80% VS. 100% rated breaker

80% rated breaker: temp. rise measured at breaker terminal must not exceed 50C rise above 40C ambient calibration temp., 90C total.

100% rated breaker: temp. rise measured at breaker terminal must not exceed 60C rise above 40C ambient calibration temp., 100C total.

80% rating or 100% rating is about heat dissipation as a safety issue, not about continuous current rating of the breaker, and when it will trip per its time current curve.

The 80%/100% rule is reflected in the NEC codebook, where it states specifically that unless the breaker is tested and labeled for use at 100% of its continuous current rating, it may have only a maximum of 80% of its labeled continuous current rating flowing through it in normal operation.

So unless the breaker is tested and labeled for use at 100% of its continuous rating, it may have a maximum of 80% of its continuous current rating applied in its installed position under normal operations.