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Old 12-12-2011, 11:13 AM   #1
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re: NM3 60 degree cable inquiry.


Hello,

I have NM3 60 degree celcius/ 140 degree farenheight cable in my walls.

I had planned to install insulation in my house where no insulation currently exists. In the past, if these wires heated up, the cavity is large enough that the heat could potentially dissipate relatively quickly.

However, if i insulate the walls, i insulate the wire, and it is the potential of overheating past the 60 degree rating.

What are some possible solutions to this problem besides rewireing everything?
Is there such thing as a 13A cicuit breaker to be installed my circuit breaker panel? (limiting the current, would decrease the maxiumum temperature.) would this actually work?

thanks


Last edited by gramps416; 12-12-2011 at 11:29 AM.
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Old 12-12-2011, 11:18 AM   #2
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re: NM3 60 degree cable inquiry.


Thats 140 degrees, so if you are overheating the wires, you need to find that problem and fix it.
That wire has been installed in insulation for years.

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Old 12-12-2011, 11:32 AM   #3
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re: NM3 60 degree cable inquiry.


Quote:
Thats 140 degrees, so if you are overheating the wires, you need to find that problem and fix it.
That wire has been installed in insulation for years.
sorry i meant 60 degrees Celsius. I am Canadian. i just corrected the original post, so yes 140 Fahrenheit. As stated originally there as never been any insulation in the wall. The person who has been living here before me was an old(er) man who only used lights to read his newspaper, and a T.V. or vacuum cleaner from time to time. He never really had a electrical demand that would max out the cable.

My extra usage and introduction of insulation is a concern.

Last edited by gramps416; 12-12-2011 at 11:35 AM.
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Old 12-12-2011, 11:52 AM   #4
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re: NM3 60 degree cable inquiry.


No need to worry all. NM cable is commonly has a 60 degree rating. Houses very rarely have 75 or 90 degree conductors unless they are installed in pipe. Which is usually overkill anyway!
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Old 12-12-2011, 12:43 PM   #5
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re: NM3 60 degree cable inquiry.


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Originally Posted by Holy Funk View Post
No need to worry all. NM cable is commonly has a 60 degree rating. Houses very rarely have 75 or 90 degree conductors unless they are installed in pipe. Which is usually overkill anyway!
NM-B type cable contains 90C conductors. This is the Romex type cable currently installed in most homes over the last few decades.
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Old 12-12-2011, 01:15 PM   #6
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re: NM3 60 degree cable inquiry.


you can insulate without problem, 60C cable have 15 amp ampacity for 14 ga wire, the 90C one has the same rating
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Old 12-12-2011, 04:58 PM   #7
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re: NM3 60 degree cable inquiry.


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you can insulate without problem, 60C cable have 15 amp ampacity for 14 ga wire, the 90C one has the same rating
That information was based on when no one had insulation in their walls. ratings change over time, but the cable still has the same stamp from when it was made.

To licensed electricians: I want to know, according to the NEC, does this statement still hold true today in 2011?

Last edited by gramps416; 12-12-2011 at 05:01 PM.
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Old 12-12-2011, 05:26 PM   #8
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re: NM3 60 degree cable inquiry.


Don't have the article in front of me but it is stated that NM cable, even rated at 90 degrees celsius, shall be derated from the 60 degree C in the ampacity table.
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Old 12-12-2011, 09:03 PM   #9
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re: NM3 60 degree cable inquiry.


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Originally Posted by Holy Funk View Post
Don't have the article in front of me but it is stated that NM cable, even rated at 90 degrees celsius, shall be derated from the 60 degree C in the ampacity table.
No, NM is derated from the 90C column if it contains THHN conductors, which most all of modern NM cable does. Its just that its derated value cant exceed that of the 60C column.

From the 2011 NEC:
334.80 Ampacity. The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The allowable ampacity shall not exceed that of a 60C
(140F) rated conductor. The 90C (194F) rating shall be
permitted to be used for ampacity adjustment and correction
calculations, provided the final derated ampacity does not exceed
that of a 60C (140F) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray shall
be determined in accordance with 392.80(A).
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 sealed with thermal
insulation, caulk, or sealing foam, the allowable ampacity
of each conductor shall be adjusted in accordance with
Table 310.15(B)(3)(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)(3)(a).
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Old 12-13-2011, 03:47 AM   #10
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re: NM3 60 degree cable inquiry.


Thanks SD. It's much easier when you actual have the article handy.
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Old 12-13-2011, 08:37 PM   #11
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re: NM3 60 degree cable inquiry.


i have tried my best to understand what the limits of the 60 degree cable is. Can someone explain this to the laymen?
-g
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Old 12-13-2011, 08:45 PM   #12
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re: NM3 60 degree cable inquiry.


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Originally Posted by gramps416 View Post
i have tried my best to understand what the limits of the 60 degree cable is. Can someone explain this to the laymen?
-g

The limit is that a #14 wire is rated at 20 amps, but code allows a max of a 15 amp breaker.
If your wires are getting close to 60 degrees, you have other problems.

That is layman terms.
Insulate the walls and don't worry about the wire.
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Old 12-14-2011, 02:34 PM   #13
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re: NM3 60 degree cable inquiry.


I didn't realize that the cable had more capacity than 15A, since that is code. That is what what I found confusing that NEC said 20A.

So you are right, I don't have anything to worry about.
-cory

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