Expanding Foam Insulation

[nizak]

I know this topic has been addressed here but still have a question hoping someone can answer.
Currently is there any code that prohibits NM cable from being totally encapsulated in expandable foam insulation? The specific situation that I am finding is as such:

(1) nm cables stapled 2 per staple w/ approximately 1/2" spacing horizontally between them

(2)cables are run thru top plate (double 2"x4") 3-cables max. per hole. Distance down from top plate to staples is 30", 12" from that point to top of panel.


Would it be advisable to suggest that this stud cavity not be encapsulated with foam, and possibly just use fiberglass in this one area? I have had inspectors in the past tell me that they did not care for this type of insulation and felt that over heating could occur.

Any AHJ's here please chime in. Thanks.

 

[GoldDigger]

I know this topic has been addressed here but still have a question hoping someone can answer.
Currently is there any code that prohibits NM cable from being totally encapsulated in expandable foam insulation? The specific situation that I am finding is as such:

(1) nm cables stapled 2 per staple w/ approximately 1/2" spacing horizontally between them

(2)cables are run thru top plate (double 2"x4") 3-cables max. per hole. Distance down from top plate to staples is 30", 12" from that point to top of panel.


Would it be advisable to suggest that this stud cavity not be encapsulated with foam, and possibly just use fiberglass in this one area? I have had inspectors in the past tell me that they did not care for this type of insulation and felt that over heating could occur.

Any AHJ's here please chime in. Thanks.

There have been experimental studies that show that the temperature of a single run of NM inside even fiberglass insulation is much higher than the free air temperature of the same run. When you add multiple cables in close proximity it just gets worse. I would not be comfortable running full rated current through NM unless I knew that it only ran for short distances through foam. Maybe something along the lines of the 10% or ten foot rule, but much stricter. The same studies showed that the cooling effect of heat transfer along the length of the wire is limited enough that even going through a caulked fire stop caused significant overheating in the NM.

 

[infinity]

I know this topic has been addressed here but still have a question hoping someone can answer.
Currently is there any code that prohibits NM cable from being totally encapsulated in expandable foam insulation? The specific situation that I am finding is as such:

(1) nm cables stapled 2 per staple w/ approximately 1/2" spacing horizontally between them

(2)cables are run thru top plate (double 2"x4") 3-cables max. per hole. Distance down from top plate to staples is 30", 12" from that point to top of panel.


Would it be advisable to suggest that this stud cavity not be encapsulated with foam, and possibly just use fiberglass in this one area? I have had inspectors in the past tell me that they did not care for this type of insulation and felt that over heating could occur.

Any AHJ's here please chime in. Thanks.

I see no issue with either 1 or 2 even if encapsulated.

 

[nizak]

I can't count the # of times that I've run across an install where there were NM cables stuffed into a 24" PVC stub out of the top of the panel. Imagine what type of heat could generated in a 2" pvc stub that has a combination of 20 or 30 cables in it. I personally have never seen one that overheated and caught fire. Also, how many homes are still standing that have 2 or 3 cables crushed under a metal staple that were installed with a 24 oz framing hammer.

 

[suemarkp]

There have been experimental studies that show that the temperature of a single run of NM inside even fiberglass insulation is much higher than the free air temperature of the same run. When you add multiple cables in close proximity it just gets worse. I would not be comfortable running full rated current through NM unless I knew that it only ran for short distances through foam.

That shouldn't be earth shattering. Isn't a raceway method installed in an insulated wall going to have the same issue (although it may take longer to heat up)? Seems to me the problem with residential wiring is that it is all concealed and much of it is in insulated walls.

 

[electrofelon]

There have been experimental studies that show that the temperature of a single run of NM inside even fiberglass insulation is much higher than the free air temperature of the same run. When you add multiple cables in close proximity it just gets worse.

