I own an old house (50s) for which the electrical is outdated (no ground, etc), so I am in the process of rewiring the whole house. The attic has tons of insulation (loose insulation atop fiberglass batts, etc), and below the insulation often it has plywood boards (subfloor). Navigating and finding anything through all this insulation is a real pain and time consuming.

Since much of the wire will be routed onto the top of the plywood (and not beside the joists at 1 1/4" from their edges) once I run the wires parallel to the joists (joists are below plywood subfloor) I would imagine that the wire would need to be protected by a conduit or wood strips; so can one use emt conduit instead of wood strips on top of the attic subfloor and could the conduit be attached to the attic subfloor wherein the wires would run (and in case someone would go the attic he/she could step on the conduits)? It would be romex (sometimes thhn) inside the conduit (and the conduit would be derated to accommodate the nm-b wiring).

By the way, the attic is accessible; and the conduit seems a much better option due to make it easier to upgrade and change the wires in the future.

  • Is the attic living space? – wallyk Oct 16 '15 at 4:25
  • wallyk: no, it is not a living space at all. – tk3000 Oct 24 '15 at 5:55
  • Still on the subject of emt conduit inside walls, I would imagine that given that the romex is not requried to be in conduits whilst inside walls then the fact that the romex is not inside a conduit all the way would then acceptable. As depicted below: ------------<br><img src="farm1.staticflickr.com/646/21915938549_249dd3e44b_b.jpg" alt="Emt conduit inside walls"> <img src="farm6.staticflickr.com/5688/21804948284_42e7d78c6c_b.jpg" alt="Emt Conduit in walls"> <img src="farm1.staticflickr.com/590/21804948314_f1214b5b3b_b.jpg" alt="Wall+Rigid Insulation+Emt conduit"> – tk3000 Oct 24 '15 at 6:44
  • (CONT...) In this particular situation the electrical box is inside the concrete exterior wall since there is a cavity in the concrete wall for the box only (and no indentation in the walls for any wire), so I can not connect the emt conduit to the box (yep, I reamed the conduit and also using bushings whenever the wires exit the conduit). Also, I don;t want to drill larger holes in the joists (at least not with the exterior walls) for the conduit, so the conduit does not go all the way to the attic; and once in the attic the wire should travel in conduits unless very impractical. – tk3000 Oct 24 '15 at 6:45
  • (CONT...) Given that the wire is not travelling 100% inside inside the conduits I assume that derating is not necessary (as well pointed out in this forum) and that I can fill the conduit even to 100%. I just want to double check and make sure that this is acceptable? – tk3000 Oct 24 '15 at 6:45

Other than the part where you are putting Romex (NM-B) in the conduit, I not only like your plan, I'm doing something like it myself.

I've seen too many rodent-chewed wires to be overly comfortable any other way, no matter if code would find less protection acceptable. I don't find it acceptable, and the cost is not all that great.

If the conduit is sized for the NM-B (as a round wire the diameter of the large dimension of the oval) you CAN do that, but it's miserable to pull and inefficient (of conduit size) .vs. just using THHN when in conduit. If leaving conduit, make a junction in a junction box when transitioning to NM-B.

While not having rodents in the house is a great idea as well, it's one that's hard to guarantee in practice over the long term...

