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Electrical Wiring in Double-Stud Wall

Isabell Norman | Posted in Building Code Questions on

I am building a passive house with double stud walls. 2×6 for exterior and 2×4 interior with a 1.5″ offset gap between the studs. I intend to put the plumbing and electrical in the 1.5″ gap between the studs, no hole drilling.

Can I have wires lay on the floor in the gap between studs rather than hang them higher up the wall? I would hang the plumbing and just lay the wires down on the floor level without hangers. My fear is this may be a code violation.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    You are usually safe as long as there is a physical barrier to protect the wires. The wires need to be in an area that is not subject to possible nail penetrations, and where they won't be subject to physical wear. I think your propose spot meets those requirements, but it's worth checking with your local building people -- some building people will resist anything "different", and it's not usually worth the hassle of trying to educate them.

    I strongly recommend keeping your plumbing out of exterior walls. The middle of a double stud wall is better, but if you're planning to insulate both sides as is probably the case, that means all those pipes "in the middle" will be running a whole lot colder than the interior of the house. Colder pipes are more prone to freezing, and that's asking for trouble. Wiring doesn't have this issue, so the only issue running wiring in the exterior walls is the need to air seal all the related holes and devices.

    Bill

  2. Tim B | | #2

    When you say plumbing, are you including the drains and vents? I believe you will need space for 3" vent and 2" drain.

    1. Isabell Norman | | #6

      You are right. I will only put the smaller supply pipes in the gap between walls.

  3. Joseph Dziedzic | | #3

    National Electrical Code requires wires to be stapled at minimum every 4-1/2 feet and within 1 foot of an outlet or junction box. Check with your jurisdiction's electrical inspector and GET IT IN WRITING if they tell you it is OK to leave the wires unsecured within the cavity.

    1. Expert Member
      Zephyr7 | | #4

      I should probably clarify in my above post that I I assumed these wires would be secured, but to the top or bottom plates within the wall instead of to the studs based on the OP. You can never just leave them loose in the wall (the only exception is if they're fished in later).

      Another possible option here would be to run a conduit in there, which simplfies the need for securing a bunch of wires (although you still have to strap the conduit every 5 feet), but it complicates all the connections to the devices.

      Bill

      1. Expert Member
        Peter Engle | | #7

        I'm thinking you could call this equivalent to cables running in a tray. IIRC, cables need to be "supported," but not necessarily physically restrained. There might be some wiggle room here. The OP will need to derate cables if they are lying together as bundles. Definitely have a good chat with the local codes people before doing it.

        1. Expert Member
          Zephyr7 | | #8

          Cable tray must be 'listed for the purpose', so a site-built wooden structure, or something pressed into service as such, isn't going to allow you to use wiring methods and materials for cable tray.

          You could use one of the cable clips that holds multiple NM cables together with a small amount of spacing. That keeps them from being in a random bundle. Note that NM cable is essentially already derated a bit since it's sized from the 60 degree table, but it's 90 degree rated wire. I would try to stay away from any need for getting into derating here though as you're likely to complicate things on a residential project. It's much better to support the cables in the normal fashion and avoid any of the "unusual" stuff that the inspectors like to fuss over. Derating is most common in conduits and cable trays used commercially, so getting into that on a residential project is likely to raise eyebrows.

          Bill

          1. Expert Member
            Malcolm Taylor | | #9

            Bill,

            We are all talking about the how but not the why. I don't see many advantage to having the cables lying between the two bottom plates. Running them through the studs means you only need support when they run vertically, and they aren't bundled where you would like some insulation to minimize thermal bridging, as they would be lying between the plates.

          2. Expert Member
            Zephyr7 | | #14

            I agree I don't really see any advantages here compared to running things the normal way, aside from maybe not having to bore holes in the studs. The issue is that laying them on the bottom plate and using it like a cable tray is probably not going to fly with the inspector, and the "tray" formed by the studs and the bottom plate won't count as a cable tray per code, so it won't let you use wiring methods normally associated with a cable tray.

            Basically I think trying to run things in that channel would be asking for trouble at inspection time, so I'd avoid it. If you tack the cables in place on the bottom plate with staples, you should be fine, but you're limited as to how many you can fit unless you use the stacking style cable clips.

            Bill

    2. Expert Member
      Malcolm Taylor | | #5

      Joseph,

      Our code here in Canada requires the same thing. I guess you could staple them down to the sub-floor?

  4. Doug McEvers | | #10

    I knew builders who would cut a v notch in the center of one end of the 2x6 studs. The electricians would run the wires through the notch above the bottom plate. It did simplify insulation as the batts would not have to be split for the wiring.

    1. Expert Member
      Malcolm Taylor | | #16

      Doug,

      I remember seeing that on a high performance Habitat for Humanity build. All the horizontal runs were on the bottom plate, with the vertical ones on the side of the studs. It uses a lot more cable, but then you have no worries about hitting wires run through studs when hanging cabinets, no cutting batts during installation, and if you ever decide to inset anything like a medicine cabinet or shelves into the walls you know you won't interrupt a run.

