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I had to install a new wire for my water heater (higher amperage/bigger wire), and was drilling through the wooden top plate to snake the wire down the wall to the pass-through. Everything seemed standard. Put the wire in and heard what sounded like wire scraping something (possibly metallic). Pulled it out, and the cable sheath had little scratches on it; not gouges, or cuts. The hole was at least 3x larger than the wire and was clean.

Now this house in Florida where metal studs are commonplace. Elsewhere in the house there have been metal bottom plates which transition to wood as we exit the exterior wall zone so I suspect under this wooden top plate there was a metal top plate - probably a few mm of metal.

I know that with metal studs anti-short bushings are used, but I am curious how this kind of situation is handled in old-work scenarios such as this where putting in an anti-short bushing isn't possible. Short length of conduit? Don't worry about it? Pulled the wire in and out a few times - more scratches but no cuts or gouges.

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    How good of access do you have to the hole that was drilled through this top plate? For example do you have direct access or was some very long drill bit used to create the hole?
    – Michael Karas ♦
    Commented Oct 21, 2019 at 11:33
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    I have not run into this problem but when you were drilling you should have seen some metal flakes if it was a plate, usually you feel nails when drilling through them but nails and hurricane straps are all I have ever hit, I would want to get advice from a local pro as this may be something your inspector looks for if it is common there.
    – Ed Beal
    Commented Oct 21, 2019 at 13:05
  • If you were drilling through metal, you'd know it...
    – Harper - Reinstate Monica
    Commented Oct 21, 2019 at 14:49
  • @Harper I was using a high powered drill and this is the kind of metal I am talking about: images.homedepot-static.com/productImages/… it is literally a couple mm thick. easy to rip through when going full blast
    – Alex
    Commented Oct 21, 2019 at 20:25
  • @MichaelKaras not enough to put my finger in to feel the other side, but enough that a spade bit cut through everything
    – Alex
    Commented Oct 21, 2019 at 20:27

3 Answers 3

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The simplest thing I could think of would be to use MC cable, the armor is pretty rugged. If you don't want to re-do the entire run, you could transition from NM to MC in a metal box mounted over the hole you made. Bring the MC in the back of the box, screw the box onto the top plate face up, and enter one of the sides with NM and a "button" NM clamp.

In my opinion MC with a right angle connector on top of the water heater makes a nicer install anyway.

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I have done something similar using a pair of Cable Feeder strips like these

These are very slippery plastic that is fairly rigid but able to bend around corners with a bit of wiggling. They will hold a bend to some degree so you can put a kink in them to help get around a tight corner. Normally you would just use electrical tape to tape the end of your cable to the end of the feeder and drag it through.

However, in one location where I had a lot of rough brick to get around and was running relatively fragile speaker wires, I sandwiched the wires between two feeders and pushed the more awkard bundle into place, then pulled each feeder free. The combination was thinner than conduit, which I didn't have room to run.

I'm in Perth, Western Australia and for some reason our dominant building style is clay bricks with a cavity external wall.

Plastic cable feeder

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Easy. Place a thin-yet-protective sheathing around your new wire's existing cable sheath. Then slide the extra-protected wire into the opening without and worries; the little scratches won't even cut into the cable sheath.

Sturdy, but stiff: small-diameter pvc tubing Sturdy yet flexible: thin plastic tubing made for aquariums. Go to your local pet store for the right diameter. Less sturdy, still flexible but cheap and easy : use duct tape!

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    That may work but if metal top plates on wood is common an inspector may be looking for a listed bushing of some type, I would want to check with an inspector or local pro because a 1 or 2 dollar fix now is better than an inspector requiring a bushing that requires another inspection.
    – Ed Beal
    Commented Oct 21, 2019 at 13:10
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    Hold on. Remember NEC 110.2 equipment must be approved, and 110.3 must be used in accordance with labeling and instructions. Your tubing suggestion is essentially "use flexible conduit, but get something random at the hobby-store instead of using a proper conduit type", I suppose that might be legal if it surrounds cable, but still...
    – Harper - Reinstate Monica
    Commented Oct 21, 2019 at 14:50
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    i think he is talking about: cabletiesandmore.com/… but yea, definitely not nec approved although I would feel perfectly comfortable using it for a 10in piece but still
    – Alex
    Commented Oct 21, 2019 at 20:26
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    A short section of smurf tube should be good enough (cheap plastic flex conduit ) I would want to check so you don’t have to rework I have gotten bit on a few really simple things like this in the past , 2 days extra for close / cover inspection over less than a dollar’s worth the plastic and the inspector was new and would not let me take a picture to verify I did the work, most take your word but I still show them photos.
    – Ed Beal
    Commented Oct 23, 2019 at 14:32

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