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Happy new year! I'm trying to replace an old rotary dimmer with a newer slider/switch and could really use your help. I searched and searched and couldn't really find someone with the same wiring conundrum I am about to explain. It's currently configured in a 3-way configuration. There is a rotary switch/dimmer (press in to turn on/off) that controls an overhead light in the dining room. The kitchen next door has a 2-gang with one switch turning the dining room light on/off. Dim level is persistent with the rotary switch - that is to say if you turn the light on from the kitchen, the dim level is that to which the rotary is currently set. On the rotary it says 'Carlon Thyrocon' and elsewhere on it 500W. I couldn't find any datasheets or anything about this device besides that it may be 'vintage' based on a couple lot descriptions on ebay. It has 2 red wires and a black wire coming out the back. Here's where things get confusing for me: the switch box seems to have a single 4-wire romex coming through - so red, white, black and bare copper ground. From the rotary to the romex it is wired black <-> black, one red <-> red, other red <-> white. bare ground is connected to the metal of the switch box. This is not what i was expecting for a 3-way configuration. As stated i have a lutron dimmer i would like to connect. Wiring available on the Lutron is one green, one red, one red with white stripe, one black wire. The Lutron instructions recommend connecting - again from switch to wiring box - black <-> tagged (i believe common) black, red <-> other black (i believe one traveler wire), red with white stripe <-> red (i believe other traveler wire), green to bare copper. So as you can see, the wiring in the box does not seem to match what is expected in convention. My guess is they just used whatever spool of wire to do the job so there is likely (hopefully) a convention in use throughout this house - but unfortunately no Rosetta Stone to translate between that and expected wiring convention. I could also not know what I'm talking about of course. I have attached a picture - i hope it helps to illustrate what i am trying to convey. Unfortunately i don't have access to any test tools here (dmm for example) so i'm going to need a little ingenuity to get this thing hooked up safely. Thank you sincerely to anyone who's reading right now (or made it this far) - I genuinely appreciate any advice you all may have.

  • Jayce

wiring from old rotary to wall box

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    Actually, what you have from the wall is perfectly normal. This is the spur end of a 3-wayy setup, and 3 wires is normal (2 travelers + common). The wall wire colors are black white red because that is how ALL /3+gnd cables are made. Those are the standard colors. By the way, the grounding is handled exactly correctly. The ground wire goes to the metal box, as required. The switch picks up ground via its mounting screws! Isn't that easy? :)
    – Harper - Reinstate Monica
    Jan 2, 2021 at 5:51
  • There's nothing "vintage" about that dimmer! It's what I had in my walls until we installed new LED lighting. They're still available for sale at my local big-box, though you have to look hard since they only support incandescent bulbs not CFL or LED.
    – FreeMan
    Jan 25, 2022 at 16:56

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Colors don't mean much

The color of wires in house-wiring cable (NM, UF) is only mildly meaningful as to their function, as you have observed, due to the fact that all such cables use the same color scheme (black/white/bare, with red for a second hot, blue for a third hot, and red/white striped for a second neutral). As a result, grouping and connectivity are the main pieces of information that convey the meaning of wires in household wiring.

This is easy, though

What you're looking at here is the end of a 3-way switch spur; the existing dimmer is wired with its black common wire to the black wire from the cable, its red traveler wires to the red and white wires from the cable, and its ground connected to the bare ground wire in the cable. The new dimmer wires up much the same way as a result: its black wire goes to the black wire from the cable, its red and red/white (traveler) wires go to the red and white traveler wires from the cable, and its green ground wire connects to the bare ground wire from the cable and a grounding pigtail to the box.

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