Question about NM Cable

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  #1  
Old 02-17-07, 01:39 AM
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Question about NM Cable

I was wondering what type of insulated conductors are actually contained inside the sheathing of NM-B cable? Is it TW?

I know from reading NEC 334.80 that the ampacity of NM, NMC, and NMS cable is determined in accordance with 310.15, and with the 60 degree Celsius temperature rating. (Although the 90 degree Celsius rating can be used for derating purposes as long as the derated ampacity doesn't exceed that for a 60 degree rated conductor.)

I've also noticed that TW and UF are listed together in Table 310.16 under the 60 degree celsius column when determining ampacities of insulated conductors.

I have no reason for asking other than simple curiosity.

Thanks!
 
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Old 02-17-07, 06:44 AM
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Many manufacturers claim that the conductors inside NM cable are actually unlabeled THHN. However, the code does not recognize the conductors individually, and the rating is not printed on the conductor insulation; therefore they are technically unrated.
 
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Old 02-17-07, 09:28 AM
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I agree with everything Ben said. One note is that you are not allowed to treat these wires as THHN. You cannot use the THHN ampacity tables, and you cannot strip the sheathing and treat the individual wires as THHN. In fact, if you strip the sheathing, the wires are unusable for anything electrical.
 
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Old 02-17-07, 06:30 PM
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I have heard the THHN story too, but it seemed that if it were true, they would want to at least consider indicating this on the sheathing.

I have friends who have used the individual conductors as if they were THxx, but I have always advised against this, even though I didn't know for sure what type of conductors they were.

I agree with you both, and have always believed, that the conductors are useless once removed from the sheathing. However, telling my friends that didn't seem to stop them from using them anyway. Even when I pointed out 334.80 and 310.15.

Anyway, thank you both for responding and sheading some light on this mystery.
 
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Old 02-17-07, 07:57 PM
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Even if it were stated on the outside sheathing, you still cannot use the individual conductors and be within code.
 

Last edited by racraft; 02-20-07 at 05:05 AM.
  #6  
Old 02-19-07, 10:42 PM
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Good point racraft. I didn't mean to imply that the conductors inside could be used without the sheathing. I just meant that it would be nice to know what's in there.

Of course, if people knew it was THxx then they might be tempted to remove the sheathing and use the individual conductors. This may explain why NM sheathing contains no information regarding the conductors?

Thanks all!
 
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Old 02-20-07, 05:08 AM
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An electrician would not be tempted to remove the sheathing and use the wires, except perhaps in a very unusual situation. An electrician would have THHN/THWN and use it, if that is what was needed.

The only ones who might be tempted to strip the sheathing would be homeowners who don;t want to to make another trip to the store or who have the NM already and don't want to or cannot return it.
 
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