3

Reference picture below.

The electrical boxes for the switches can be adjusted in/out for different thickness of drywall.

I want to run conduit from the 2-gang switch box though the top-plate and into the attic for about 3 feet. The reason is so that I can come back later and run NM cable to the switch box. The conduit must allow the box to be adjusted in/out to meet the thickness of the drywall or other wall covering.

  1. Does the NEC allow this?
  2. If so, what options are available?

enter image description here

Revision:

I am undecided on the wall covering at this point. At least 5/8 Type X Drywall. Had considered maybe 2 x 5/8 Type X Drywall for sound.

The box is Arlington Industries FA102:

enter image description here

  • We cant see the box, Does it accept a connection with conduit ? Or is it a plastic box with tabs ? You do not know what thickness your drywall or other wall covering is going to be ? Run your NM cable like the others just leave the whole ( uncut or enough to get to its destination ) roll up in the attic so you can send it where it needs to go. – Alaska Man Jun 1 at 20:44
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    Why not just run a couple lengths to a junction box in the attic and call it good? – isherwood Jun 1 at 20:55
  • Yes, can you post a link to what sort of box you put in? That's going to be the major determining factor as to how much work you have to do – ThreePhaseEel Jun 1 at 20:58
  • I had considered pre-running wire but unsure if I would later need 14/2 or 14/3 or 12/3, etc. or quantity of each. This is an outside wall and will be insulated. If was an interior will, I would not worry too much about it. – Turtle Turtle Jun 1 at 21:32
  • Is having a JB in the attic to serve as a distribution box an option? – ThreePhaseEel Jun 1 at 22:52
5

NM-B ("Romex" is a brand name...) in conduit is a pain. Conduit fill calculations treat "oval" cables like a round cable of the largest dimension, so if you need more than one you need HUGE conduit.

So, use a junction box in the attic to transition from NM-B to THHN in conduit, which makes life MUCH easier.

Properly attached rigid conduits (which can be a confusing term, since "rigid" (RMC) is the heaviest class of steel conduit, but in this case I mean "non-flexible" to include IMC, EMT and PVC) will not allow much, if any flex, since it's supposed to be attached near boxes/termination points.

So, you need a flexible product. ENT or "smurf tube" is a corrugated plastic conduit. ENT picture from Carlon

Flexible metallic conduit (steel or aluminum) is also reasonable. Flexible aluminum conduit picture from home depot

Liquid-tight flexible conduit is probably acceptable, but insanely expensive and something you don't need, so it's unreasonable.

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  • I did not realize that "Smurf Tube" could be use for electrical wiring. I thought it was limited to only low voltage stuff like coax and Ethernet. – Turtle Turtle Jun 2 at 2:35
  • 1. Nice idea: ". . . use a junction box in the attic to transition from NM-B to THHN in conduit, which makes life MUCH easier." 2. I did not realize that "Smurf Tube" could be use for electrical wiring. I thought it was limited to only low voltage stuff like coax and Ethernet. 3. And is it okay to use the flexible metal conduit with plastic junction boxes? 4. Which flexible conduit would be easier to use to pull THHN though it. 5. I have not looked at the price of Liquid-tight, but it is an option if it is just easier to work work with. – Turtle Turtle Jun 2 at 2:43
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    5. Liquid-tight is not easier. 4. Metal would be my preference. 3. If the plastic boxes have appropriate connection ports for conduit connectors. 2. Orange/Yellow ENT is commonly used for LV/network/telephone/fiber, and similar-looking slit loom tubing is as well but ENT (which is not slit) is a listed conduit type (commonly used in concrete, but does not appear to be required to be used only that way.) Blue (thus smurf) is the usual color for normal wiring. – Ecnerwal Jun 2 at 13:51
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    @TurtleTurtle 5. Liquidtight is going to be more expensive/harder to bend for 0 gain in this application 4. THHN wires will pull quite readily through any appropriately sized conduit. 3. FMC is generally only used with metal boxes due to the need to bond the conduit, your current box isn't set up to do that. 2. ENT is legal as a mains wiring method anywhere NM is (and then some, even) 1. Your box does indeed have the correct KOs for conduit usage, so using ENT for this should be pretty much a snap. – ThreePhaseEel Jun 2 at 21:06

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