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Summary

I'm trying to replace a broken adjustable switch fixture in my house. I'd like to replace the switch myself. However, I am a total novice, so I am reluctant to proceed without understanding what I'm seeing. Unfortunately, the wiring in this (old?) switch box does not seem to match any diagrams I can find. So, I'd greatly appreciate any explanations as to what the different wires are in this switch box.

Details

The house is old, built in 1939. Some of the old cables are metallic sheath. The box for the switch is metal. This adjustable switch was functional, but the mechanical parts of the switch broke. That's why it is being replaced.

The current (broken) switch fixture has two wires connected to it. As Image 1 shows below, there are three wires on the top right, and two wires on the top left.

There are two wires connected to the old adjustable light fixture. A reddish wire connects directly from the top right of the box to the right of the old fixture. (Image 1.) My guess is that this is the live line in. (Q1 - does this seem right?)

On the left of the old switch fixture, a whitish wire seems to connect via a cap to BOTH a wire from the top right and top left of the box. (Image 1.) The white wire seems like it must be the load out, but I'm confused why it connects to two wires, which seem to go in both directions out of the box. All the diagrams I have seen just show the load out as one wire. (Q2 - what are these two wires connected to the white wire on the old light fixture?)

Image 1 - Wires to Fixture

Image 2 shows three additional wires that are capped together, and do not appear connected to the old light fixture. One of these wires comes from the top right of the box. Another comes from the top left. The last appears to connect somehow to the bottom of the box. I assume these are neutral wires, or maybe ground wires. However, I haven't seen any diagrams that have three wires for either purpose. (Q3 - what are these wires?)

Image 2 - Additional Wires

Bonus Question: I want to connect a new single-pole fixture to this box. It has connections for live, load, and ground. Where would I connect each? My current guess is that I would just replace the connections to what I assume are the current live and load. For the ground, could I connect to the screw in the back of the box?

Thanks for your help with this novice's question.

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  • When you say "adjustable switch fixture", are you talking about a light dimmer, or...? – ThreePhaseEel May 29 at 19:58
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    Yes - a dimmer switch. – Novice1234 May 29 at 20:20
  • How many lights does this dimmer switch control? – ThreePhaseEel May 29 at 20:21
  • It controls one chandelier with four bulbs in it. This chandelier was put in recently, a few years ago. It replaced an older chandelier with maybe 5 candle-style bulbs in it. I'm not sure what might have existed before that. – Novice1234 May 29 at 20:24
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This is a pretty typical install

This is about what you'd expect to see in a situation where power is brought to the switch first, then carried to the light along with the switched (dimmed, in your case) hot for the fixture. The wire junction with the white pigtail to the dimmer sticking out of it is that always-hot power I mentioned, while the reddish wire is the switched/dimmed hot going off to the chandelier, and the wire junction that does not connect to the dimmer is neutral.

So, your new dimmer should just be able to be wired up with the existing pigtail on one hot screw and the reddish wire on the other hot screw, provided it doesn't have any smarts, of course. Grounding is done through the screws mounting the dimmer's yoke in this case, since the box is sort-of-grounded through metal cable armor from the looks of things.

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  • Thanks - I think I follow that explanation. Any thoughts on what might be a reasonable way to ground? Could I connect ground to the screw in the box? – Novice1234 May 29 at 21:40
  • Looks like the box is plastic. Connecting a ground to the box wouldn’t do any good anyway. Does the old cloth wiring have a ground anyway. Don’t think it does. – user137119 Jun 8 at 20:46
  • The style of cableclamp visible in the boxes in the photo is not found in plastic boxes in North America, and "cloth wiring" is not specific enough of a descriptor -- I can see the tails of spiral BX/old-AC armor in there, which provides a not-great-but-maybe-better-than-nothing grounding path – ThreePhaseEel Jun 8 at 23:11
  • When I tested for connectivity from the live wire to the screw visible in the first picture, I got the expected voltage. So I figured maybe it would work as a ground. I ran the ground from the new switch to under this screw. I'm not sure how great an idea that is. If I understand correctly, seems like eel is saying "maybe better than nothing." I guess that is what I'm aiming for. – Novice1234 Jun 9 at 2:38

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