This whole thing seems silly to me. Not disagreeing with the quote of course, but just how much ambiguity and uncertainty there is with this whole wiring in insulation derating issue. Has no one ever tested this and come up with some actual numbers? Maybe Ill have a few beers, crank up the tunes, grab a can of great stuff and test it myself this weekend. Does anyone know what were the test conditions or assumptions for the "non-free-air" ampacity charts? Does fiberglass insulation result in conductors exceeding their temperature rating? How many bundled conductors? Repeat for closed cell and open cell spray foams..... #14,12, and 10 are already heavily derated so how does this factor in with the test results? With the increased use of spray foams I have been curious about conductor temperatures but havnt seen any hard data, just vague statements and waffling code.

 

[GoldDigger]

I read the report and I believe looked an associated video from a link in this forum. I will have to try to find it again.
If I recall correctly, the temps in fire caulk did exceed the design limits, while the temps in fiberglass were above the free air (in stud cavity) temps.

Tapatalk...

 

[mgookin]

...Currently is there any code that prohibits NM cable from being totally encapsulated in expandable foam insulation? ... Thanks.

The question is "is there any code that prohibits...".

Looks like the answer is no. Unless someone can come up with an enforceable code section.

Ratings are what something is tested to; they're not limits. You don't get fire or failure at 91C on something rated 90C. You don't vaporize at 56mph when you're in a 55mph zone.

Anything is going to hold more heat (dissapate less heat) when in an insulated environment. That means the insulation is doing its job. Tests and studies should show higher temps in this environment.

 

[ggunn]

This whole thing seems silly to me. Not disagreeing with the quote of course, but just how much ambiguity and uncertainty there is with this whole wiring in insulation derating issue. Has no one ever tested this and come up with some actual numbers? Maybe Ill have a few beers, crank up the tunes, grab a can of great stuff and test it myself this weekend.

Irrespective of the results of your test DO NOT get that stuff on your skin or clothes! I don't know what solvent they use, but I tried alcohol, acetone, gasoline, turpentine, and mineral spirits. Nothing I tried cuts it after it sets.

 

[kwired]

I can't count the # of times that I've run across an install where there were NM cables stuffed into a 24" PVC stub out of the top of the panel. Imagine what type of heat could generated in a 2" pvc stub that has a combination of 20 or 30 cables in it. I personally have never seen one that overheated and caught fire. Also, how many homes are still standing that have 2 or 3 cables crushed under a metal staple that were installed with a 24 oz framing hammer.

Most of your circuits in a dwelling have a low enough diversity factor that there is not that much significant heating, and with your 24 inch nipple, you still have a place for air circulation, now plug the top with expanding foam and you will trap more heat. You never see much heating effects in a dwelling unit loadcenter unless something is wrong, with the exception of electric heating or air conditioning circuits that have been running for hours. With those it is more common to find a warm breaker or cable, simply because of the continuous or near continuous demand. Most other circuits in dwellings either are not that loaded or if they are the demand is only for short durations. There apparently have been enough incidents where they are not willing to take that chance with NM cable though, as often the true load diversity is not really known.

That shouldn't be earth shattering. Isn't a raceway method installed in an insulated wall going to have the same issue (although it may take longer to heat up)? Seems to me the problem with residential wiring is that it is all concealed and much of it is in insulated walls.

It isn't that there is no heat developed in the cables, it is the fact that these spray foam insulations hold more of that heat in place for longer duration than other insulation types. It also isn't that we can not embed an NM cable in such insulation, but if we do we must derate its ampacity to help prevent overheating it. Just so happens no deration is necessary for a single cable or for two cables, just like no deration is required for less than 3 current carrying conductors in a raceway wiring method. I am a little surprised it just says two cables instead of mentioning number of current carrying conductors. What if you had a single cable that contained more current carrying conductors, than say three two conductor cables? Which one potentially creates more heat?

 

[Gregg Harris]

I am a little surprised it just says two cables instead of mentioning number of current carrying conductors. What if you had a single cable that contained more current carrying conductors, than say three two conductor cables? Which one potentially creates more heat?

