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There are, to my knowledge (if there are other types, I'd appreciate being educated about them), three different kinds of NM-B cable clamps that go into a knockout in a junction box:

  1. The "traditional" screw in, screw clamp type of connector. It has a threaded lock nut that must be tightened down inside the box, and looks like this:

enter image description here
Image from lowes.com, sample link for more detail

  1. The "push-in" (or "snap-in"), screw clamp type connector. It simply pushes into the knockout, then you screw it down to clamp the cable, like this:

enter image description here
Image from lowes.com, sample link for more detail

  1. The "push-in", plastic clamp. It pushes into the knockout, then you push the wire through, no screws involved anywhere, like these:

enter image description hereenter image description here
First image from lowes.com, sample link for more detail. Second image from homedepot.com, sample link for more detail

Obviously, all 3 types are UL-approved for use in the USA, but UL approval doesn't mean there aren't some drawbacks. "Backstabs" (or "quick connect", as I've seen it listed on packaging material) are a UL-approved method for wiring an outlet, but aren't always 100% reliable. Many here strongly advocate against using them. Ever.

The first connector is the most time consuming to install - you have to push the connector through, get the nut started, hold the clamp so you can maintain access to the clamp screws, then tighten down the lock ring. When properly installed, I'm sure they hold very securely, but I've always struggled to get the lock right tight enough (though I see they make wrenches specifically designed for tightening the nuts on these).

The second connector is much easier to use - align the wire clamp portion so you have access to the screws, give it a push to get it into the knockout, maybe whack it with a screwdriver or hammer if your thumbs aren't strong enough and Bob's your uncle. It still takes extra time to tighten down the clamp screws around the cable, so it's not as fast as it could be.

The third type looks to be, by far, the easiest to use. Simply push the plastic ring into the knockout then pull the cable through the hole.

Are there any notable drawbacks to using the simplest method - i.e. the snap in, pull through, leave the screwdriver in the tool belt plastic cable clamp?

Are there other pros/cons of the three types that I'm missing out on?

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  • Yes, I should have done more research before commenting. The plastic boxes with knockouts I was thinking of are for low voltage use. I've never used the plastic ring type; in my small scale projects robustness is usually the priority over ease of installation :)
    – Armand
    Commented Jan 30, 2023 at 17:29

2 Answers 2

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The big difference I'm aware of is that as you go down the list, rework gets harder.

The thread-together ones can be infinitely reused. I have a bag of them I've pulled out of various work around the house over the years that I'll no doubt reuse at some point in the future.

The clip-in ones are hard to remove from the box, but easy to change the wire on. They're not as easy to keep for reuse later. I think they are a pretty good compromise all told.

The plastic ones almost always fit tightly. They're both hard to remove and hard to change the wire in. I love putting them in but hate having to change anything about them. For that reason I tend to avoid them.

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  • Both you & Harper provided good tips and essentially the same info. I think he's got enough rainbows and unicorns, so I'll send the extra few points your way. :)
    – FreeMan
    Commented Jan 31, 2023 at 16:02
  • That happens more than you might think. Usually I end up giving Harper's answer an upvote as well.
    – KMJ
    Commented Jan 31, 2023 at 16:09
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When properly installed, I'm sure they hold very securely, but I've always struggled to get the lock right tight enough

It's super simple. You need a flat blade screwdriver and a hammer. You put the screwdriver on one of the castellations around the edge of the nut, and bap it with the hammer. That's what they're for.

And when I say "hammer" I mean the side of your multi-tool, or the box cover plate, or whatever. It doesn't take that much, really. It's just a little too much for your bare fist lol.

If you've been reaching into the box trying to do surgical work with a gripping tool, no wonder you think they're awful!

The others fit when they fit, but when they don't fit they're just horrible. In particular the plastic job is sensitive to cable size. I'm no fan of plastic, they don't really fully seal and they're flammable.

And the clip-in types are more difficult to undo/remove.

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  • The "bang a flat-blade screwdriver with a <something>" is the technique I've used. I've always had a hard time keeping the wire clamp screws aligned so I can actually reach 'em. I've never tried using one of the little wrenches, though I could see where holding the clamp part with my linesman's pliers to keep it where I want it, then turning the nut with the little wrench could be easier... Maybe I've always tried to make them too tight.
    – FreeMan
    Commented Jan 30, 2023 at 20:10
  • @FreeMan or put a vise-grip on the outside part just to add additional mass to it so it resists rotating in the milliseconds of the impact. Commented Jan 30, 2023 at 21:55
  • @FreeMan my life got easier when I discovered Xeno screwdriver tips.
    – KMJ
    Commented Jan 31, 2023 at 7:38
  • 3
    "Any tool can be a hammer if you use it wrong enough."
    – Moshe Katz
    Commented Jan 31, 2023 at 15:43
  • 1
    @FreeMan Yeah I was just thinking of the vice-grip as a way to add mass to the part you don't want to move. I didn't think about bracing it. Think of it this way: jack up a car wheel so it can spin freely. Try removing the lugs with a ratchet - the tire just spins. But try it with an impact - comes right off! The impact is too short to get the tire spinning. Commented Jan 31, 2023 at 20:30

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