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I recently had an existing old work box (metallic, installed with the supports that look like 'F') where there was a cable coming in from the top and another from the bottom. I wanted to add a third cable from the back of the box. So I took my trusty hammer and screwdriver, aimed the screwdriver at the part of the back knockout away from the tab, smacked it, and broke the box tabs right through the drywall... :(.

Once the box was out of the wall, it was still pretty much impossible to knock out the knockout from the inside of the box. I then flipped it around and tried from the back and it came out super easily.

  1. Are knockouts typically "one direction only"?
  2. Is there a different way to get them out in a situation like this? Or is it standard to have to take the existing cables out, the box out, punch them out, then put everything back together?
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    Your problem description made me laugh out loud. Not in spite or schadenfreude, but in sympathy. That sounds exactly like something that I'd do... – Stian Yttervik Nov 20 '20 at 13:45
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A lot of handy boxes and old work "gem" boxes have knockouts that are made to knock from the outside in. This picture of a Steel City handy box shows it clearly

Steel City handy box

If you have to remove these from the inside, there is a trick. You can't pull the KO with a hammer. But if you drive a small self drilling screw into the KO from the inside

self drilling screw

then back it out a few turns, you can pull on that screw with pliers to remove the knockout. If you can't get it with pliers, try a mini prybar

mini prybar

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    @batsplaterson FYI I just tried this. I got the screw in, then pulled with all my might with pliers to no avail. But then I tried a little mini pry-bar on the screw and the knockout came right out! Thanks again! – David Doria Nov 20 '20 at 2:05
  • @DavidDoria The back-end of a claw hammer should work too ;-) – Mast Nov 20 '20 at 8:44
  • @DavidDoria that's great, I am glad it worked for you. I will edit the answer to include that method. – batsplatsterson Nov 20 '20 at 10:44
  • just be sure there is not some surprise romex behind where you're drilling. – Z4-tier Nov 20 '20 at 15:15
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I recently had this issue and solved it this way:

Drill small hole in the center of the desired knockout (from the inside).

Drive a long self-tapping metal screw into the newly drilled hole until it is firmly in place.

Take a claw-hammer or kitty-paw to pull on the screw until you can get pliers onto the knock-out enough to twist//work it out.

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Yes, they are one way unless you really want to bend up the box. When the box is installed, I usually take a small screwdriver, slotted, and tap it into the crack between the knockout and the box and twist the knockout out a bit. Then grab it with a pair of needle nosed pliers and twist it completely out... be careful not to damage the insulation on existing cables when twisting out the knockout. If there is room between the wall and the box, you might be able to get a small screw driver in there and tap the knockout into the box and then grab it with your needle nosed pliers.

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Yes, knockouts are often one direction only, esp. in outlet boxes. If you can see that the KO is not even with the rest of the box, you need to take a something like a flat blade screwdriver, put it in the recessed side and give it a couple of medium hits with a hammer. Then once you get it started, use a pair of pliers (whatever type is best, maybe needle nose) to pry on the KO until it breaks loose.

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  • @isherwood Surprised you said that?! I would think that the last part of my answer "...you really need to knock it out from the recessed side" I think that qualifies as a "how". If not please tell me how to improve my answer. The "that's how" at the end is something my autistic grand-daughter says practically at the end of most of her sentences (7 years old). Just threw it in there for comic relief. Take care, you are doing a great job here. – George Anderson Nov 18 '20 at 18:57
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    @isherwood Thanks for the advice, I edited and hopefully substantially improved my answer. – George Anderson Nov 18 '20 at 19:08
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I've gotten a few out with just a flat head screw driver at an angle and using a hammer to tap it into the crack or to make a gap. Once it's in, you can pry or twist with the screw driver to widen the gap and get something bigger in to finish the job.

It's not exactly easy, but it works without extra hardware or a drill.

And yes, like the others say, they are intended to be one-way from the outside in. I'd have to assume they didn't want the knockouts to be accidentally knocked out while the installer was putting wires in or a repair being done. Trying to fish one back into place would be even harder than trying to pull one out.

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  • They make special snap-in plugs to replace missing KOs for that reason :) – ThreePhaseEel Nov 19 '20 at 0:34
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    @ThreePhaseEel, that makes sense. I'd assume those are also useful for when someone removes a circuit/wire/something and leaves a knockout empty. – computercarguy Nov 19 '20 at 0:47

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