NM-B wire vs THHN


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Old 05-27-24, 10:40 AM
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NM-B wire vs THHN

I plan on running an 8' cable from a Halex disconnect to a heat pump outside. I will use 1/2" liquid tight and plan to use some left over solid 12 ga wire stripped from some left over NM-B cable on hand. My other option is to buy a small roll of THHN. I would prefer to use the free NM-B wire. Is this acceptable?

Can I use black electrical tape instead of paint to mark a hot white wire?
 

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05-28-24, 01:05 PM
MiamiCuse
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I have some NMB cables that contain individually marked labeled THHN conductors, those can be stripped and reused.
 
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Old 05-27-24, 11:04 AM
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You really should use THWN in damp locations (and outside is always considered a damp location, even inside liquid tight),

The wires inside NM-B are not labeled for use except in the cable assembly.

You can buy THWN by the foot at any big box store or electrical supply house.
 
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Old 05-27-24, 11:30 AM
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Should you do it? No. As Carbide posted, the wires in Romex are designed to be used as a cable assembly and may not be rated for a wet location. Also, you may only reidentify a white wire as a hot in a cable, not in a conduit.

Can you do it?
 
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Old 05-27-24, 12:47 PM
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The job will be inspected so I don't want to have to redo it. I bought some THHN. Not cheap, had to wait 20 minutes until an employee unlocked the wire cage and then escort me to the cash register to make sure I didn't steal $16 worth of wire.

Shoplifters should be beaten with a 2'X4'in the parking lot and then run over by a forklift.
 
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Old 05-27-24, 02:21 PM
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I'm not offering this as an argument just a comment about my wire confusion. I don't have any arguing ammo. Just trying to understand why it has to be so muddled. At least it is in my muddled mind.

"THHN and THWN are the same" except that they aren't.
Allied Cable web site

"THHN and NM-B wires are the same" except they aren't.
Nassau Cable Web site and IEWC web site

From a couple of other cable web sites -
"In residential and commercial wiring, Romex (NM-B) and THHN are two of the most common types of cables used. The confusion is that NM-B wire is not that far from THHN in terms of construction. Whereas THHN is a cable with a single thermoplastic high heat-resistant nylon wire, NM-B wire is a sheathed cable with multiple conductors. The conductors inside the NM-B wire are either insulated or uninsulated, and the former is actually THHN, which commonly leads to confusion between the two types of cables.

I also discovered that good old Romex is not code in some localities and is prohibited from residential use.


It also leads to my confusion.
 
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Old 05-27-24, 03:03 PM
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"THHN and THWN are the same"
They aren't. The "W" in the designation specifies acceptable for wet conditions. So there's a big difference.
BUT, most single strand wire produced is dual-rated THHN/THWN. So chances are, the wire you purchase is in fact THWN rated. But it's important to understand, since you could run into THHN wire, which would not be acceptable for wet conditions and could degrade.

​​​​​​​"THHN and NM-B wires are the same"
Similarly, most (all?) NM-B is made out of that same THHN/THWN wire. Manufacturing-wise, it doesn't make sense to produce a bunch of different types of wire. So stripped NM-B should be the same THWN wire. BUT, it's not marked as such. So there's no guarantee that it is, and if it's inspected closely, being not marked means that it can't be approved for that use.


So at the end of the day, it would 99% be ok. But it's still wrong if used in the wrong application.
 
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Old 05-27-24, 04:58 PM
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Pretty good chance the THHN you bought will be dual labeled as THHN/THWN; just look at the printing on the wire. If it is, you're golden. If it's THHN only, well, it will depend on the inspector....
 
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Old 05-28-24, 01:05 PM
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I have some NMB cables that contain individually marked labeled THHN conductors, those can be stripped and reused.
 
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