Do cables that are not NM type need to be derated when used as NM?

Question

I am a newbie in this forum but have been looking at some of the wiring in my house especially some XHHW-2 wiring used for the electric range. I need to have some clarification of cable terminology.

When someone refers to non-l(or NM) cables they are referring to a specific type of cable. What I mean by this is that the term NM does not include for example XHHW-2 cable. Correct? (I think I'm reading too much into the term "non-metallic")

I have an NM-B, copper two conductor 14 AWG with one 14 AWG ground cable running through the same hole in a floor joist as an aluminum XHHW-2 three conductor 6 AWG with 1 unshielded 6 AWG ground wire cable. There is no sort of filler and insulation in the hole.

Does this mean I do not have to derate the ampacity of these cables? Looking at various forums and reading the code I get that impression but then I notice almost all those forums are explicitly talking about NM cable - which takes me back to my first question.

If someone could help clarify things that would be much appreciated.

Answer 1

Neither cable needs to be derated here, for multiple reasons

First off -- you only have one NM cable (the 14/2 W/G) in this hole, as the other cable (the 6-6-6-6) is a type SE cable; the two cables (NM and SE) are distinct in their constructions and usages, and are covered by different articles in the NEC (NM in Article 334, SE in Article 338). As a result, the requirement in NEC 334.80 paragraph 2 that requires derating of multiple NM cables bundled through an insulation-packed hole in wood framing does not apply here to this situation, as there's only one NM cable to be had:

Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.

Furthermore, this requirement only applies to sealed openings in wood framing; since your opening is unsealed, this passage would not apply even if you had 20 NM cables in the same opening. Finally, this passage does not apply to SE cables anyway, as while SE cables are required to be treated as NM cables when used indoors by 338.10(B)(4)(a):

(a) Interior Installations. In addition to the provisions of this article, Type SE service-entrance cable used for interior wiring shall comply with the installation requirements of Part II of Article 334, excluding 334.80.

For Type SE cable with ungrounded conductor sizes 10 AWG and smaller, where installed in thermal insulation, the ampacity shall be in accordance with 60°C (140°F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, if the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.

, this passage explicitly excludes 334.80 from its requirements, as the wire insulation (often XHHW-2 or full THHN/THWN-2) on wires in SE cables is more robust than the PVC-only insulation used on the individual wires in a NM cable.

Answer 2

To add onto TPE's answer

When someone refers to non-metalic (or NM) cables they are referring to a specific type of cable. What I mean by this is that the term NM does not include for example XHHW-2 cable. Correct? (I think I'm reading too much into the term "non-metalic")

NM refers to sheathed cable containing individual wires (Romex is the trade name for Southwire's NM cable in the US). NM cable is available in a variety of wire sizes (14-6 gauge is available in NM cable from Southwire).

XHHW-2 is a single wire by itself, coated in cross-linked polyethylene (or PEX in plumbing terms). It's also commonly stranded, instead of solid core like NM. Unlike NM, XHHW-2 can handle higher heat (up to 90 degrees Celsius) and can be used in wet locations (although it's not direct burial and needs to be in conduit). You would more commonly find this on circuits higher than 30 amperes.

Answer 3

But just so you know, if they ran >24" in a conduit or raceway, the plain old NEC 310.15(b)(3)(a) derate rules would still apply.

With split-phase service (NOT 2-of-3-phases like NYC or Europe), all circuits contain 2 wires that count as far as this derate. (because other wires carry only differential current, e.g. a MWBC neutral, or travelers in a 3-way circuit). That would be 4 wires total in both cables.

4-6 wires (2-3 split-phase circuits) calls for an 80% derate, but off the maximum thermal rating of the wire, and NM and UF cables get to use 90C for this calculation due to an exception. The exception is why NM and UF cables are not listed in the 310.15(b)(16) chart. So:

• 14 AWG wire (25A@90C) derates to an absolute maximum 20A, but it's already limited to 15A due to NEC 240.4, so that's that.
• 6 AWG wire (75A@90C) derates to an absolute maximum 60A.

Answer 4

I just passed the electrical inspection for the 220V outlet and wiring. Thank you all for the directions and assistance! Jim