NEC 310 and NM Cable Inner Conductors

[wwhitney]

310.1 (Scope) says in part "These requirements do not apply to . . . conductors specifically provided for elsewhere in this Code."

310.4 says "Insulated conductors shall comply with Table 310.4(A) and Table 310.4(B)," but the inner conductors in NM cable don't match any of the entries in Table 310.4(A). Indeed 334.112 says "The insulated power conductors shall be one of the types listed in Table 310.4(A) that are suitable for branch-circuit wiring or one that is identified for use in these cables." The listing standard for NM cable identifies the insulation used on the power conductors in lieu of complying with Table 310.4(A).

310.8(A) says "All conductors and cables shall be marked to indicate the following information, using the applicable method described in 310.8(B):" But NM cable inner conductors are not marked and so don't comply with 310.8(A).

My conclusion is that NM cable inner conductors only comply with Article 310 via the exclusion in the scope in 310.1 for "conductors specifically provided for elsewhere in this Code," in this case provided for by Article 334. That scope exclusion waives all requirements in Article 310.

Therefore NM cable inner conductors are not subject to 310.10(H) on parallel conductors, and it is OK to parallel NM cable inner conductors in any size.

Comments? : - )

Cheers, Wayne

 

[tom baker]

Is NM cable available in the minimum size required to parallel conductors?

 

[wwhitney]

Is NM cable available in the minimum size required to parallel conductors?

That my whole point, based on the logic chain given, NM cable is exempt from Article 310, so the limits in 310.10(H) on parallel conductors don't apply to NM cable.

Cheers, Wayne

 

[Dennis Alwon]

Wayne, you are incredibly smart, IMO however I think you have delved into this too far and it would never fly. My point is why go this route? Btw, nm cable is labeled on the jacket and 334.100 tells us what temp. rating these wires are

 

[wwhitney]

Wayne, you are incredibly smart, IMO however I think you have delved into this too far and it would never fly.

Maybe. It's certainly not the intent. But the language in 310 needs tweaking to avoid this logical paradox.

Do you at least agree that NM cable does not comply with 310.8(A) as written?

I mean, if 310.8(A) said that when the cable is marked, the individual conductors don't have to be marked, then all would be copacetic on that point. But it doesn't say that, it says the cable and conductors shall be marked.

Cheers, Wayne

P.S. This line of thought arose out of responding to the thread https://forums.mikeholt.com/threads/15-minute-fire-resistance-rating-for-nm.2582778/ I was going to say that if you run NM cable in EMT, you don't need article 334 on NM, you just need article 358 on EMT. But then I saw that NM cable is not listed in Table 310.4(A), so that doesn't work. Which led me to consider the consequences are from that omission.

 

[tortuga]

FWIW messenger supported wiring is the same way, the XLPE conductors don't need to meet 310 requirements, they don't even need to be listed or use a NEC ampacity table.
Cable can be pulled into EMT, as long as a fill calc is done. And that fill calc has to consider shape of the cable, not the conductors in it.
358.22
I think we should move on & get some folks on here to comment on all your great PI's

 

[tom baker]

That my whole point, based on the logic chain given, NM cable is exempt from Article 310, so the limits in 310.10(H) on parallel conductors don't apply to NM cable.

Cheers, Wayne

Ok so what is the minimum size conductor to parallel cables?

 

[wwhitney]

Ok so what is the minimum size conductor to parallel cables?

If 310.10(H) doesn't apply, then there is no restriction. Unless some article other than 310 has a blanket prohibition on paralleling conductors unless otherwise allowed in the Code. Which I'm not aware of.

Cheers, Wayne

 

[wwhitney]

FWIW messenger supported wiring is the same way

Agreed, Table 396.10(A) permits "Other factory-assembled, multiconductor control, signal, or power cables that are identified for the use." The scope of 310.1 then exempts such cables from the requirements of Article 310. So if you had two such cable assemblies that are messenger supported, you could parallel #8 Cu wires, for example. At each end of the messenger supported segment, if you transition to a wiring method subject to Article 310, you'd have to stop paralleling and splice to a single wire per conductor.

