Checks BOTH steady running load and the surge when the largest motor starts DOL with everything else already running — the starting case is the one most often overlooked.
Standard method — stated scope limits apply
Connected Loads
Largest Motor (DOL Start)
Define loads and calculate.
About This Calculation
Sizing a standby or prime-power generator means checking it against several genuinely different load conditions, not just total connected kW: continuous running capacity for steady load, the much larger and shorter-duration surge current needed to start the largest motor on the system without the generator's own voltage collapsing, and the generator's transient voltage dip response to that starting event. A generator correctly sized for steady-state running load can still fail to start a large motor if its transient reactance and voltage-regulator response aren't checked against that motor's starting kVA specifically.
Frequently Asked Questions
Why is motor starting kVA usually the real sizing constraint, not total running load?
A large motor’s starting current (5–8× full-load, at a low, mostly-reactive power factor) briefly demands far more kVA than the same motor’s running load — a generator sized only for steady-state kW can have its voltage collapse badly enough during that surge to stall the very motor it’s trying to start, or trip on undervoltage before the motor accelerates.
What does subtransient reactance have to do with generator sizing?
Subtransient reactance (X″d) governs the generator’s voltage response in the first few cycles of a sudden large load step, like motor starting — a lower X″d generator holds voltage up better during that transient, meaning the same generator can start a larger motor (or the same motor with less voltage dip) than a higher-reactance machine of similar steady-state kW rating.
How much voltage dip during motor starting is generally acceptable?
A commonly used guideline allows 15–20% momentary voltage dip during motor starting on a genset, though this depends on what else is connected to the same bus and how sensitive it is — always check against both the generator manufacturer’s guidance and the tolerance of other equipment sharing the supply.
Is a generator’s standby rating the same as its prime (continuous) rating?
No — standby ratings assume limited annual run hours at variable load with no overload capacity margin beyond the rating itself, while prime ratings are for continuous, unlimited-hours running and are typically set lower (often around 90% of the standby rating) precisely because of that duty-cycle difference.