Sizing a Generator Set

Sizing a generating set is not difficult, but it is done badly more often than almost any other calculation in building services, and the reason is that people size for the running load and forget that the machine has to start things. The running kilowatts are the easy part. The starting transients decide the frame size.

The method below produces a defensible number from a load list. It is deliberately conservative in the places where being wrong is expensive, and it ends with the derating corrections that most quotations quietly omit.

Step 1: build the load list

List every load that will be on the generator, with its rating in kilowatts and its power factor. Separate the essential loads from those that can be shed. For motor loads, note the starting method, because a 75 kilowatt motor started direct on line and the same motor on a variable frequency drive are two completely different problems for the machine.

Apply a diversity factor only where you can justify it. Lighting and small power can usually be diversified; a fire pump cannot, because it will run exactly when everything else is running.

Step 2: convert to kVA

Generating sets are rated in kVA at a power factor of normally 0.8 lagging. Divide the total real power by the actual power factor of the load to get the apparent power. A 300 kilowatt load at a measured power factor of 0.85 is 353 kVA of apparent power, and that is the figure the alternator has to supply.

Step 3: the motor starting check

A squirrel cage motor started direct on line draws roughly six to seven times its full load current for the first seconds, at a very poor power factor. In the worked example, a 75 kilowatt motor represents about 93 kVA at full load and therefore something near 600 kVA of transient demand at starting.

An alternator does not have to be rated for that figure, because the demand is brief, but it does have to absorb it without the voltage collapsing. As a planning rule, an alternator can typically accept a starting demand of about two to two and a half times its own rating while holding the voltage dip within roughly 15 per cent. Six hundred kVA of transient therefore calls for at least 240 to 300 kVA of alternator on that criterion alone, which in this example is comfortably covered by the running load.

It is not always so comfortable. On installations with a small running load and one large motor, a pumping station is the classic case, the starting check governs the whole selection and the machine ends up two frame sizes above what the running load would suggest.

Step 4: step load acceptance

Loads are rarely applied all at once, and they should not be. ISO 8528 defines performance classes, G1 to G4, that set limits on the transient voltage and frequency deviation the set may show when a load step is applied, and on how quickly it must recover. Class G3 is commonly specified where sensitive electronic loads are present.

The practical consequence is that the order of starting matters. Bringing the largest motor on first, while the machine is unloaded, is easier for the set than bringing it on last, on top of everything else. Where a transfer sequence can be programmed into the controller, that sequencing is free capacity. The test that proves the behaviour afterwards is described in the load bank testing guide.

Step 5: margin, then derating

Add a margin for growth, commonly 20 to 25 per cent, before derating rather than after. Then apply the site corrections: roughly 1 per cent of output lost per 100 metres above 1,000 metres of altitude, and roughly 2 per cent per 5 degrees Celsius above 40 degrees ambient, with the manufacturer's own correction table taking precedence.

In the worked example, 353 kVA of running load plus a 20 per cent margin gives 424 kVA, and a site at 45 degrees and 1,200 metres pushes the requirement to around 455 kVA. The next standard frame is a 500 kVA set, and it should be bought on its standby rating if the duty is standby and on its prime rating if it will run regularly, a distinction set out in the generator set section.

Step 6: check it against a real machine

Take the required figure to the platform pages and see where it lands. Around 500 kVA sits comfortably on a 19 litre inline six, which on the imported platforms means the range described in the Cummins page and, on the domestic platforms, the mid-range blocks covered in the Shangchai page. Then read the datasheet in the order recommended in the technical datasheet page, correcting each candidate to your own conditions before comparing anything.