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Thread: Plastic electrical boxes - how are wires clamped?

  1. #1
    Join Date
    Sep 2006
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    Las Cruces, NM
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    Plastic electrical boxes - how are wires clamped?

    Plastic electrical boxes that hold 1 electrical receptacle have rectangular "knock-outs" in them, so the circular types of wire clamps used to clamp wires into metal boxes can't be used. Are the knock-outs also supposed to work as clamps? Or are there special clamps that work with plastic boxes?

    One Youtube video I saw says to "staple the wire within 8 inches of the box", which i assume means to staple the wire to a stud. That wouldn't be hard to do in new construction, but stapled wires could make it hard on someone trying to rewire an existing situation.

  2. #2
    Join Date
    Feb 2008
    Location
    E TN, near Knoxville
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    12,298
    The tab you punch/pry out will act like a clamp and prevent the wire from pulling back out easily IF you don't break it off! I sometimes break them off on purpose. I don't worry about it, just fasten the romex near the box as you mentioned.

    I'm no electrician, but I always thought one reason for stapling the wires to the studs was to keep them in the center of the wall where a misplaced sheet rock nail/screw wouldn't hit it. Not usually a problem in Old Work where you have to snake wires in existing wall cavities. I don't worry about that either, just cut a hole for the old-work box, snake the wire, and forget it. If you really want to staple the wire you could always cut out a section of sheet rock, staple, then repair (but I wouldn't).

    When I wired my first house I had it inspected in stages. The electrical inspector said he wasn't strict on stapling the wires since that made rework inside the walls very difficult. The best thing to do, of course, is ask your local electrical inspector. Never rely on electrical advice from a forum!

    JKJ

  3. #3
    Quote Originally Posted by Stephen Tashiro View Post
    Plastic electrical boxes that hold 1 electrical receptacle have rectangular "knock-outs" in them, so the circular types of wire clamps used to clamp wires into metal boxes can't be used. Are the knock-outs also supposed to work as clamps? Or are there special clamps that work with plastic boxes?

    One Youtube video I saw says to "staple the wire within 8 inches of the box", which i assume means to staple the wire to a stud. That wouldn't be hard to do in new construction, but stapled wires could make it hard on someone trying to rewire an existing situation.
    The box's rectangular openings have integral 'spring gate' type covers (that you don't break or remove!); typically 4X in a 1-gang box. NM-type (Romex) wire passes thru this gate and is pinched in place to prevent withdrawal. This is the only clamping/strain-relief required in the boxes.

    I'll leave the staple issue in old-work vs new-work to the licensed folks.

  4. #4
    Join Date
    Sep 2016
    Location
    Modesto, CA, USA
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    9,889
    New work it is 8" to first staple for plastic boxes 12" for metal boxes with proper clamps. After that is is 4.5 feet between staples. If it goes through a stud that is considered as good as a staple. In rework there is no way to staple so it is not required.
    Note that cable has to lay flat under the staple not on edge. That rule does make sense.
    Bil lD.
    Last edited by Bill Dufour; 05-27-2019 at 2:25 PM.

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