I just met a WA inspector on a 12Kw standby generator inspection, and I quizzed him on generator feeder/breaker sizing. A popular topic right now. This is a synopsis of his comments, is this representative view?
"The generator on-board breaker is sized by the manufactirers UL listing, it is to protect the generator only. Generator output feeders are protected by the backfed breaker in the load panel. Thus the panel breaker and feeder conductors can be sized by the generator output rating (at 125% for continuous duty), not the generator onboard breaker. Eg. if generator is 12Kw @ 240V, then output is 50A (continuous), so size output feeders at #6 THHN and 60A breaker in the panel. Even though that generator has onboard breaker of 70A. This provides selective coordination also."
"The generator on-board breaker is sized by the manufactirers UL listing, it is to protect the generator only. Generator output feeders are protected by the backfed breaker in the load panel. Thus the panel breaker and feeder conductors can be sized by the generator output rating (at 125% for continuous duty), not the generator onboard breaker. Eg. if generator is 12Kw @ 240V, then output is 50A (continuous), so size output feeders at #6 THHN and 60A breaker in the panel. Even though that generator has onboard breaker of 70A. This provides selective coordination also."