# NM vs THHN Ampacity



## nap (Dec 4, 2007)

sirsparksalot said:


> Guys, I've been getting different answers from local electricians on the ampacity of 8 guage wire. Some say it will carry 50 amps, others (including the NEC charts) say 40 amp. Apparently, there's a difference between whether it is NM Romex or THHN.
> 
> So, my question is this: if one stripped the cable from 8/3 Romex (rated for 40 amp), and simply ran the individual wires as THHN, would this make a difference in the ampacity? Stated differently, are the wires within the Romex cable the same size as those sold as THHN in stores, and would that allow a 50 amp circuit?
> 
> I have licensed Master electricians telling me that 8/3 is good for 50amps because the wires within that romex are THHN, so I'm just interested in other opinions.


you cannot (legally) strip the conductors out of NM and use them let alone consider them to be THHN. NM is a cable and must be used as a cable. The components of the cable are not designed nor intended to be used individually.

this is not a matter of opinion (which your master electrician appears to be wrong anyway). The nec clearly states that NM is rated using the 60º conductor rating (334.80). For purposes of derating, the 90º rating can be used but the resulting ampacity must still be no more than the 60º rating.

that means; #8 nm is rated for 40 amps. It can not be used (in typical installations) for a circuit rated for more than 40 amps.

and to further confuse you; #8 THHN is actually rated for 55 amps, not 50 but that is a lesson for another day.


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## sirsparksalot (Oct 16, 2010)

OK, for underground wiring in conduit, would it be permissible to run THHN? 

I'm just guessing that the prohibition against Romex underground (along with the prohibition against stripping the outer sheath of Romex, and running it underground) has to do with the bare ground wire getting wet. If we run an insulated THHN ground wire underground in conduit, I assume that would be permissible. But if this is the case, how is it that the bare wire to a ground rod isn't similarly impacted by wet locations?

I'm not trying to argue anyone's advise, or belabor the rule; I just want to understand this, and I do appreciate your help.

Would anyone elaborate?


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## secutanudu (Mar 15, 2009)

Underground in conduit has to be THWN, not THHN. Most wire sold at your local stores will be rated for both uses (Look for THHN/THWN on the wire), and you choose the appropriate column in the NEC based on how you are using it.


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## oleguy74 (Aug 23, 2010)

the wire alone is not the only ampacity factor.it is the insulation that is used.basicly copper is copper.it would be highly recomended that you diy guys get an uglys electrical refference book.has wire tables and other good to know things.


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## nap (Dec 4, 2007)

sirsparksalot said:


> OK, for underground wiring in conduit, would it be permissible to run THHN?
> 
> I'm just guessing that the prohibition against Romex underground (along with the prohibition against stripping the outer sheath of Romex, and running it underground) has to do with the bare ground wire getting wet. If we run an insulated THHN ground wire underground in conduit, I assume that would be permissible. But if this is the case, how is it that the bare wire to a ground rod isn't similarly impacted by wet locations?
> 
> ...


 often times you can run a bare ground wire. 

the prohibition against stripping NM and running the individual conductors is exactly what I said previously. It is a cable. It is UL listed as a cable. It is designed to be used as a cable and any use of it outside of it's designed use is outside of it's UL listing and that means you cannot use it other than as a complete assembly.


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## McSteve (Dec 8, 2009)

And also the individual conductors in NM cable are generally not marked in any way. They may look like THHN, and in some cases may even _be_ THHN, but they're not labeled as such. So they cannot be used except as part of the cable, which is marked.


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## sirsparksalot (Oct 16, 2010)

Guys, I appreciate the comments and advise, and I don't disagree with what has been said. I agree that it's illegal to run NM underground, in conduit or not. I also agree that stripping the cable jacket from NM, and running underground is illegal. 

With that said, however, my question referred to _why_ this is so, and some have answered that for me, while others seem to be saying "just don't do it". That type of reply doesn't really help the untrained DIY, because, being untrained, we will naturally ask "Why?". 

Also, referring us to books isn't as valuable either, because we come here to hopefully get some quick replies for projects that might not allow the time needed to read books. And, too, the books generally don't go into the "whys" either. 