  • On the subject of romex, I know it is likely more cumbersome to route it through the conduit, but it offers a little bit extra protection and allows me to use it in situation whereby a conduit may not fit (some sections of concrete walls) Revelant calculation of the derating factor for the romex wires in emt conduits since there is not derating table for nm-b wires on NEC (Chapter 9 of the NEC lacks any tables with acceptable dimensional areas for romex cables for the purpose of area filling inside conduits given that most people don't use romex inside conduits), as follows: (next post) – tk3000 Oct 16 '15 at 23:22
  • => Cross-sectional area of 14-2 calculation: ((7/16”)/2)^2*pi = 0.1503”, thus the cross-sectional area for a 14-2 romex is about 0.1503”. => According to NEC's chapter 9 tables the inside cross-sectional area of a 1/2” EMT conduit is equals 0.30385. Considering that the 14-2 romex is seens as a single conductor one can use the 53% for the conduit fill area which then renders: 0.30385(.53)=0.161” – tk3000 Oct 16 '15 at 23:25
  • And since 0.161” > 0.153” one can use nm-b 14-2 inside 1/2” EMT conduits. But similar calculations indicate that the nm-b 12-2 would not be acceptable inside a 1/2” EMT conduit. – tk3000 Oct 16 '15 at 23:25
  • Yep, rodents is always a concern for me too. I still have to find a house that is completely rodent prove. But easy of upgrading and change over time is also a major plus. – tk3000 Oct 17 '15 at 0:46
  • 2
    Also note that if you're not installing a "complete conduit system", you can technically fill to 100%. So for example, if you only have to cross the floor for a couple feet. You could pull the cable through a short piece of conduit, just to protect it for that short span. In this situation, it's not considered a "conduit system". See NEC chapter 9 Notes to Tables (2) Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections of conduit or tubing used to protect exposed wiring from physical damage. (CONTINUED)... – Tester101 Oct 17 '15 at 0:46

If you're running the wiring along the floor of the attic; or within 7' of the floor, you'll have to protect the wiring within 6' of the point of access. Unless the attic is accessible via a permanent staircase or ladder, then you'll have to protect the wiring run across the floor (or within 7' of the floor) throughout the entire attic.

I've quoted the actual code on this elsewhere on the site

  • This attic is not a livable area and it is full of loose insulation, plus it mostly have a very low celing clearance. But at some point in time someone decided to install plywood sheets onto the joists in portions of the attic probably to make it safer move around there. The attic has pull down ladder for access. So, I have to protect the nm-b wires within 6 feet from the attic entrance opening (where the pull down ladder stays) by means of conduits. Does that apply for situations where the wire runs besides the ceiling joists? And when the wires running onto the attic subfloor? – tk3000 Oct 16 '15 at 23:40
  • 2
    @tk3000 No. Because there's a pull down ladder, you have to protect the cable throughout the entire attic. A pull down ladder is a 'permanent staircase or ladder'. Securing the cable to the face (side) of the joist, is considered protecting it. You'll only have to add protection, where the cable crosses over joist or the insulation/subfloor. – Tester101 Oct 17 '15 at 0:36
  • "livable" or "not livable" makes no difference, the term used is "accessible". In this case "accessible" is defined as "accessible by permanent stairs or ladders". – Tester101 Oct 17 '15 at 0:39

There is no need to run conduit in a home for most of your outlets, Romex is fine for outlets/ lighting circuits in the crawl space walls and attic as long as it is stapled to the trusses and within 6"? To the box, if you run , THHN it is required to be in conduit for protection for 14 & 12 gauge wire there are exceptions in my area for larger feeders to a sub panel but if in a living space it is way cheaper to run Romex than the additional cost of even plastic conduit

  • I realized that it not required by code in most situations to run romex in conduits. I was considering running it into conduits to further protect them and make it easier to upgrade. But then the issue of the attic partial subfloor came into play too. Since there are parts of the attic with plywood sheet on top of the ceiling joists (subfloor) the wire would not be able to run besides the joist, (unless I would break the drywall ceiling below which would be too much) but only top of the plywood sheet (subfloor) making them more exposed . – tk3000 Oct 16 '15 at 23:41

you cannot leave the outside jacket on if you are running it thru conduit. it is a code violation and you will fail inspection if you are getting it inspected

  • 3
    That is not correct -- leaving the jacket on a cable in a conduit sleeve is actually required by Code as the individual wires in NM or UF are not intended for use outside their jacket. – ThreePhaseEel Jan 14 '17 at 18:23

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