All NM Cables

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knot stable

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Just was wondering if anyone knows what type of insulation is on the conductors of NM cable?( i.e. TW, THHN,THW) Not the outside jacket the internal conductors. Are they 60 degree, 75 degree or 90 degree rated? I think outside the box to many times and sometimes I get lost.:grin:I have not checked the book yet.It's in my truck.
 
All NM Cables

knot stable said:
Just was wondering if anyone knows what type of insulation is on the conductors of NM cable?( i.e. TW, THHN,THW) Not the outside jacket the internal conductors. Are they 60 degree, 75 degree or 90 degree rated? I think outside the box to many times and sometimes I get lost.:grin:I have not checked the book yet.It's in my truck.
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NM cable is 90 degree listed. But look at 334.80 it will tell you to use the 60 degree conductor rating for ampacity. You can use the 90 degree rating for derating.
 
Romex (NM) cable has been around since 1926 the first insulation on conductors were rubber. In the mid 1950's the wire industry transitioned to thermoplastics of either 60 or 75 degrees. In the mid 1980's 90 degree cable was mandated for NM-B cable. This information is from Some History of Residential Wiring Practices in the U.S. by David Dini P.E. Underwriters Laboratories.
 
This was discussed in another thread.
However there still has not been a industry reason for the ampacity limitation the NM has. I realize many feel it is an inferior type of wire but is that really the reason?
 
Sierrasparky said:
This was discussed in another thread.
However there still has not been a industry reason for the ampacity limitation the NM has. I realize many feel it is an inferior type of wire but is that really the reason?
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My guess is that it has to do with the frequency of installation into attic spaces which tend to be a bit on the warm side half the year.
 
From Southwire's Technical data on NM:
Conductor insulation is 90?C-rated polyvinyl chloride (PVC), nylon jacketed

You will note it is only described as an insulated conductor with no desigination such as THHN, etc. In that the conductor itself is not a type recgonized in 310.13 and is therefore not allowed as an ungrounded wiring conductor when stripped from the NM.​
 
augie47 said:
From Southwire's Technical data on NM:
Conductor insulation is 90?C-rated polyvinyl chloride (PVC), nylon jacketed

You will note it is only described as an insulated conductor with no desigination such as THHN, etc. In that the conductor itself is not a type recgonized in 310.13 and is therefore not allowed as an ungrounded wiring conductor when stripped from the NM.​
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So we should carry some solid THHN for pigtailing, right? :cool::grin:
 
augie47 said:
From Southwire's Technical data on NM:
Conductor insulation is 90?C-rated polyvinyl chloride (PVC), nylon jacketed​


You will note it is only described as an insulated conductor with no desigination such as THHN, etc. In that the conductor itself is not a type recgonized in 310.13 and is therefore not allowed as an ungrounded wiring conductor when stripped from the NM.​
Click to expand...

Thanks for that Gus. I had a guy pull some out of an in slab conduit and told him that it's not listed for that use and of course I got the "we do it all the time" story. So he brought in a piece he had and even though it looked like THHN, I told him it's not marked.
 
LarryFine said:
I can't help but wonder why they bothered.
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The designation change from NM cable to NM-B cable was made for identification purposes.
When the standard changed requiring the 90C rating of the conductors within the sheathing, there was no easy way for people (installers, supply houses, and inspectors) to be sure which cable they had, the existing style NM cable or the new style NM cable. So the new style cable was designated NM-B.
 
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