Does NEC 251.3 continuous current derating apply to the frame rating or the trip setting?

[pv_n00b]

I'm used to derating OCPD to 80% for continuous current but the NEC is not specific about if the derating applies to a circuit breaker frame rating or the trip setting. If I have a 2,000AF/1,600AT circuit breaker can I run it at 100% up to the 1,600A trip setting or do I have to limit the continuous current to 1,280A? Seems to me that the derating should only apply to the frame rating and not the trip setting.

 

[electrofelon]

I'm used to derating OCPD to 80% for continuous current but the NEC is not specific about if the derating applies to a circuit breaker frame rating or the trip setting. If I have a 2,000AF/1,600AT circuit breaker can I run it at 100% up to the 1,600A trip setting or do I have to limit the continuous current to 1,280A? Seems to me that the derating should only apply to the frame rating and not the trip setting.

I would say the trip setting. If you can find a breaker where the manufacturer says "100% rated when used at or below xyz trip setting" than go for it

 

[wwhitney]

So how do adjustable trip setting breakers work? Does a "2,000AF/1,600AT" breaker end up matching the long term trip curve of a 1,600A thermal magnetic breaker, including the dependency on ambient thermal conditions in the breaker enclosure, and the possibility of nuisance tripping on continuous loads of the breaker rating, if not in a specially designed enclosure?

Cheers, Wayne

PS Are hydraulic breakers and electronic trip unit breakers subject to the above effects? If not, does that mean that if any manufacturer tested them in their standard enclosures to the testing standard required for 100% rated breakers, they would necessarily pass? If so, seems like the listing standard should give those breaker types a free pass, or the NEC should be updated to only require the 125% factor for thermal magnetic breakers.