I have a metal electrical junction box with different size knockouts. Can I repurpose the larger size knockout by drilling a smaller standard hole within the same larger knockout? I need additional (small) holes than what's available per side. I will use standard NM cable clamps for the smaller holes too. See image below. enter image description here


1 Answer 1


The KO won't survive drilling. Just use standard reducing washers.

enter image description here

Used 2 at a time, bumped sides inward, so they wrap around the box metal, are held centered, and are clamped by the threaded connector and conduit nut.

The only issue you'll ever see with these is they're not valid ground paths, but that's only an issue when using conduit shell as a ground path. It's not a NM or UF problem.

Speaking of "NM problems", don't forget to include a pigtail to the metal box in your ground cluster. Or if they give you enough ground screw sites, land every cable's ground on one! Or install a breaker panel's ground bar. Easy mode LOL.

I don't know what a "standard" cable clamp is, you'd have to use this style here so it has the thread and conduit nut. I know there are knock-in cable clamps, but no good with washers.

enter image description here

  • Great! Thanks for the solution. I wasn't confident that drilling a smaller, hole, or using a smaller cutout saw would pass inspection, so I asked here for help. Thanks for clarifying on my use of "standard cable clamp". I meant standard nominal size NM cable clamps. Jan 22 at 3:21
  • On using a pigtail for grounding all the circuits, my wiring method will be to ground the box with only one of the circuits coming into the box. If there's a short (a live wire) touching the box, the return path is back to panel using the single ground wire which opens the breaker of the shorted circuit. So, why ground all grounds to box? Jan 22 at 9:24
  • Wouldn't this trip all the circuits breakers for all the circuits running to this box instead of the actual shorted circuits? Jan 22 at 9:34
  • Others will certainly be more lucid, but a short only trips the affected circuit. Imagine 3 water hoses with circuit breakers at their source: if one hose breaks open, it has no effect on the other two… excess water just flows to the, uh, ground. Jan 22 at 14:01
  • Connecting all grounds is so if one is removed afterwards, ground path is still available.
    – crip659
    Jan 22 at 15:29

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