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It's my first time doing this kind of work around my house so I just want to confirm my understanding before attempting anything.

I want to replace one of the 3-way switch in a room with a 3-way dimmer switch. I started by shutting off the breaker and removing the faceplate to inspect the wiring of both switches. Here's how it's currently setup:

Current 3-way switches wiring

I checked at the back of the current switches and the common wires look to be the bottom red one for top switch and the bottom black one for the bottom switch.

So my questions are:

  1. Is it normal that my common wires are not the same color in both switches? My understanding would be yes since one is going the source/panel and the other one to the light.
  2. Both switches are not hooked to the ground wire. However, both are in metal wall-boxes that look to have a bare copper wire under a screw at the back of the box. So, when hooking up the new dimmer switch, should I screw the ground wire in the back as well or just cap it with a plastic wire connector?

Thank you !

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  • Yeah, 3-way wiring is a royal mess, it's one of the most confounding things in home wiring. In particular, the colors are totally randomized, as are the terminal positions on 3-way switches. Look for screw colors (black=common brass=travelers), and grouping of wires into cables. Mar 7, 2022 at 19:35
  • @Harper-ReinstateMonica Yes I looked at the screws, they seem a different color but they are not black, they look like brass but a bit darker maybe. Don't know if it's just because my switch is old and standards changed since then? But behind the switches it's written "COMMON" at the bottom close to the screw so I figure this is a good indication? Mar 7, 2022 at 20:16
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    Can you post photos of the insides of the boxes involved please? Mar 8, 2022 at 4:16

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Is it normal that my common wires are not the same color in both switches?

Completely normal. 3-way wiring is an insane barf of random wire colors. In fact, Code requires the white wire be used in a certain priority list: as neutral; as always-hot; and as traveler (never switched-hot). I've seen 4-way wiring where there were three runs of /3 cable, with travelers different color-pairs in each of the runs!

Both switches are not hooked to the ground wire. However, both are in metal wall-boxes that look to have a bare copper wire under a screw at the back of the box.

That is the correct way to wire a metal box. Ground the box first; let the mounting screws carry to ground to switches, and receptacles which are marked "self-grounding" or installed in a certain way.

So, when hooking up the new dimmer switch, should I screw the ground wire in the back as well or just cap it with a plastic wire connector?

If the dimmer's yoke (metal frame) will have good contact with the mounting screws, you are all set and can just cap it off. If not, make sure the dimmer has a listing from a reputable testing lab - UL, CSA or ETL. Many dimmers bought mail-order (Amazon in particular) do not. And if so, ground the wire to the metal box using a grounding clip or a ground screw designed for that purpose. You can add it to the other grounds in a pigtail.

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  • Thanks a lot. I installed the new dimmer and I was a lot more confident doing it with all this info. Mar 8, 2022 at 13:34

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