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Generator Sizing Calculator

Most generator calculators add up your running load and stop there. That is not what decides the size. On a site with motors, the set is almost always chosen by the largest motor starting, because the current surge at start drags the voltage down. This one works both out and tells you which is governing.

Running loadTotal running loadKilowatts. Everything that will be running at once, motors included (not the sum of everything installed).Overall 0.8 is the usual assumption for a mixed industrial load, and is what generator sets are rated at.Line voltageVolts, three-phase.

Largest motorRated outputKilowatts, of the largest single motor that starts against the generator. Enter 0 if there are no motors.Starting methodDOL starting multipleTimes full load current.Motor efficiencyPer cent.Acceptable step loadPer cent of the set's rating that may be applied in one block. A standard alternator will usually take somewhere around 60%; your supplier will give the real figure for the machine.

Duty and site conditionsDuty ratingNamed in ISO 8528-1. A set rated 100 kVA standby is worth roughly 90 kVA prime or 70 kVA continuous, so the harder you work it, the bigger it has to be.Site altitudeMetres above sea level.Maximum ambient temperatureDegrees Celsius.

Results

Running demand--

Motor starting kVA--

Peak while starting--

Derating applied--

Minimum set size--

Governed by--

What "governed by" means. If it says running load, the set is sized by what the site draws day to day and the motor start is comfortably within it. If it says motor starting, you are buying a bigger machine than the running load needs, purely so the lights do not dim when the big pump kicks in. That is normal, and it is the case worth knowing about before you order.

ISO 8528 Generator Sizing & Transient Step Load Formulation

ISO 8528-1 (Duty Ratings) & ISO 8528-5 (Transient Performance)

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Pumping station worked example: 45 kW running load at 0.8 pf on 400 V, with a 15 kW pump starting direct on line:

Running demand = 45 / 0.8 = 56.3 kVA
The 15 kW pump draws approx 30.1 A FLC. A 6.5 × DOL start draws approx 195 A (about 135 kVA step load).
At a 60% step limit, the set must be at least 135 / 0.6 = 226 kVA (governed by motor starting step load).
If started star-delta, the step drops to 45 kVA, reducing the required set size to approx 80 kVA.

A first-pass estimate for comparing options, not a specification. Real sizing depends on the alternator's reactance, the order loads are brought on, non-linear loads such as UPS and inverters, and the voltage dip the installation can actually tolerate. Confirm the final selection with the supplier before ordering. Derating figures used here are common rules of thumb and vary between manufacturers.

Standards reference: Duty ratings (ESP, PRP, COP) are defined in ISO 8528-1; transient performance classes in ISO 8528-5. Neither standard is reproduced here: consult the published documents for the definitions and limits.