The "whys" are important to know for the sake of teaching our helpers who might be hard headed, and won't follow our advice when we can't anser the "why". 

In my case, I'm trying to help my buddy with his project, and in reply to my statement that it's illegal, he says, _"I don't care, I done mine that way, and it works; the worst that can happen is that a breaker will pop"._ Well, I don't know if that last part is true or not: maybe someone can help there, as well. 

While I intend to do my work legal and safe, I do appreciate knowing the consequences of the doing it the other way.

*On a side note*, would someone give me the NEC Code article number that disallows this so that I may refer it to one of my Master Electrician friends, because he (and other electricians) believes that we can strip the NM cable and run it as THHN in conduit? Of course, this doesn't make sense to me because if water damage is a concern, even when ran through conduit, stripping the outer jacket can only make it worse if the inside conductors of the NM are not insulated in the same way that THHN is. Also, is there a legal difference between the definition of "buried cable" and "cable ran in a raceway"?


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## oleguy74 (Aug 23, 2010)

the book uglys that i reffered you to is used by a lot of electricians.it is a refference book.it contains the same wire tables and other material that is in the code book.it is a field refference.if you look the wire striped from romex is not marked.if not marked or labeled,it cannot be used in place of rated wire.thhn,thwn,tw ect the insulation is rated for a specfic use.it can be confusing at times.along with the answers you get on here,get the uglys book.for 12$ it is a good ref.we are all here to help the best we can.


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## J. V. (Jun 1, 2007)

sirsparksalot said:


> *On a side note*, would someone give me the NEC Code article number that disallows this so that I may refer it to one of my Master Electrician friends, because he (and other electricians) believes that we can strip the NM cable and run it as THHN in conduit? Of course, this doesn't make sense to me because if water damage is a concern, even when ran through conduit, stripping the outer jacket can only make it worse if the inside conductors of the NM are not insulated in the same way that THHN is. Also, is there a legal difference between the definition of "buried cable" and "cable ran in a raceway"?


With all these professional electricians at your disposal, why are you here in an internet forum looking for answers? Master Electrician friends? 

That said, article 334 is designated to NM cable and that's where I would tell your so called masters where to start.


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## nap (Dec 4, 2007)

> In my case, I'm trying to help my buddy with his project, and in reply to my statement that it's illegal, he says, _"I don't care, I done mine that way, and it works; the worst that can happen is that a breaker will pop"._ Well, I don't know if that last part is true or not: maybe someone can help there, as well.


Ya, best of luck with that. Part of what we do is so the breaker will pop when needed. Doing things wrong can definitely cause a situation where a breaker won't pop when needed.

I guess he can explain to the insurance company that he didn't care if it was legal or not. He was going to do it his way.


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## sirsparksalot (Oct 16, 2010)

J. V. said:


> With all these professional electricians at your disposal, why are you here in an internet forum looking for answers? Master Electrician friends?


OK, so it's just one, but he IS in fact a Master who is also an EC. He does this for a living.

I feel kinda awkward going behind his back to ask these questions here, but I'm of the "trust but verify" persuasion, so that is why I come here.

So, a related question: if 8/3 Romex is okay for 40 amps, is it ok to use it for a 40a portable SPA? My concern would be startup current which would trip the breaker.


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## Techy (Mar 16, 2011)

the motor in a spa isnt the big draw, it's the heaters... they dont have a large 'kick-on' startup current, #8 is fine for a 40A spa


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## FutureSparky (Apr 15, 2011)

Check 110.2,110.3 (B), 334.80, and 310.11. Refer the "master electrician" to these codes also.


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## Saturday Cowboy (Nov 29, 2009)

This is my opinion, but I believe I am right. Romex is not allowed in WET locations. This prohibits its use in underground or outside conduits, and outside. I have been told and have ever reason to believe, that the paper inside romex love to wick up water, and there is a bacteria that then grows in this environment that is corosive to the insulation and will even begin to corrode the copper. I have seen some romex that has been very wet that makes me believe this story is likely.


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