TCC @ 3 hours @ Rated Current

[ron]

Most molded case breakers are 80% rated, meaning that a load where the maximum current is expected to continue for 3 hours or more shouldn't exceed 80% of the breaker rating.

The time current curve linked below shows the tripping characteristics out to almost 3 hours (10,000 seconds), and still doesn't seem to indicate that it would trip at 80% of the rated current. Any thoughts why? Do you think from 2-3/4 hours to 3 hours it takes a dramatic swing over to 80% (where it isn't published) and then continues on its way further up the Y axis?

http://static.schneider-electric.us...it Breakers/QO-QOB Circuit Breakers/730-4.pdf

 

[don_resqcapt19]

I don't think it will trip at 80% unless you are in a high ambient area. All breakers are tested as single units in a 40°C ambient (I think testing is in open air and not in an enclosure, but not sure) and must carry the rated current forever under those conditions. The 80% is just a CYA for multiple breakers in a common enclosure where the thermal unit temperature might exceed 40°C. The combination of multiple heavily loaded breakers in a common enclosure and maybe in an small room with other heat producing equipment, may increase the temperature enough to shift the trip curve to the left. I have seen that type of breaker carry 20-25% of nameplate for well over 3 hours a number of times.