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Tuesday, November 17, 1998
Question:
I have landed a #4 THHN Cu. circuit that feeds a calculated, noncontinuous load of 83 amperes. After the job was complete, the inspector turned it down because the circuit breaker didn't have a temperature marking on it. I complained that the wire was rated for 95 amperes at 90 degrees C, I was only using it at it's 75 degrees C rating, and besides, all circuit breakers are rated for 90 degrees C. Who was right?
A
Answer:
The inspector was! Section 110-14(c)(1) exception 2 allows you to use the 90 degrees C wire at the 75 degrees C rating IF the circuit breaker is marked for 75 degrees C. The wire size is not large enough for the breaker to automatically have a 75 degree C rating without being marked for it (if not marked, it is rated for 60 degrees C). As a side-bar though, most circuit breakers have a 60/75 degree C marking, even the smaller ones but I don't know of any that have a 90 degree C rating in the 600 volt and under range.