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MasterTheNEC

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That would be awesome! :)

I guess I am wondering what is holding NM back to 60*C when it could in theory handle 75*C. Type AC cable can run at 75*C when not in thermal insulation, and my understanding MC is regardless.

Well that I am not old enough to answer my friend...lol...I know that the 60C has been in the NEC a long time and while we do have studies to show the affects of Type NM Cable installed in thermal insulation to warrant an adjustment I would guess the roots of the 60C stem from UL 719 and the STP's that worked on that during it's development.

If time permits I am sure I can research the answer to that question. However as it stands now the main limiting factor is the UL 719 which of course led to the language in 334.80.

I do believe their are some proposals coming up (UL not NEC) to mandate a temperature rating on the sheathing of Type NM..but I can tell you based on the material it will be equivalent to or exceed that of the 60C...just based on the 30mils thickness and compounds...but i will look into it for you my friend and post what I determine.
 

mbrooke

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Well that I am not old enough to answer my friend...lol...I know that the 60C has been in the NEC a long time and while we do have studies to show the affects of Type NM Cable installed in thermal insulation to warrant an adjustment I would guess the roots of the 60C stem from UL 719 and the STP's that worked on that during it's development.

If time permits I am sure I can research the answer to that question. However as it stands now the main limiting factor is the UL 719 which of course led to the language in 334.80.

I do believe their are some proposals coming up (UL not NEC) to mandate a temperature rating on the sheathing of Type NM..but I can tell you based on the material it will be equivalent to or exceed that of the 60C...just based on the 30mils thickness and compounds...but i will look into it for you my friend and post what I determine.

That would be of great help. I am really interested in this, it peaks my curiosity.

Thank you :)
 

MasterTheNEC

Host of ElectricianLIVE.com
Location
McKinney, Texas
Occupation
Master Electrician & Director of Codes and Standards
That would be of great help. I am really interested in this, it peaks my curiosity.

Thank you :)
@mbrooke-

After doing a bit of digging I did find out a few things. Currently the thermal limits on the PVC material being used for Type NM-B sheathing has a usable threshold of -40C to 80C while some premium PVC can range even lower and higher. The recommended lower and upper limits for "installation" (with a 24 hour acclimation rate) is -10C (14F) as the lower installer temp thresholds. The compounds themselves in Type NM-B products used for the jacket are of sufficient mils thickness (the thinnest being 30 mils)...

So the insulation on the conductors themselves is in many cases thinner than the PVC jacket so operating the conductors within their ratings would also limit the exposure to the nonmetallic sheathing also. i can only assume that this is why no PVC jacket ratings are required...limiting the use to the 60C for Type NM-Cable by proxy protects the sheathing also....when the thermal conditions are violated on the inner conductors insulation the outer sheathing is not designed to add any thermal protection...it is a protective sheathing so it breaks down under the exceeded thermal condition at a parallel pace.

If I find any additional info I will gladly post for you.....FYI to others....when I post @someone.......it's for them.....;)
 

mbrooke

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So in theory at minimum the outer jacket can go up to 80*C, thus 75*C wire will heat the insulation, but it will dissipate heat accordingly?
 

MasterTheNEC

Host of ElectricianLIVE.com
Location
McKinney, Texas
Occupation
Master Electrician & Director of Codes and Standards
So in theory at minimum the outer jacket can go up to 80*C, thus 75*C wire will heat the insulation, but it will dissipate heat accordingly?
I believe that the PVC Sheathing (jacket) would be equal or exceed the use rating of the Type NM-B Cable. Again I am not aware of any test for that in terms of UL 719 but that is that is expressed in terms of the PVC material used for the sheathing.

I have not gotten more details as I was swamped last week and sick all weekend. I will dig more this week.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
I believe that the PVC Sheathing (jacket) would be equal or exceed the use rating of the Type NM-B Cable. Again I am not aware of any test for that in terms of UL 719 but that is that is expressed in terms of the PVC material used for the sheathing.

I have not gotten more details as I was swamped last week and sick all weekend. I will dig more this week.

Sounds good! And I hope you feel better. :) I was sick a while back, it sucks.
 
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