Electrical Inspector before commissioning; for installations above 650 V, regulation 47 requires interlocks that prevent inadvertent paralleling or backfeed between supplies not meant to run together. See CEIG approval for DG sets.
• Fuel. Diesel storage has its own safety and licensing thresholds; see PESO licence for diesel storage.

How Secured Engineers helps

We design the LT and HT distribution, the changeover scheme and the DG room in-house, and carry MEPF and solar under one contract, so the DG, the solar controller and the APFC panel are designed together. CEIG, Fire NOC and DISCOM filings are prepared by our own team. We work from offices in Noida (Delhi NCR) and Ludhiana, on 535+ projects across 18+ states, with 89.77% repeat clients.

Send your load list, single-line diagram and a recent bill, and we will review your DG sizing file: the critical panel, the largest start and the restart sequence. Begin at transformers and DG sets, then message us on WhatsApp, call +91 70099 85817, or ask for a free proposal.