Electrical Wire Derating for Wires in Air Tight Spaces
I’m finishing a builder-framed bonus room above my garage. I live in Zone 4 in Sussex County, Delaware. The garage walls, doors and ceiling are insulated.
I’m insulating from the knee walls in including the floor, walls and ceiling. The area behind the knee walls will remain as an unconditioned space. The envelope between the knee walls will be cooled with a mini split. Heating TBD.
The bonus room floor is sheathed, and the joist bay underneath between the knee walls is insulated with blown-in cellulose( by the original builder) and no air-sealed drywall seams as I can’t get to them. The ends of the bays are covered with foamed edge polyiso. All fine and good. Where is the issue?
The code says I can’t run bundled electrical wires (15/20 amp service) through an airtight space without derating the service. I can’t derate the service economically as it’s a vast array of wires that run all over the house. I must either derate or not seal the bay. There are only two bays with this issue out of 16.
Suggestions? Am I reading the code correctly?
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> The code says I can’t run bundled electrical wires (15/20 amp service) through an airtight space without derating the service.
Do you mean circuits (i.e. not service)?
> Suggestions? Am I reading the code correctly?
Would be good for you to post a code reference.
Also, having a hard time understanding the setup because you mention the bays being sealed but then you mention it’s not an issue with all the bays.
Patrick,
Thanks for jumping in. Lets see if I can help you here. I cant attach a pic as this site wont take it. By service I mean circuits. The wires that run from the Electric Panel. The code is NEC 334.8 and table 310.15, B, 2, a. The code does not specifically use the term airtight. I gather I have to use the table and see which wires and how many are running together.
The wires are not separated. They all run together unsecured.
I sealed the bays at each end at the knee wall location with 2 inch polyiso per other articles on GBA. The bays in question are partially sealed with cut outs to clear the wires from the polyiso by 3 inches. That 3 inches I was going to cover with drywall up to the wires and then fill with fire-rated putty. While not completely airtight, it would be pretty close.
I hope this helps.
Short answer: that doesn't apply to you.
Long answer: The NEC differentiates between "conductors" and "cables." A cable is what most people think of as wire, a piece of Romex is a cable. Cables are made up of conductors, a piece of Romex will have several conductors within it.
The table you cite is "Adjustment factors for more than three current carrying conductors in a raceway or cable."
A raceway is conduit or similar, it's not the inside of a wall. So that doesn't apply to you. The ground conductor is not a current-carrying conductor, so regular Romex isn't covered by this section either.
> The wires are not separated. They all run together unsecured.
So how are they supported? It's typically difficult to run NM-B cable with proper support while also causing a "bundling" situation.
> The code does not specifically use the term airtight.
Then don't use it. :-) One important thing to realize about the NEC in particular is that it should be thought of as a "permissive" code. If it doesn't forbid you from doing a certain thing, or dictate the way to do a thing, then you can probably go ahead and do it.
Thanks all for the education guys.
Patrick, it appears the wires rest on the drywall covering the garage ceiling below. I can't see into the joist bay far enough because of the insulation. I would have to cut a piece of subfloor out to see. The builder is still in the area. I'll go look at their in-progress builds and see if they do it. In any case, I have enough info to move forward one way or another.
Doing home improvements right is not a chore for the timid. Happy Holidays.
>Suggestions? Am I reading the code correctly?
You're not reading the code correctly. Seal away.
Cheers. Homer moves forward. Happy Holidays