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This light switch broke. (Plastic paddle physically broke and fell off). It is a Decora switch, and behind it I found an old BX cable. The armor is clamped securely to a metal box and a red anti-short bushing is present where the armor was cut. The cable has three conductors, insulated with rubber and red,black,white cloth. The red is connected to the ground terminal on this light switch.

enter image description here

The question is to anyone with experience wiring or rewiring during the 1950s, 60s, 70s, 80s .... when BX was still used and Decora switches were also in use. Why would a three-wire conductor be run to this light switch (it is not a three-way switch), why would one of them be red, and why would that be connected to ground? Was there some common practice of using a cable this way?

There is no continuity between red and the box. There is no continuity between any pair of wires or between any wire and the box.

Should I cap off the red conductor with a wire nut or connect it to the ground terminal of my replacement switch as it was with the old one?

I realize this is not common and there is no obvious answer. I'm looking for some "wisdom" from anyone who's seen anything like this.

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  • Off hand would say somebody had 3 wire BX and did not trust bx for good ground(or did not know).
    – crip659
    Oct 18, 2021 at 18:08
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    Any chance of seeing the other end of the cable? Were they using red for ground.
    – crip659
    Oct 18, 2021 at 18:52
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    I wonder if someone had 3 wire BX and used it to connect a 1950s-era switch that did not even have a ground terminal, leaving the red wire unused and unterminated in the box, and later someone replaced the switch with this Decora one, found the red wire half way through the job, didn't know what to do with it so connected it to the only thing that was left to connect it to, the ground screw? Leaves me wondering what is happening at the other end, whether my theory is correct or @crip659 's.
    – jay613
    Oct 18, 2021 at 18:54
  • There is no chance of seeing the other end. If there was I would have a LOT more info to share! This is a 1st floor powder room and the cable travels upwards out of the box. It might go to the light fixture, but I replaced that fixture about 5 years ago and do not remember having an extra wire to puzzle me. Maybe I'll take down the new fixture and have another look .... weekend project.
    – jay613
    Oct 18, 2021 at 18:56
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    So far red and bx not connected. Red might not be connected to anything or bx is not making ground past the box.
    – crip659
    Oct 18, 2021 at 21:05

1 Answer 1

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There will be continuity between red and the metal box, once you install the switch into the box.

The mounting screws on a switch are a perfectly legitimate way to ground a switch. The green screw on the switch is connected to the yoke on the switch, which in turn is connected to the screw, the metal box and the jacket on the BX.

So it's possible someone is doing something weird, like using the red as neutral and letting it path to the panel's neutral via the grounding system.

However, I note two things. First, insofar as Photoshop's "Levels" function will reveal, there is only 1 cable coming into this box, so it is absolutely, positively a switch loop. Its only possible destination is one of the lights or fans it controls, unless there is an intermediate box somewhere which has been "buried", and it's illegal to "bury" boxes. All wires must terminate in boxes, and all boxes must be accessible without removing any of the building's finish materials.

Second, I note the wall staining and tiles. I would guess this is a bathroom. Bathrooms are very often installed with 3-wire+ground cable between switch and light/fan. This is to allow separate control of the light and fan using a dual switch.

So I would investigate any light and fan boxes further to see if a red wire shows up.

Alternately, a /3 "switch loop" can be used to comply with post-2011 requirements, which require always-hot, switched-hot and neutral to be carried on switch loops. The neutral is to empower smart switches, motion sensors, lighted switches and countless other applications that need power at the switch.

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  • Hmm. By stating some obvious (to me) things you've given me some additional motivation to go back and remove the light fixture I installed several years ago. The other end of this cable is almost certainly there. I really don't remember an extra wire, and I would not have ignored it. You're right, it's a powder room, there is only one cable.
    – jay613
    Oct 19, 2021 at 0:03
  • @Jay sweet, so you might even be able to get an upgrade out of the deal :) Oct 19, 2021 at 0:06
  • As for 2011 requirements, I'm curious why you say that. The cable is probably from the 1950s or earlier and the broken switch was, I'm guessing, between 1980 and 1995 based on the condition of the plastic.
    – jay613
    Oct 19, 2021 at 0:06
  • An upgrade? Hmm ... ok ... here's a plausible explanation. At some point in its life there might have been a vanity unit in this room with two different lighting options controlled by a double switch. When that was replaced with some subsequent decor and just one light, they attached the red to ground just to store it somewhere. Perhaps the thinking was that if it ever became accidentally energized, the popped breaker would flag it.
    – jay613
    Oct 19, 2021 at 0:13
  • Marking this correct until proven otherwise, which may be never, for deducing it must be a switch loop and that being in a bathroom the red probably controlled something else previously.
    – jay613
    Oct 19, 2021 at 14:04

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