310.15 Ampacities for Conductors Rated 0?2000 Volts.

(3) Adjustment Factors. (a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
334.80 Ampacity.
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)(3)(a).

 

[kwired]

I am a little surprised it just says two cables instead of mentioning number of current carrying conductors. What if you had a single cable that contained more current carrying conductors, than say three two conductor cables? Which one potentially creates more heat?

310.15 Ampacities for Conductors Rated 0?2000 Volts.

(3) Adjustment Factors. (a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
334.80 Ampacity.
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)(3)(a).

True, for some reason I blocked that out when I replied, and trust me I was thinking about it and almost did mention it in some way as part of the earlier part of my reply then somehow got fixed on the "two cable" wording.

 

[Gregg Harris]

True, for some reason I blocked that out when I replied, and trust me I was thinking about it and almost did mention it in some way as part of the earlier part of my reply then somehow got fixed on the "two cable" wording.

I can relate, I only posted to clarify to others reading.

 

[JDBrown]

Since NM-B is rated for 90 degrees but you have to use the 60 degree column, can you derate from the 90 degree column the way you would with, say, THHN that's being landed on 60 deg. terminals? If that's the case, then for a 20A circuit wired with #12's, you could have an adjustment factor of 67% and still be fine.

 

[ggunn]

Since NM-B is rated for 90 degrees but you have to use the 60 degree column, can you derate from the 90 degree column the way you would with, say, THHN that's being landed on 60 deg. terminals? If that's the case, then for a 20A circuit wired with #12's, you could have an adjustment factor of 67% and still be fine.

Are we still talking about expanding foam? How can you make any assumptions at all when you have conductors encased in insulation with far lower heat transfer than the native insulation?

 

[infinity]

I am a little surprised it just says two cables instead of mentioning number of current carrying conductors. What if you had a single cable that contained more current carrying conductors, than say three two conductor cables? Which one potentially creates more heat?

310.15 Ampacities for Conductors Rated 0?2000 Volts.

(3) Adjustment Factors. (a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
334.80 Ampacity.
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)(3)(a).

It says more than two cables. You could conceivably have two-four wire cables and 8 CCC's and still not require derating.

 

[JDBrown]

Are we still talking about expanding foam? How can you make any assumptions at all when you have conductors encased in insulation with far lower heat transfer than the native insulation?

I was more asking about derating NM-B in general, but I think it's relevant to the current discussion. As I understood the OP, the cables will be passing through expanding foam in the wall space as they go down from the attic to the load center.

I was just wondering, if you're derating your cables because they're passing through expanding foam, can you start your derating from the 90 deg. column since NM-B has 90 deg. insulation? Or are you not allowed to do that with NM-B?

 

[GoldDigger]

I was more asking about derating NM-B in general, but I think it's relevant to the current discussion. As I understood the OP, the cables will be passing through expanding foam in the wall space as they go down from the attic to the load center.

I was just wondering, if you're derating your cables because they're passing through expanding foam, can you start your derating from the 90 deg. column since NM-B has 90 deg. insulation? Or are you not allowed to do that with NM-B?

334.80 [2011] is pretty clear that with NM-B (90 degree) you must not exceed the basic 60 degree ampacity, just as for lower temp original NM, but you can start from the 90 degree ampacity column for derating corrections and adjustments.

 

[JDBrown]

334.80 [2011] is pretty clear that with NM-B (90 degree) you must not exceed the basic 60 degree ampacity, just as for lower temp original NM, but you can start from the 90 degree ampacity column for derating corrections and adjustments.

Thanks for clarifying. What I was getting at is that the OP will be able to use a derate factor as low as 67% before he would need to start up-sizing his conductors, using #12 NM-B copper on a 20A circuit.

 

[kwired]

It says more than two cables. You could conceivably have two-four wire cables and 8 CCC's and still not require derating.

You still have to derate with 8 CCC's, just so happens that normally the result of the deration doesn't require an increase in conductor size - especially when derating a 60 deg termination application while using 90 deg insulation conductors.