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The answers to this question indicate that NM can be run in conduit: Can Romex (NM-B) be run through conduit?

I was hoping to enclose 12-2 NM-B in EMT for about 20 feet from the breaker box up my garage wall and into my attic to protect it from physical damage as needed when running perpendicular to the joists in my accessible attic. Then, the NM cable would just run the rest of the way unenclosed and parallel to a joist to a non-metallic box with integral cable clamps to feed my garage door motor: enter image description here

However, it looks like I may not be able to do so because of section 312.5(C).

Section 312.5(C) lists some additional restrictions which seem to indicate that cable cannot be fully enclosed in raceways (such as EMT) for a length greater than 10 ft if the raceway starts at a cabinet, cutout box, or meter socket enclosure -- I understand these definitions to include things known as service panels, breaker boxes, or load centers in language of the typical homeowner. I know there are other options to achieve what I want that do not raise this question (e.g. use something other than EMT for protection, run THHN inside the EMT and then use a junction box to transition to NM), but I would like to know if I'm correctly interpreting section 312.5 in this situation. I have quoted section 312.5(C) from the 2014 NEC below:

312.5 Cabinets, Cutout Boxes, and Meter Socket Enclosures. Conductors entering enclosures within the scope of this article shall be protected from abrasion and shall comply with 312.5(A) through (C).

(C) Cables. Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure. Exception: Cables with entirely nonmetallic sheaths shall be permitted to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 450 mm (18 in.) and not more than 3.0 m (10 ft) in length, provided all of the following conditions are met:

(a) Each cable is fastened within 300 mm (12 in.), measured along the sheath, of the outer end of the raceway

(b) The raceway extends directly above the enclosure and does not penetrate a structural ceiling.

(c) A fitting is provided on each end of the raceway to protect the cable(s) from abrasion and the fittings remain accessible after installation.

(d) The raceway is sealed or plugged at the outer end using approved means so as to prevent access to the enclosure through the raceway.

(e) The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than 6 mm (1⁄4 in.).

(f) The raceway is fastened at its outer end and at other points in accordance with the applicable article.

(g) Where installed as conduit or tubing, the cable fill does not exceed the amount that would be permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto. Informational Note: See Table 1 in Chapter 9, including Note 9, for allowable cable fill in circular raceways. See 310.15(B)(3)(a) for required ampacity reductions for multiple cables installed in a common raceway.

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    I would strongly suggest EMT the whole way (and then THHN makes a lot more sense than NM-B) - not as a code issue, per se, but if you are capable of working with EMT at all, the protection from things with teeth is nice, and you can USE MUCH smaller EMT if you are not doing the wire fill based on NM-B - which will make it cheaper, too.
    – Ecnerwal
    Dec 21, 2016 at 18:07
  • @Ecnerwal - Honestly, I like EMT, but NM-B is much easier to work with for complex bends and I'd rather not go through the hassle of making the last few complex bends. You are correct that conduit fill is definitely more of an issue cable assemblies, but a single 12-2 NM-B cable can generally fit inside 1/2" EMT (smallest trade size of EMT), no? The largest outer dimension of the 12-2 NM-B I am using is 0.45" (which calculates to a fill of 0.159 in^2) with a 53% (single cable assembly) conduit fill of 0.161 in^2 allowed for EMT.
    – statueuphemism
    Dec 21, 2016 at 19:14
  • it's fun playing with numbers like that, but that's one miserable pull, and leaves no expansion room. The right way is the junction box and the THWN-to-NM splice. This also gives expansion room for the future.
    – Harper - Reinstate Monica
    Dec 22, 2016 at 0:32
  • @Harper Its not actually 52% fill, that's just the code calculation treating it as though the elliptical cable is a full circle of the same diameter. I've done a pull of 12-2 through about the same length of 1/2" EMT (with 2 90 degree bends) for protection from damage and personally didn't find it to be too much of a bear.
    – statueuphemism
    Dec 22, 2016 at 1:39
  • I have covered the wire space with 1/2" plywood up a wall above the box that was accessible & on a 2nd floor to the attic. Using NM. I was able to use scrap and it passed inspection. This may not be the look you want but it is the easiest and cheapest way to go even if you have to purchase a sheet. The toughest part for you would be fishing the NM through a clamp in the back of the box. Raco makes some nice snap in clamps that are pushed in from the inside of the box I think they are called inside connectors. The connectors work with 14-2 through 10-3 with ground.
    – Ed Beal
    Dec 22, 2016 at 23:15

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After some reading, the interpretation in the question is correct.

In every edition of the NEC since 2002 when the clause was introduced, you cannot run NM-B in conduit for more than 10ft if the conduit run starts at a cabinet, cutout box, or meter socket enclosure -- this includes "breaker panels".

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  • I think this interpretation might be incorrect. It sounds like you're basing your assumption on the exception to 312.5(C), which says "Cables with entirely nonmetallic sheaths shall be permitted to...". This is simply saying that if you're using NM cable, you can enter the enclosure through a 10' section of rigid conduit without securing the cable to the enclosure.
    – Tester101
    Feb 8, 2017 at 13:21
  • @Tester101 To my knowledge, there is not a listed device for securing NM cable to the enclosure while also running it through conduit. Are you thinking using something like a UL listed zip-tie and a screw hole (e.g. globalindustrial.com/p/electrical/race-ways/cables-and-ties/…) to secure the NM cable to the enclosure inside of the enclosure itself?
    – statueuphemism
    Feb 8, 2017 at 19:05

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