eprice- I think you are mistakenly using 310.15(A) because 334.80 specifically says that "B" applies and using your approach it would rarely be the case where there would be derating of NM cables. And I do understand the Exception in "A" of 310.15.
George Little,
Well, here is how I think these sections work together. I agree that 334.80 does not reference 310.15(A). It tells us how to calculate the ampacity that would apply to the part of the circuit that is in the hole and it uses 310.15(B) to do that. When we're done with that step, we have two (maybe more) portions of the circuit that have different ampacities. Now comes the step where we determine the ampacity of the circuit as a whole. To do so we have to go to 310.15(A). When we get there, we find that the exception to 310.15(A)(2) nullifies the calculation we just did in the first step, because it allows us to apply the ampacity of the portion of the circuit that is not in the hole, to the portion of the circuit that is in the hole. I agree with Ryan, in that, IMO, the second paragraph of 334.80 will hardly ever (only when we have a very short total circuit length) have any relevance, because of the allowance in the exception to 310.15(A)(2).
Edit to add: I do believe that because of the exception in 310.15(A)(2), the derating of NM cable is not necessary as often as many believe. If they are bundled for say 24", then the length of conductors not bundled would need to be less than 20 feet for derating to be necessary. I don't know that I would call that circumstance rare, but it is often not the case.
[This message has been edited by eprice (edited 03-23-2006).]