On Fri, 28 Dec 2007 20:24:41 -0800 Paul Hovnanian P.E. wrote: | Chuck wrote: |> |> > The best reference I have found seems to be NEC article 310.16 regarding the |> > ampacity of wire. There are charts and footnotes indicating that one must |> > derate bundled cables. One reference is to THHN wire with a 90 degree |> > Celsius rating showing that bundled cable with 10 to 20 current-carrying |> > conductors should be derated at the rate of 50%. The chart shows these |> > conductors carrying capability to be 30Amps, thus when bundled in a group |> > between 10 to 20 current carrying conductors the circuits should be derated |> > to 15Amps. This is for 12/2 wire. The derating factors are 80% for 4 to 6 |> > conductors and 70% for 7 to 9. |> >
|> > So, I'm going to run with that. Thanks. |> >
|> |> That will certainly be conservative. If |> I'm not mistaken, the Code includes an |> exception to the effect that derating |> factors shall not apply to conductors in |> nipples having a length not exceeding 24 |> in. Seems difficult to understand why a |> 1.5" wood stud requires derating but a |> 24" nipple does not. | | That's a good point. I don't think the derating factors apply in this | case.
I suspect that it is considered that the extra heating of the bundle is able to reasonably dissipate if that heating is only for a short length. That level of heat can be handled better with a metal nipple up to some length, and in the wood up to some shorter length. The case of a bundle of great length would have no opportunity to have extra dissipation by means of the spread of wires out either end. These are not huge levels of heat, but enough to warrant adjustment to make sure all the other extra margins of safety are maintained.
|> The only safety downside to a |> conservative interpretation is that |> structural issues may eventually surface |> if too many holes are bored in the studs. | | Have you ever seen plumbers drill a 2" hole through a 2x4 (effectively | 3.5" wide). There's not much wood left. If you need to drill a series of | 3/4" holes through a stud, keep them in a vertical line. This will leave | a maximum amount of wood on either side in the form of two columns. As | long as you separate the holes by a few inches, the stud should retain | most of its bending strength.
I would not want such a hole in a load bearing stud. OTOH, I plan to have at least exterior walls double thick with staggered studs, so it would be possible to run wires horizontal, where any runs are needed (not likely), zig-zagging back and forth between the layers. My flooring structure will also be double layer with a 2x4 layer over a 2x16 layer at 90 degree angle so there will also be places to run wires without drilling a single hole. The main support beams will definitely be out of bounds for any drilling. My house design is a post and beam design with walls generally not doing any load bearing, but they may be used for structural stability.
And, FYI, my intent is to use the new combo cable that meets both AC and MC requirements.