I have been trying to understand the rules about the allowable loading on a circuit breaker, specifically the requirement that the circuit breaker rating be 125% of the total continuous load placed on a branch circuit. (The same language is used in several places, and seems to apply both to branch circuits and to sub-panel feeders, and presumably in other areas.)
I can understand this as providing a useful bit of safety margin, and (probably more importantly given the various safety margins already built into component ampacity) a useful bit of avoidance of nuisance tripping. However I have also seen it suggested that the circuit breakers themselves are only listed for application up to 80% continuous loading or 100% non-continuous loading, and that the circuit breakers themselves could not tolerate a 100% continuous load unless specifically rated.
What I want to figure out is what to expect when a high continuous load is placed on a circuit breaker. Say, for example that a receptacle branch circuit is loaded to 100% capacity with halogen lamps, which are simply left on. Because the branch circuit has no permanently connected loads, no attempt is made to limit the load to 80%.
I would expect the various components in the circuit (wires, receptacles, circuit breaker, etc) to heat up, but that this heating would be expected and designed for in the selection of components.
Will the (presumably 80% rated) circuit breaker to heat up, do the temperature compensation thing, and eventually simply (and safely) trip? Or will the circuit breaker not trip, but instead overheat in some component part? Or will the circuit breaker eventually trip, but suffer some accelerated wear in doing so?
For the small molded case breakers used in residential applications (15-30A), are these devices usually 80% rated or are they usually 100% rated? The same size cases are often used for much larger breakers, and the terminals on these breakers can often take much larger wire than is needed for the circuits being protected.
Thanks
Jon
