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Joined: Sep 2003
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There _are_ qualifiers to 310.15(B)(2), in the form of the exceptions that permit certain conductors, which are clearly expected to carry current, but which are not _counted_ as current carrying conductors. Eg. the permission to not count a neutral which only carries the imbalance current in a multiwire circuit, with the qualifiers that insure this is only used when the neutral actually doesn't carry current.
The example set of 12ga wires (which under current code _must_ derated to 15A) is different from a 14ga switch loop in a very important fashion. If any _one_ of these conductors is actually carrying 20A, then the other conductors cannot carry much current or the OCPD will trip. In other words, if any one of these conductors is actually carrying the full circuit current, the other conductors are _not_ carrying current.
If I stuff a conduit with a single 'hot', a single 'neutral', and 11 unused conductors, then under current code none of the conductors would be derated.
-Jon
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Joined: Mar 2001
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I understand that exceptions exist, but the exceptions wouldn't pertain to this example. I believe that current carrying conductors are current carrying conductors, regardless of the amount of current they are carrying.
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Joined: Sep 2003
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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
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Joined: Jul 2004
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When in doubt of the mind of the CMP and NFPA, write a proposal.
I like George's idea. ... Where multiple switched legs of a single branch circuit occupy the same raceway they may be allowed to be considered a single conductor for the purposes of derating ... or words to that effect.
Greg Fretwell
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Joined: Feb 2005
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How about derating based on the quantity of circuits?
Larry Fine Fine Electric Co. fineelectricco.com
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Joined: Sep 2005
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How about an exception for single family dwellings, like the state of Wisconsin has in their state code.
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Posts: 404
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