Generator Sizing from Connected Load

Connected load is the sum of every nameplate on the site. Demand load is what actually runs at the same instant, and it is always less, because kitchens do not run every appliance at once and shops do not start every machine together. The ratio between the two is the demand factor. Multiply the connected load by it, add a margin for the loads nobody has thought of yet, and you have the continuous kilowatts a generator must deliver. Two hundred and fifty kilowatts connected, running two-thirds of it at once, with a quarter again for growth, wants a set good for about 203 kW.

The demand factors themselves are tabulated by occupancy and load type in the code, and they are not reproduced here — look up the ones that apply to your building. What this page can tell you is which way the errors go. A demand factor entered as 65 instead of 0.65 gives an answer a hundred times too large, which is at least obvious; a demand factor borrowed from the wrong occupancy is not obvious at all and shows up years later as a generator that never gets above quarter load, burning fuel to wet-stack itself.

Two things this arithmetic does not do. First, generators are rated in kW but limited in kVA — the alternator's copper carries amps regardless of phase angle — so divide this kW answer by the set's rated power factor (usually 0.8) to get the kVA the machine must be rated for. Second, on a motor-heavy site the running load is rarely what sizes the set: the starting inrush of the largest motor is, because the generator's voltage sags under it far harder than the utility's does. Size for the running load here, then check the starting case, and let whichever is larger win.