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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

Always

AlwaysLearning
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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

Harper - Reinstate Monica
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  • 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. – AlwaysLearning Jan 22 '22 at 03:21
  • 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? – AlwaysLearning Jan 22 '22 at 09:24
  • Wouldn't this trip all the circuits breakers for all the circuits running to this box instead of the actual shorted circuits? – AlwaysLearning Jan 22 '22 at 09:34
  • 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. – Aloysius Defenestrate Jan 22 '22 at 14:01
  • Connecting all grounds is so if one is removed afterwards, ground path is still available. – crip659 Jan 22 '22 at 15:29
  • Your right ... I had it backwards. Just need to figure out how that will look/work when I have multiple circuits coming in and out of the box using DIN rail connectors, so it doesn't get messy. – AlwaysLearning Jan 22 '22 at 18:33
  • @Always grounds don't work that way. Ground wires are a safety shield "around" the wires, like they would be if all your wiring ran in EMT conduit. You want the best safety shield you can get, to maximize the chance of an instant trip if you have a bolted fault (a point where you will very much want instant trip). Ground does not go through the circuit breakers so they don't care about current moving on ground. They only monitor hot/live (and neutral if xFCI) so they only trip the faulting circuit. Don't guess or freestyle. Follow Code. – Harper - Reinstate Monica Jan 22 '22 at 18:52
  • Also, I could just run a green wire from the box back to panel. And since I will have multiple boxes, daisy change the boxes together using the single green wire back to the panel. Any issues with this? – AlwaysLearning Jan 22 '22 at 18:54
  • Me bad I had it backwards ... the short going back to the breaker ... which is wrong! – AlwaysLearning Jan 22 '22 at 19:00
  • @AlwaysLearning Also it sounds like you are experienced with other types of electrical projects, like automotive or silicon electronics. No offense but such people tend to be a disaster in home wiring. They ignore Code and do things "their" way based on what they know from electronics... they stubbornly argue very sophomorically with Code experts and demand every reason for every rule be explained and then they argue with it... the result is a safety-compliance dumpster fire. No permits pulled. Later they have trouble selling. I don't recommend this obviously. – Harper - Reinstate Monica Jan 22 '22 at 19:05
  • @AlwaysLearning Failing to connect the Romex ground wires would be unexplainable. You have to do something with them, and you're only allowed to do one thing lol. – Harper - Reinstate Monica Jan 22 '22 at 19:12
  • Also, thanks for the reminder ... I will definitely triple check my work, so it's correct. I am not that stupid! I obtained a permit for the work I am doing, and making sure everything meets code. But, I will not rely on the inspection to bail me out! And arguing with the inspector doesn't make my install any safer! – AlwaysLearning Jan 22 '22 at 19:28
  • FYI, all circuits' ground wires will be connected to the ground bar in the panel per code. It's making sure the box is grounded correctly. Thanks! – AlwaysLearning Jan 22 '22 at 19:36
  • @Always yeah I wasn't thinking you were like that, but I did want to mention the worst case because it leads to such a bad destination. I am an electronics nerd like that, but have found every rule in NEC has an interesting story, and it's taught me a lot of stuff that I bring back to electronics. – Harper - Reinstate Monica Jan 22 '22 at 19:37