Sizing an ATS, and the transfer time question
The transfer switch is rated for the mains, not for the generator, and the interval nobody specifies is the one the load actually notices.
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News
What a load-bank test proves that a no-load run does not
A monthly off-load exercise proves the battery is charged and the starter turns. It does not prove the set can carry the building.
Prime, standby, and why oversizing a set costs you
A bigger set is not a safer set. Lightly loaded diesels wet-stack, and the rating label you choose decides what you are allowed to do with the machine.