Redsy,
I completely agree. 1) Under present code this exception does not exist. 2) I believe that this situation is a safe situation and thus a reasonable exception to add to the code 3) I believe that this situation is unlikely enough as to not be worth the time to add such an exception.
This was more of a thought experiment that goes back to an earlier discussion in this thread, and a much more common situation: if you have a panel with limited feeder capacity, does this feeder capacity in any way influence the derating of bundled conductors fed by this panel. The answer is that there is no provision in the code to adjust the derating to account either for load diversity or feeder capacity. The question then becomes: is there a way to describe a _safe_ exception to 310.15(B)(2) that would account for such load diversity.
I don't know that there is a good way to build such an exception without restricting it to a few number of extreme cases as to make the exception essentially useless.
To get back to load diversity in residential panels with 'stove pipes' (big bundles of romex coming off the panel), perhaps a way to calculate the 'effective number of current carrying conductors in the worst case' would read as follows: 1) take the result of the service calculation as per article 220, 2) allocate this current to circuits operating at their rated current, starting with the smallest capacity circuits, 3) count the number of conductors used in this 'worst case' situation, 4) use this number for determining derating using 310.15(B)(2)a
Again, this is _not_ code; this is a vague outline of a suggestion for an exception to add to the code.
To work the above example, consider a residence with 200A service 240V/120 , a calculated load of 77A and a 40 circuit panel with a bunch of 20A general lighting and receptacle circuits. If all of the conductors were stove-piped, there might be 60 current carrying conductors in that single pipe. The worst case scenario for current heating would be if the load current were concentrated in a minimum number of circuits, so 8 120V 20A circuits being used to capacity.
I believe that it would be 'reasonable' (but not code) to count the above as 16 current carrying conductors for 310.15(B)(2).
There are clearly holes in the above; with a 200A service and calculated 77A load, what happens when the load increases (as it most certainly will)?
-Jon
