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#191813 01/14/10 03:51 PM
Joined: Jul 2007
Posts: 85
I'm seeing alot of new residential buildings with complete spray foam applications on tv. I'm wondering if anyone has come across a situation where the homeowner has wanted new wiring after the fact. this material looks as though it may not lend itself to being snaked. are electricians in the future gonna have to be like the plumbers and open up all walls where "our work" will be. typically people are kind of suprised at how few holes a seasoned electrician has to actually make to accomplish his task, i wonder if this new insulation technique is gonna prohibit this in the future.

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
You're not too far off! Only under certain lucky circumstances are you able to 'burrow' through the foam. One method some have suggested is the use of very hot metal balls to melt a chase through the foam - just remember that foam also burns real well.

The code is silent on the issue, but as a design choice I think even 'smurf tube' is a much better method than Romex for these applications.

Joined: Oct 2006
Posts: 745
Something tells me that Wiremold is going to start getting popular again.


"But the guy at Home Depot said it would work."
Joined: Mar 2005
Posts: 1,803

Whatever method you choose DO NOT try melting through PU foam to create ways. PU foam burns really well, ignites easily and gives off very toxic fumes when ignited - including CO, isocyanates and hydrogen cyanide. It starts to break down chemically at only 470F, a temperature easily reached by a holesaw drilling enthusiastically through sheetrock. I'd guess the ignition temperature will be about 600F. The dust is not nice either.
I once ran some copper pipes through a hole we had bashed through a granite wall here. I decided to fill the void with a can of pu foam before sheetrocking over. Then I had the 'brilliant' idea crazy of speeding up the cure with a gas torch! The speed and intensity of the resulting blaze was frightening, and if Mrs B had not kept calm and grabbed an extinguisher....

I'd say the only method would be to drill a line of holes slowly into the sheetrock with a holesaw, at intervals of maybe 8", and pick out a way with one of those breadknife-like rockers saws. Keep the discs and spackle them back in to repair the damage, but it is really difficult to hide the 'bumps'! I slipped a 1" x 4" piece of 1/2" ply through the hole and fixed discs with 2 buglehead screws when running a new satelite coax under polystyrene foam-backed plasterboard. We are still finding the beads!

Last edited by renosteinke; 01/14/10 07:54 PM. Reason: context
Joined: Feb 2008
Posts: 21
Thanks for the info,question will cables have to be derated
now that the cable is inclosed in the foam?I'am an electrical contractor and a lot of my work is trying get a new cable in
with out a lot of holes in drywall.I watch Holmes on Homes once in a while and he is a big fan of spray foam insulation
and I feel it will make our work much harder,as more homes are
done this way.

Joined: Jun 2006
Posts: 613
There is another problem with the spray foam but I don't have much more than sketchy memories. The foam must be applied under very strict and narrow conditions. We have seen the jacket of Loomex (Canadian Romex) damaged by foam that was either too hot or cold when sprayed in? The jacket became brittle and would break or crack at bending. Sorry it is a little cryptic. If I had a preferred way it would be ENT (smurf tube) and pull wires in after. As for derating? Not until there are more than 3 - 2 wire cables in contact for at least 2 feet. NMD90 is approved for installation in thermal insulation so single cables would not require derating.

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Derating? What, me worry?

NEMA did a study that concluded that simply having Romex encased in foam was no realson to have to derate the conductors. Go ahead, use the 75 degree table.

THEN the NEC decided that if the cables pass through a framing member that needs to be firestopped (like the top plate, you need to derate. Hello 60 degree table.

The reality> The NEC already massively derates the table values for the smaller wires (#'s 14, 12, 10), so the effect should be minimal. In the usual house, this means #6 instead of #8 to the range, and maybe #8 to the air conditioner.

Joined: Mar 2007
Posts: 165
Originally Posted by jackolsen
...I watch Holmes on Homes once in a while and he is a big fan of spray foam insulation...

Same here. Not to take anything away from Mr. Holmes, but he seems to proceed on the concept that nobody is ever going to have to come after him and change or add. I'm not sure that's 100% reasonable.

Joined: Jul 2004
Posts: 9,800
Likes: 17
I wonder if you can bore through this with a divirsabit going slow in a battery drill? Just be careful you don't hit an existing wire wink
If you got a hole in there I bet you could use it like a chase but you would have to start with something rounder and softer than the end of a cut romex. Maybe suck some jet line through with a vacuum and pull your cable.

Greg Fretwell
Joined: Apr 2002
Posts: 7,336
Likes: 7

From the '08 Handbook..
"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 60C (140F) conductor temperature rating. The 90C (194F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60C (140F) 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)."

Am I missing something, based on your comments above?

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