      The problem is under our code 2"x6" studs need to be fastened with three nails, and I'm not sure how you would do that with a V removed. Maybe run everything between 6" and 8" above the plate and just split the bottom of the batts?

  5. Isabell Norman | | #11

    Thanks for responses. When first presented with the idea of having 1.5 inch gap between the double studded walls I thought "great I can run plumbing supply lines and electric wires in this space without having to drill all the holes in studs", big time saver. Then I wondered how to staple the romex. The exterior part of the double studded wall will have plywood on it so you can't get a staple and hammer in the 1.5 inch space to set the staple. Resting the cable on the bottom of the cavity seemed like a easy solution. Now after reading everyone's responses I see it may be wise to not have it there. If placement is to keep it away from future nail pokes and drill bit damage I remember more than once drilling from crawl space to the bottom of the wall cavity to run after the fact wiring. This would surly hit a wire resting in the bottom of the cavity if drilling from below.

    Another solution would be to support the wire with those zip ties that have a screw hole in the end. They could be drilled into the side of the stud and reach back to the 1.5 inch cavity to grab the cable. My only concern is if the zip tie satisfies the code definition of a staple. Must get a buy in from the inspector.

    1. Expert Member
      Zephyr7 | | #15

      Code doesn't specify staples, it just specifies that the cable be supported. You could make the argument that the "must be listed for the purpose" code lingo applies to any support you use, but I think a zip tie with a mounting hole would probably qualify. You might have an issue if you bundled a bunch of cables with a single zip tie though. I'd run this by your local inspector just to be safe.

      If the wall hasn't been built yet, you could stagger the interior side studs with respect to the exterior side studs, then you could easily nail into the exterior side studs from the interior side. I'm assuming the plywood/sheathing is on the far exterior side of the double stud wall. If there is plywood in the gap as Peter mentioned might be the case, you can staple the wires to the plywood -- code doesn't require the cables be secured to studs, it just requires that they be "supported".

      Bill

  6. Joseph Dziedzic | | #12

    Check out Gardner-Bender's Multi-Cable Staples model MCS-20W. They're right around 1-1/2" tall so it wouldn't necessarily fit between the two studs, but you could always nail a small block of 2x4 to the side of the exterior stud and mount the MCS there. You can find these at the "orange" box store or likely any electrical supply store (go to Homer's Web site and search on that model number to see a picture).

  7. Expert Member
    Peter Engle | | #13

    Not sure I understand where the plywood is. If it is on one of the sides of the 1.5" gap, then you could use the plywood as a running board and staple the wires to the plywood pretty much anywhere you want. Also, you can consider whether any of your issues are improved by offsetting the studs. There is no reason that the studs on the inner and outer wall must line up. In fact, it's a bit better thermally if they don't. And finally, your choice of insulation might make a difference in this discussion. Some insulation conforms better to wires and other stuff.

  8. Stephen Sheehy | | #17

    We have an air barrier membrane on the exterior side of the inner stud wall. So we avoid most penetrations through the air barrier for wiring or plumbing and the plumbing is protected from frost. After rough wiring and plumbing, stud bays filled with batts. Outside the inner stud wall we used blown cellulose.

  9. Isabell Norman | | #18

    Plywood is on both sides of the exterior wall. The plywood on the inner side of the exterior wall is the air barrier. I was told I can staple into it to hang the wires but I would rather not. I was told as long as I don't remove the staple the air barrier should be good, but I would rather play it safe and not penetrate the air barrier. This exterior wall will be pre-filled with high density fiber insulation. After the wiring and plumbing the interior wall, high density fiber will go into this interior wall including the 1.5 inch gap.

    This is an interesting construction if anyone is interested. My contractor will let me do any of the work myself so I do wiring, plumbing, drywall, exterior siding, kitchen and bathroom to keep costs down. The wall construction is the interesting part. The contractor will have a CNC board cutter. It spits out boards as fast as you can stack them. Each board is cut to size and marked with a number and has marks identifying where to line up studs and cross members. After the pile of lumber is finished the walls are nailed together on the warehouse floor. Each board you pick up from the top of the stack is the next board you need in the assembly. The walls are built fast and will have windows, doors pre-installed and insulation for exterior wall will be pre-filled. Then it all goes o a truck and a crane puts the walls in place on the foundation. Drywall will be set on the first floor before the walls go up, and the same on the second floor. Framed and roofed in a couple days with drywall stacked inside waiting for me to finish my part.

    1. Expert Member
      Malcolm Taylor | | #19

      Isabell,

      If the interior wall and gap will be filled with blown insulation, and is separated from the exterior one by your plywood air-barrier, I would install both the plumbing and wiring conventionally - as you would if it was any other wall. I don't see any advantage to varying from that.

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