334.80 Ampacity for NM cable seems like it may make installation more difficult and time-consuming by possibly requiring ampacity adjustment of wiring in insulated walls for residential construction. At the very least, it could mean drilling more holes to pull wires through. What happens when you bring all of your Home Runs down an insulated exterior vertical stud wall bay to a panel board in a basement? Seems like it would be necessary to apply the ampacity adjustment factor of Table 319.15[B][a] to all conductors.
I guess unless you have some sort of rack to maintain spacing, it will somehow be necessary to separate the cables so you don’t end up having to run 10/2 for the small appliance branch circuits. I can't imagine that builders and insulation contractors are going to be thrilled with electricians filling up the whole stud bay with wiring and making it more difficult for them to get the required R-value.
If they are bundled together or stuffed in a conduit, then yes you need to derate. I rarely bore holes larger then an inch. Sometime like at the panel requires several small holes. I see it quicker and less ugly then a large hole plus you do not need to nail plate it.
They get around a lot of this in Florida by putting the panel in the garage which is not under "air" so they don't need to insulate the wall. In places where they are really serious about the R value of the wall we should probably be surface mounting the panel and running the wires up within the living space so you don't have that big hole in the insulation. The panel itself probably has an R value of 3 or less. Is there a condensation issue there? (cold panel, warm moist inside the house air)
It seems that even though they made a change to 334.80, it still refers to 310.15[B][a], so it should allow the practice of running multiple wires through the same large diameter hole to enter and exit the top and bottom plates of a stud bay without any ampacity reduction, since the 24-inch allowance is intact. It appears that wires running vertically inside the stud bay itself or horizontally through holes in multiple studs of insulated walls could be where 334.80 wants more attention to separation during installation. I think since most of us already use those NM stackers or something similar for support and to maintain clearance from the stud edges, it should pretty much be business as usual when running wire vertically in insulated stud bays. I could see a possible problem though, when running wires horizontally through multiple stud bays in insulated walls. Maybe just drilling more holes to separate the wiring would sufficient.
Not sure, but 334.80 doesn’t seem to differentiate between types of thermal insulation. It just states “where multiple type NM cables are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15[B][a].” I think the “spacing between cables” could be the troublesome issue.
From what I can gather, the concern over the spray-foam insulations may have been what started this revision, but when NEMA found no significant difference with the use of NM cables embedded in different types of insulation materials, it looks like they just decided to apply it generically across the board.
I can see yet another possible installation issue with regard to insulated ceilings. Electricians who just like to lay all their NM cable runs on top of furring strips [strapping] in ceilings of new residential construction, will have to somehow maintain wire spacing. I guess now they might just as well staple the NM to the sides of the ceiling joists and be done with it.
2 X 3 wires or 3 X 2 wire through the same holes #10 or larger by it self. As many holes as required. Every job and been doing that way for 25 years, Never seen anything else by any contractors here. The insulators just deal with it without a bitch ever.
I was just reading about this in Mike Holt’s NEC 2008 Changes book. He says that this requirement has no effect on conductor sizing if you bundle nine or less #12 or #14 current carrying conductors using the 90-degree ampacity for THHN conductors in Table 310.16 at 70% adjustment in Table 310.15[B][a]. This is probably more wires than most electricians would run through the same holes anyway, so shouldn’t be that big of a problem. It should have no effect on doubling up NM cables under the same staple either since that would be less than nine conductors. I can see the need to address the issue of maintaining spacing between conductors though when more than nine conductors are run through the same stud bay or ceiling area, such as when running home runs from a panel. This should really be much of an issue for most of us to apply in the field.
Read Jim Pauley's proposal for the 2008 and the comments. NFPA is making it clear they mean any kind of insulation that touches NM, even if the insulation is not in the hole itself, only around the hole on the surface of the stud. I think they have been sniffing the foam too long but that may just be me. They also added a line that says 310.15(A)(2) exception does not apply in the 2008 so they are saying the wire will not carry away any of the heat from a "hot spot".
I do agree that you would need 5 NM-B in a hole to have this rule kick in if there are no other derating factors present (>9 CC conductors) so it really doesn't affect may installations. If you are foaming in a huge hole in a non bearing member just be sure you separate the cables before you shoot.