Originally Posted by
Rick Christopherson
Yes Roger, your analogy is pretty accurate. The "Thermal" part of a Thermal-magnetic breaker is based only on the temperature buildup within the breaker itself. If your load center was located inside of a freezer, then all of your circuit breakers would increase their trip points. For example, a 20 amp breaker would not trip until the current reached, say, 25 or 30 amps--regardless what temperature the rest of the circuit was operating at.
A common problem I encounter in my line of work is where a portable load center is placed out in the hot sun, and circuit breakers begin tripping at only 10 or 15 amps. I have also seen situations where the heat from an adjacent circuit breaker causes nearby circuit breakers to trip prematurely, even though their own circuits are not near capacity. (Hydraulic-magnetic breakers are less temperature dependent).
So no, the circuit breaker has absolutely no knowledge that the downstream circuit is operating above its rated temperature.
I am not sure if this is what you meant in one of your statements or not, but as the wires get hot, their resistance will increase very slightly, and the current will decrease, but only by a very tiny amount.