DG Set Sizing Calculator

Two calculations, and you need the larger

Most DG quotations do one sum: add up the load, divide by power factor, add a margin. That covers the set running steadily. It says nothing about the moment a 30 kW motor is switched on, which is when gensets actually misbehave.

Running requirement. Connected load × demand factor, divided by power factor, plus whatever margin you want for future load.

Step-load requirement. Everything already running, plus the starting kVA of the largest motor divided by the transient capability of the alternator. A brushless alternator holds roughly 2.5 times its continuous rating for the second or two a motor takes to run up, with the voltage dip staying inside the 25 % that contactors tolerate. Ask for more than that and the dip drops out every contactor on the board — including the one feeding the motor you were trying to start.

On a plant with one large DOL motor, the second number is usually the larger, and a set chosen on running load alone will fail in service while being entirely correctly sized on paper.

Starting kVA

A motor draws about 6.5 times its full load current on DOL, at a miserable starting power factor. Expressed against the shaft rating, that works out at roughly:

The commercial consequence is worth spelling out: fitting a soft starter to the largest motor can drop the genset a full frame size or two. The starter is almost always cheaper than the difference — and the smaller set then runs at a healthier load factor for the rest of its life.

Why kVA, not kW

A genset has two ratings and two limits. The engine delivers kW; the alternator is limited by current, so it is rated in kVA at 0.8 power factor. A 100 kVA set gives 80 kW. Run it at 0.7 power factor and the alternator hits its current limit at 70 kW while the engine is still loafing — you paid for engine capacity you cannot use. Correcting power factor on a DG-fed plant frees real capacity.

Note that capacitor banks and gensets need care together: a large fixed bank on a lightly loaded set can drive leading power factor and make the AVR unstable. Switch the bank out when running on DG, or use a controller that does.

Site derating

The engine is an air pump. Thin air means less oxygen, and less oxygen means less power. Standard practice:

• About 3 % per 300 m of altitude above 1000 m.
• About 2 % per 5 °C of ambient above 40 °C.

A set installed in Shimla in summer can be 15 % down on its nameplate. The derating applies at the site, so the ordered rating has to be larger than the calculated requirement, not equal to it.

Check the rating definition too. ISO 8528 distinguishes standby (emergency use, no overload capability), prime (unlimited hours with a varying load), and continuous (unlimited hours at constant load). The headline kVA in a quotation is usually the standby figure and the prime rating is around 10 % lower. If the set is your regular supply rather than a backup, size against the prime rating.

Load factor, and why undersizing is not the only sin

An oversized diesel is not harmless. Below about 30 % load a diesel runs cool, fails to burn its fuel completely and wet-stacks: unburnt fuel and carbon build up in the exhaust, the injectors glaze, and maintenance costs climb. Aim for 60–80 % load in normal running.

Fuel consumption is roughly 0.25 litres per kWh at full load, rising towards 0.30 and beyond as the load drops — the figure shown here uses the calculated load point. Over a year of daily running, that gap between a well-loaded set and a lightly loaded one is usually larger than the price difference between the two sets.

Step load, and the dip the calculation does not show

Starting kVA tells you whether the alternator can supply the current. It does not tell you whether the engine can absorb the torque, and those fail differently. Apply a large block of load to a running diesel and the speed falls before the governor recovers it, taking frequency with it — a dip to 46 Hz on a 50 Hz set is not unusual on a hard start.

ISO 8528 grades sets by how well they hold up. A G2 set is permitted a transient frequency deviation of around 10 per cent and several seconds to recover; a G3 set is held to about 7 per cent, and G4 is negotiated for loads that will not tolerate either. Most general-purpose sets sold for building services are G2, which is entirely adequate for pumps and lighting and marginal for anything with a UPS behind it.

That is the trap worth naming. A double-conversion UPS watching its input frequency will reject a supply that dips below its window and transfer to battery, so the generator starts, the biggest motor starts, and the UPS drops the load it was installed to protect. Widening the UPS frequency window usually fixes it; staging the motor starts so no single step is more than about a quarter of set rating fixes it properly.

The generator that is too big for its load

Diesel engines need to run warm and loaded. Below roughly 30 per cent of rating, combustion temperature never reaches the point where fuel burns completely, and unburnt diesel and lubricating oil accumulate in the exhaust — wet stacking. The symptoms are visible: black oily deposits around the manifold, a persistent smoke haze, and oil weeping from exhaust joints.

The damage is real rather than cosmetic. Deposits glaze the cylinder bores, which raises oil consumption permanently, and they foul injector tips so the spray pattern degrades and the problem accelerates. A set that has been idling at 15 per cent load for a year does not recover by being loaded properly afterwards; it needs the deposits burning off under a load bank, and sometimes a top overhaul.

This is why oversizing has a cost that a sizing calculation never surfaces. A set chosen with a generous margin for a load that never arrived spends its life below the threshold, and the reliability it was bought for is exactly what it loses. Where the standby load genuinely is small most of the time, two smaller sets in parallel, or a set with a load bank, is the honest answer.