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I was simply replacing a existing dimmer switch for my kitchen lights with a newly bought Feit electric smart 3 way dimmer switch. After studying, I have located where the dimmer switch should be replaced, the load side, as written on the instructions it came with (rather than the line side).

After rereading the instructions, I finally understood what I missed because the problem was the new dimmer would work only if the switch on the other side was turned on. It was that I did not connect the line wire (common wire) with the traveler wire as instructed by the instructions. I simply didn’t think I had to modify the configuration on the other switch. (I thought well the original dimmer switch was working, so if I wanted to make it a smart dimmer, all I had to do was replace the dimmer with the new one.).

Anyway it all works perfectly now, meaning the lights can be turned on or off on either the dimmer side, or the switch side.

My question is, in order for my switch to work, I had to clamp the traveler wire to the switch using the side screw as the line wire was already plugged into the back and there was no second hole. And because both wires are 12 gauge, I couldn’t jam them both into the hole in the back. So all I could do was leave the line wire plugged into the back and I clamped the traveler wire to the side and screwed it in. Is this safe?

enter image description here

UPDATE: Thank you for all your help. I ended up taking all of your advice and fixed the switch! I found that the previous person who owned this house, or the previous electrician just did some questionable things after learning from you all! Here's the final image: enter image description here

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    Most switches/outlets should be able to use 14 and 12 gauge wires under the screws. Bare wire showing is not good, except right under the screw. The holes in the back have a bad name because of the problems they cause. Would pigtail with a wirenut the two black wires and connect them by the screw with another short black wire.
    – crip659
    Sep 21 at 12:47
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Using the hole(s) in the back is called a "back stab", and, while technically code-legal, they're frowned upon because they can come loose and cause arcing and, if left alone long enough, fires.

The side screws are actually the preferred method and are applicable for either 12 or 14 gauge wire. However, the way you've done it is missing on 2 points:

  1. You have the wire on the "out" side of the screw.

    When the wire is on this side of the screw, the friction of the screw turning will tend to push the wire out of the screw/plate area as you tighten the screw. This can (as in this case) force some bare wire out of the pocket, potentially exposing it to other wires in the box, causing a short.

    If the wire is on the other side of the screw, it will be pulled into the screw as it's tightened, making for a better electrical and mechanical joint.

  2. The wire should be bent into a "shepherd's crook" before being installed.

    Making a bend in the wire (I use a pair of needle nose pliers to make the bend) gives you a hook that holds the wire in place between placing the wire and picking up the screw driver. This makes your life easier as you don't have to hold it there once the wire's in place.

    It also increases the electrical contact surface between the wire and the switch, and it gives a very secure mechanical bond between the wire and the switch.

Should you choose to remove the other black wire from the back stab, the logical thing would be to put both of them under the screw. However, you MUST NOT do this, as the screw isn't designed to hold two wires. Instead, you would put both of these wires and a third wire (about 6" long and stripped at both ends) into a wire nut (make sure you have one designed for three 12 gauge wires), then bend your shepherd's crook at the other end of the third wire and screw it under the screw. This is called a "pig tail" and is the ideal way to have this wired.

One additional note: Some outlets, and I presume switches, (usually marketed as "commercial grade" or similar) have what are called "side clamps". With these devices, you can put two wires under one screw! You strip the wire, do not bend it, and stick it between a fixed plate and a moveable plate under the screw. The moveable plate has small bends in it for the wires to fit into. Then you simply tighten the screw down to the manufacturer recommended torque setting. Because the screw doesn't actually touch the wire, you can have a wire on either/both sides of the screw and you'll get the same tight fit.

IMHO, it is well worth the extra dollar or so to invest in these for any expansion/replacement work as this clamp system makes life much easier - you get the speed and convenience of the "back stab" with the safety of the screw.

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    You can also use the LOOP hole in a standard pair of wire strippers.
    – LShaver
    Sep 22 at 14:40
  • Good point. I don't carry that type of stripper with me - I've never found them particularly useful or handy. It's just me, I'm sure. I've used them a fair bit recently since I saw that post and discovered they will cut screws!
    – FreeMan
    Sep 22 at 14:42
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  • @LShaver TBH, now that I look at the new pic the OP posted, I'd guess that the black wire was bent around needle nose pliers with that nice, smooth curve, and the red wire was bent with something else because of the sharpish kink. IMHO, the black wire is a better hook, but I'm not a pro, so it's just my opinion.
    – FreeMan
    Sep 23 at 14:17
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    Thanks, @bishop, I knew that was here somewhere, but didn't spend the energy to go look for it. :(
    – FreeMan
    Sep 23 at 14:18
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Yes, I see several problems.

  • Wire just stuck under a screw, and not using a proper shepherd's hook with a more than 180 degree bend (i.e. squeezing it together after forming it. Watch a Youtube video on how to put a wire on a screw.

  • Insulation excessively stripped, leaving bare bits of wire sticking out beyond the back surface of the switch. Not OK. When you use backstabs, you must strip the wire to exactly the length shown in the strip gage - no more, no less. And then, push it in all the way so no bare wire remains exposed. That's not just to avoid this - it's also to inform you that you have pushed it in far enough. Otherwise, how would you know?

  • Unfortunately backstabs cannot be reused (pulling the wire out damages the spring) so you must now pigtail to the side screws, or buy a modern, spec-grade "screw-and-clamp" type switch (3 dollars) which accepts 2 wires under each screw. They would be a good fit for you. Tighten the screw hard.

  • Use of backstabs with #12 wire. Normally this is forbidden (and the hole won't fit the wire). Older switches did allow it, however UL banned that for a reason. Backstabs are generally unreliable even in ideal conditions.

If someone is a novice to doing electrical work, I won't recommend a method which is unreliable, like backstabs. There will be enough "learning moments" already, you don't need to learn that too!

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Yes it can and it is the best way to do it. Those holes in the back are commonly called backstabs and have a long history of failing. After you turn off the power, you can remove those wires by sticking a small screwdriver into the slot next to each hole and pulling the wire. You can also hold the wire and twist the switch back and forth while pulling the wire.

Shepherd hook the red wire and hook it around the screw clockwise and tighten. Pigtail the two black wires with a 6" extra piece of wire with a red wire nut. Hook the other end of the extra wire around the remaining screw.

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