What size DG set do you need?

Because starting current — not running load — usually decides the rating. A direct-on-line motor draws roughly seven times its running kVA for a few seconds; a star-delta starter cuts that to about three times; a VFD nearly eliminates it. Two sites with identical running loads can need very different gensets.

Two reasons: a genset should not run continuously at the top of its prime rating (sizing so running load sits near 75% leaves margin for surges and future loads), and the set must absorb the largest motor’s starting surge without a damaging voltage dip. The calculator sizes for both, plus engine kW and any UPS or drive loading, and tells you which one governed.

It is how far the voltage may sag for a moment when the biggest motor starts. Stamford’s guidance allows 35% at the motor but reserves 10% for cabling, leaving 25% for the set — the default. Many consulting engineers specify 20% for mixed loads; medical imaging and other sensitive electronics call for 10%. A tighter dip always means a bigger set, because the alternator can deliver less starting kVA before it sags that far.

Their rectifier inputs draw distorted current, which heats the alternator and distorts the voltage for everything else. Cummins guidance keeps drive loads under about half of generator capacity to hold distortion below 15%, and sizes rectifier loads at 1.15 times nameplate for 12-pulse inputs, rising steeply for older 6-pulse designs. The calculator applies those factors rather than the plain arithmetic.

Yes, and it is an expensive mistake twice over. Cummins application guidance is not to run a diesel set below about 30% of rated load for sustained periods: chronic light running glazes cylinder bores and passes unburnt fuel into the exhaust (wet stacking), which shows up later as slobber, fouled injectors and shortened life. If a starting surge forces a set your running load will barely touch, the calculator shows what a soft starter on that motor would save.

Dramatically. Starting everything in one step forces the set to swallow every surge at once; stepped starting — largest motor first, then the rest — usually allows the smallest set for the same steady load. Two rules from Cummins application practice: life-safety loads may have to be up within about ten seconds of mains failure, and any motor that cycles on and off in service must be checked as if it starts last, with everything else already running — which is exactly what this calculator assumes.

No — treat it as a well-informed starting point. It does not know your motors’ actual code letters, your alternator’s exact transient curve, phase balance, or whether a compressor starts loaded. Send the load list on WhatsApp and an engineer confirms the sizing free of charge, the same day, against the Cummins data sheet for the specific model.