Thread: Wiring in spray foam insulation

I have learned a lot in this forum about bundling wiring and derating because of heat dissipation issues. It occurred to me that wiring in the walls of homes with spray foam insulation may also have the same difficulty with heat dissipation. Would it not be logical, then, that this situation should require derating? What am I missing?

Darrel Hood
DILIGENT PROPERTY SERVICES

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Darrel,

I think this answers your question:
- From the 2008 NEC. (underlining and bold are mine)
- - 334.80 Ampacity.
- - - The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
- - - Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.
- - - Where more than two NM cables containing two or more current-carrying conductors 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)(2)(a).

Table 310.15.(B)(2)(a) is the derating table.

Thus, first there is the derating for ambient - which ALWAYS applies and ALWAYS applies first (in severe cold climates, derating still "applies", it is just that there is no derating "to apply"), then the derating for bundling or lack of maintaining spacing applies, AND, if there is no bundling or lack of maintaining spacing - which is what triggers Table 310.15(B)(2)(a) in 334.80 Ampacity - then there is no harm and no foul.

This tells me that they ran tests and determined that, as long as the cables are spaced out, they are suitable for the same ampacity in thermal insulation. That is because the NM-B uses 90 degree C rated insulation, which apparently can withstand the accumulated heat of normal allowable current flow within the conductor. The normal allowable current flow in a #12 is 20 amps overcurrent protection, with normal less than 80% on the circuit if a continuous load, if not a continuous then there will not be the same heat build-up to be concerned with.

Jerry, thanks for reminding me about that new addition to art 334.80 (2005) however, u failed to include "where bundled togeather" when passing thru framing members to be insulated.
If the cables are not bundled togeather between the studs then this derating requirement would not apply.
Note that thru out the code, that bundleing togeather means held togeather 2ft or more.
Bob Smit, County Electrical Inspector

Originally Posted by bob smit

Jerry, thanks for reminding me about that new addition to art 334.80 (2005) however, u failed to include "where bundled togeather" when passing thru framing members to be insulated.

Bob,

I didn't forget that as you are referring to the paragraph above the one I highlighted. See below in red for what you are referring to.

If the cables are not bundled togeather between the studs then this derating requirement would not apply.

Which is what I said. See blue text below.

Originally Posted by Jerry Peck

Darrel,

I think this answers your question:
- From the 2008 NEC. (underlining and bold are mine)
- - 334.80 Ampacity.
- - - The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
- - - Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.
- - - Where more than two NM cables containing two or more current-carrying conductors 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)(2)(a).

Table 310.15.(B)(2)(a) is the derating table.

Thus, first there is the derating for ambient - which ALWAYS applies and ALWAYS applies first (in severe cold climates, derating still "applies", it is just that there is no derating "to apply"), then the derating for bundling or lack of maintaining spacing applies, AND, if there is no bundling or lack of maintaining spacing - which is what triggers Table 310.15(B)(2)(a) in 334.80 Ampacity - then there is no harm and no foul.

That's what i get for always being in a hurry Jerry.
One step further...i think it always wize to include the fact that any mention of derating should be followed by mentioning the derating starts at the 90 degree C column for NM cable but cannot exceed the 60 degree rating, and of course most panel and device terminations are 75 C so are usually not an issue.
Thhn conductors are 90 C rated yet the terminations bring them down to atleast the 75 C rating, unless after derating from the 90C column one might end up with less ampacity than the 75C column.

I know your aware of these facts, however, many who visit this site need to know of some of the other rules. But what point do u cut off at, right?
I've been in the big-box stores many times and overheard the employee telling a homeowner "use this size for 30 amp" etc. without any knowledge of the application and use. I would sometimes add my help if wanted, but would usually end up spending alot of time designing their whole job.
I've learned to keep my mouth shut so I can get in and get out of there.
Trouble is Jerry, we give a damm, else we wouldn't be using our valuable time posting on sites such as this.

As if I don't do enough of it doing electrical inspections everyday:{
What's wrong with us...
Bob Smit

Originally Posted by bob smit

One step further...i think it always wize to include the fact that any mention of derating should be followed by mentioning the derating starts at the 90 degree C column for NM cable but cannot exceed the 60 degree rating, and of course most panel and device terminations are 75 C so are usually not an issue.
Thhn conductors are 90 C rated yet the terminations bring them down to atleast the 75 C rating, unless after derating from the 90C column one might end up with less ampacity than the 75C column.

And yet another step further, one I mention almost every time I mention derating, is that ambient temperature derating *IS ALWAYS* done, regardless of whether or not any bundling or lack of maintaining spacing is present, and if the conductor run down an exterior wall, are exposed into a garage or other unconditioned area (to a lesser extent), or run up into an attic (to the greater extent), then derating is positively necessary - and that is the precise reason that NM went to NM-B ... to allow for that ambient derating and still allow 15 amp 20 circuits with #14 and #12 respectively. Which means that derating for bundling or lack of maintaining spacing does not start at 30 amps for #12, it starts at the lower derated-for-ambient-rating.

I have found that most electricians skip the derating for ambient and use the full 90 degree C rating for derating for bundling purposes, which gives them a false sense of 'Yeah, that's okay' when in fact 'it is not okay'.

I've been in the big-box stores many times and overheard the employee telling a homeowner "use this size for 30 amp" etc. without any knowledge of the application and use. I would sometimes add my help if wanted, but would usually end up spending alot of time designing their whole job.
I've learned to keep my mouth shut so I can get in and get out of there.

Same thing here, and I only interrupt on safety things, like one time when a customer was talking about their pool deck box and its conduit being rusted out and the 'salesperson' was being helpful - 'All you have to do is use some silicone to glue that new metal box down into the concrete, you don't need to worry about that rusted off metal conduit or anything else ... " I said something like "Ummmm ... excuse me for interrupting, but ... just who is it you are trying to help him kill in his pool?"

Good point on the ambient temp derating Jerry.
Like i stated, where do we cut-off?
Especially when u find yourself repeating portions of previous threads.
Bob Smit

What about that pesky spray foam insulation actually sprayed INTO switch/j-boxes for "infiltration" How ridiculous is that?...

At an Electrical Inspectors' meeting late last year, a spray foam electrical draft stopping product was passed around which did not have a fire rating.
Now I can't remember the name of the product I so desperately committed to memory. Anybody know?
When I see a foam agent surrounding and always protruding into enclosure crevices, I ask to see the 'can'.

On a related note the IAEI published a report back in Sept. 2003 about the oversite of nm-b bundling and bundling in fire draft applications. The results are rather interesting. It was the general conclusion that bundling in shorter lengths than 24 inches could also be a major issue as ambient and conductor loading rise on any given day. I can't rmember if I posted this in the npast but here it is again just in case. It is an interesting read but it does take some time to absorb and get through the details. Since that article some of the issues have been addressed by current code such as derating in conduits exposed on rooftops.

Anyway here it is for those interested ....

Ambient Temperature Ampacity Corrections for Cable Bundling and Direct Solar Exposure | IAEI Blog

Thank you gentlemen. This was a very knowledgeable conversation.

Much appreciated

Mr. Smit, that could be an open cell foam manufactured by Icynene, But I dont want to change the direction of this thread...

AWESOME information gentleman, very informative!