Some North American electrical boxes (usually nonmetallic ones) lack conduit knockouts, as they are designed solely to clamp NM cable. Unfortunately, some devices (the RIB series of enclosed, UL listed relays, for instance) use their own enclosure with a conduit nipple providing a wiring chase to the box they're connected to.

Does one need a conduit knockout provisioned in the box in order to attach a nonmetallic conduit (nipple) to it, or can one drill an appropriately sized hole in the box, then use a non-metallic locknut to attach the conduit to the box? What Code provisions apply?

  • @Mazura -- the device is in a nonmetallic box with a nonmetallic, threaded nipple molded into it -- it has no facility for grounding as it doesn't need one. Jan 4, 2016 at 12:45
  • @Mazura -- I retitled the question based on your feedback Jan 4, 2016 at 23:07
  • No laws, I can think about,other than command sense. Do not so many holes ,Can not put on lock nuts or box weakens.
    – user101687
    Jun 13, 2019 at 21:57

1 Answer 1


It's OK to drill a hole in plastic, in fact a lot of them don't even come with knockouts - for example


The zip boxes and others with integral NM clips - I doubt they are listed for use with conduit but even if they are, I'd use one like that linked above.

  • we regularly drill plastic boxes, the only thing I can think of the box needs to be large enough for the "box fill" and needs to be accessible, I.E. not covered with sheetrock/ buried in a wall.
    – Ed Beal
    Jan 4, 2016 at 14:32

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