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#192300 - 02/06/10 03:25 AM Foam insulation cable derating  
Check Pilot  Offline
Member
Joined: Nov 2005
Posts: 141
Edmonton Alberta Canada

It's been a while since I've had to ask a question here, now that I have retired from the EC business. (and my other job as a Civil Aviton Inspector as well - it's nice to get older and retire).

However, once the neighbours know about your past, I get questions and asked for advice about electrical installations. One of them has me flummoxed since I had not run into this one before.

He is doing a new basement installation to add a new living space in the basement and the 2X6 walls are being wired for all kinds of stuff including CAT 5 and every other kind of who knows what its for, cable including the basic electrical outlets, switches and fairly elaborate lighting system in a suspended ceiling system.

However there are a few places where 4 NMD 90 cables could be run up the 2X6 studs from just above the concrete floor level to the ceiling and these are on outside walls that he would like to spray with foam insulation.

I have checked both the CEC and NEC codes ad nauseum, about spraying insulation into the interior of the wall stud cavities and can find about zero information about any requirement to derate the cables for that kind of insulation. We all know about the usual batt insulation stuff but it seems to me that maybe the same rules that might not apply since there is no air flow in spray insulation as in fiberglass or rockwool insulation.

Has anyone here have information, experience or a code requirement about spray foam insulation applied on outside walls that require derating of the conductors?

Last edited by electure; 02/06/10 10:24 AM. Reason: Double Posting

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#192303 - 02/06/10 11:35 AM Re: Foam insulation cable derating [Re: Check Pilot]  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
The NEC does not differentiate between fiberglass or spray in foam. If you look at 334.80 in the 2008 NEC, the last paragraph requires an ampacity adjustment.

If you do the ampacity adjustment, you'll start in the 90 degree column and adjust that with a 70% multiplier for 8 current carrying conductors. That works out to 21 amps for #12 and 17.5 amps for #14, both of which exceed the maximum circuit breaker size for both sizes.

I'd say that there is no problem running 4 two conductor cables.

Tom


Few things are harder to put up with than the annoyance of a good example.

#192304 - 02/06/10 11:47 AM Re: Foam insulation cable derating [Re: Tom]  
leland  Offline
Member
Joined: Aug 2007
Posts: 853
Lowell area, Ma. USA
[quote=Tom]The NEC does not differentiate between fiberglass or spray in foam. If you look at 334.80 in the 2008 NEC, the last paragraph requires an ampacity adjustment.

If you do the ampacity adjustment, you'll start in the 90 degree column and adjust that with a 70% multiplier for 8 current carrying conductors. That works out to 21 amps for #12 and 17.5 amps for #14, both of which exceed the maximum circuit breaker size for both sizes.---


My guess is it will soon. My only caution with the foam is,it gets hot as it cures,so any low volt use a good PVC/Poly insulation.I have had times where the cheap typical phone cable has sufferd damage.
I'd say that there is no problem running 4 two conductor cables.


Last edited by leland; 02/06/10 11:48 AM.

#192314 - 02/06/10 07:22 PM Re: Foam insulation cable derating [Re: leland]  
Obsaleet  Offline
Member
Joined: Apr 2003
Posts: 362
Pa
I would be more concerned if the the foam would react with the rx insulation.

Ob


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#192317 - 02/06/10 08:07 PM Re: Foam insulation cable derating [Re: Obsaleet]  
Texas_Ranger  Offline
Member
Joined: Dec 2001
Posts: 2,405
Vienna, Austria
Yes, conduit seems like a good idea. Some kinds of foam insulation (particularly the stuff sold in small cans to the homeowner) are supposed to react with PVC and soften it. Better safe than sorry!


#192319 - 02/06/10 08:36 PM Re: Foam insulation cable derating [Re: Texas_Ranger]  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Over here in New Zealand we use a different grade of PVC cable for these sorts of installations.
It's called Non-Migratory PVC.
Here's a link to the manufacturers specification:

Clicking this link will open a PDF file


#192322 - 02/06/10 10:15 PM Re: Foam insulation cable derating [Re: Trumpy]  
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
Derating of Romex cable starts at the rating of the cable: 60 degrees Centigrade.

This reduced rating vs the underlying THWN-2/ or THHN reflects the fact that the cable sheath insulates...

The rise in super-insulated homes may well cause the Code Committee to revisit this issue. But as it stands, no NEC provision exists to derate based upon spray in foam. For my money it should be treated as if were running underground.


Tesla

#192323 - 02/07/10 12:19 AM Re: Foam insulation cable derating [Re: Tesla]  
leland  Offline
Member
Joined: Aug 2007
Posts: 853
Lowell area, Ma. USA
1981- 2010- derated once.

Never ever to this day had a problem. In any application.


#192331 - 02/08/10 12:26 PM Re: Foam insulation cable derating [Re: leland]  
mbhydro  Offline
Member
Joined: Feb 2003
Posts: 347
Winnipeg MB Canada
If you watch any of the Mike Holmes renovations shows from Canada if he has an open outside wall open on the inside, he foams it.

I am sure I have seen him order foam for existing walls with the old "fabric" romex, and you know how anal he is about doing things to right to code.


#192332 - 02/08/10 02:46 PM Re: Foam insulation cable derating [Re: mbhydro]  
KJay  Offline
Member
Joined: Nov 2007
Posts: 763
MA, USA
As far as I can tell, the derating requirements of 334.80 only apply to more than two NM cables with two or more current carrying conductors if you don't maintain spacing.
IMO, since the spacing is not defined, it could be 0 to 1mm or even 10-feet... there is no specific distance required top to bottom, or side to side, whether thermal insulation is involved or not. Likewise when any number of NM cables is installed through the same hole in wood framing whether it is to be draft stopped or not.
So, I guess if you have metal framing... no worries?

The way I see it, you could drill a single 1-1/4-inch hole in a wood top plate and run as many NM cables through it as will fit without derating, as long as the cables are evenly stacked top to bottom and side to side. There also doesn’t appear to be anything preventing you from changing direction of the cables to a different position once they pass thought the hole in the framing, as long as they all transition in the same direction with the same spacing, which in this case, could be any desired amount.

I don't write it... I just read it.


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