Cheers, Wayne

 

[tortuga]

310 only applies to conductors, like THHN on a spool you pull in a raceway, not factory made cables.
but 300.3(B)(1) reaches thru 310, bypassing the scope of 310, and calls out 310.10(G). [2023 code refs]

 

[wwhitney]

310 only applies to conductors, like THHN on a spool you pull in a raceway, not factory made cables.

Well, it still applies to the conductors within those cables if they are one of the types listed in Table 310.4(A) and we don't have separate permission from another article that allows the use of those conductors (like Article 396 on Messenger Wiring and Article 334 on the unlabeled inner conductors of NM cable).

but 300.3(B)(1) reaches thru 310, bypassing the scope of 310, and calls out 310.10(G). [2023 code refs]

OK, that works for cables with non-310 conductors that have a ferrous sheath, as the only way to comply with 300.3(B) with parallel conductors is via 300.3(B)(1). But if the cable has a non-ferrous sheath, we don't need to use 300.3(B)(1), we can just use 300.3(B)(3).

Cheers, Wayne

 

[tortuga]

Well there you go, I suppose you could check the listing of the cable and fall back to 110.3(B)

 

[electrofelon]

The mystery of NM inners is on my list of annoying NEC things that would be nice to have cleared up (its partly an issue with the product standard). I Am not sure why NM cant just have THHN inners that are marked, then that would eliminate all these issues of stripping and running in conduit, using pieces of inners for pigtails etc. Better yet make the inners THWN and then you can strip and run through outdoor conduits. That would also, in general, allow NM in wet locations if in a raceway. seems like a no brainer to me. But no, for some reason the product standard has to say things like "the conductors shall meet the same specifications as THHN" but dances around actually saying they must be THHN (dont remember if that is the exact wording, but its some thing like that).

 

[don_resqcapt19]

The mystery of NM inners is on my list of annoying NEC things that would be nice to have cleared up (its partly an issue with the product standard). I Am not sure why NM cant just have THHN inners that are marked, then that would eliminate all these issues of stripping and running in conduit, using pieces of inners for pigtails etc. Better yet make the inners THWN and then you can strip and run through outdoor conduits. That would also, in general, allow NM in wet locations if in a raceway. seems like a no brainer to me. But no, for some reason the product standard has to say things like "the conductors shall meet the same specifications as THHN" but dances around actually saying they must be THHN (dont remember if that is the exact wording, but its some thing like that).

The product standard, UL 719, permits the conductors to be of "THHN construction" and meet all of the insulation thickness and other applicable requirements for Type THHN thermoplastic-insulated wire, but prohibits marking the conductor with "THHN", "-B" or any ampacity or temperature marking. It also permits the manufacturer to use other conductor insulations that are electrically and mechanically similar to THHN.
So we really have no idea of what any given manufacture has used in the construction of their NM cable.

 

[tortuga]

I run into allot of old romex that has the inner conductors marked 'TW', and also has green insulation on the EGC.
But yeah NM is a bargain basement product.
I think they should allow joist pull 'JP' tray cable to be used anywhere NM is and give it its 75C rating.
Then when you have situations that call for a higher quality cable contractors can use JP tray cable.

 

[letgomywago]

I run into allot of old romex that has the inner conductors marked 'TW', and also has green insulation on the EGC.
But yeah NM is a bargain basement product.
I think they should allow joist pull 'JP' tray cable to be used anywhere NM is and give it its 75C rating.
Then when you have situations that call for a higher quality cable contractors can use JP tray cable.

I'd be happy to use tray cable instead of UF for outdoor lights ect... way better product for above ground damp locations

 

[mtnelect]

This all started with the Federal Housing Authority requiring AHJ adopt their standards to be eligible for federal funding on low-income housing, back in the 60's.