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I am working on adding outlets to my garage and decided to open up this strange double switch box that is 6 or 7 feet up on the wall.

There are two separate circuits in this box, one that the kitchen and dining room lights are on, and one that the garage lights and existing outlets are on. From top left to right:

  • Cables 1 and 3 are on the dining lights circuit (cable 1 is the power feed, hots were already separated in this pic)
  • Cables 4 and 5 (bottom right) are 3 wire cable and part of the 5 way switch wiring for the garage lights (lights can be switched on and off from 4 different locations).
  • Cable 5 is the one bringing power into the box, where the travelers (black and red) are spliced to the black and red from cable 4.
  • The white from cable 4 is bundled with the other neutrals in the box (which includes neutrals from the dining lights circuit), and the white from cable 5 (power) is connected to the black from cable 2.
  • The white from cable 2 is part of the neutral bundle.

As you can tell from my description, I was able to dissect the box and identify the cables, and I successfully replaced the box and connections properly and everything works. But... I am confused about the wiring on cables 2, 4, and 5. Why is cable 4 picking up a neutral in this box? And why is cable 2 being powered off of cable 5? Lastly, is it appropriate for these two separate circuits to be sharing neutrals? I hope you are able to follow my post, I know it's a lot to read. Thanks in advance.

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  • What is on the other end of cable 2? – A. I. Breveleri Mar 4 '20 at 16:56
  • I can’t be sure. I don’t know if cable 2 is going directly to the lights or if cable 4 is. They both go up and into the attic at the same point. I wasn’t able to get up there and dig around. – clwhoops44 Mar 4 '20 at 16:59
  • Are cables 1 and 3 on the same breaker as everything else? Or a different breaker? – Harper - Reinstate Monica Mar 4 '20 at 18:31
  • Cables 1 and 3 are on a different breaker, the one that controls the dining room and kitchen lights. 2, 4, and 5 are on the breaker that controls the garage outlets and lights. In the original configuration, 1,2,3, and 4 were sharing neutrals. – clwhoops44 Mar 4 '20 at 18:42
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OK, so it sounds like Cable 2 is your actual power supply to this circuit. Black is hot and white is neutral.

Cable 5

goes off to a 3-way switch. There may also be one or more 4-ways in between the 3way and here. The 3 wires in cable 5 are: (all the way through any 4-ways to the far 3-way):

  • Always-hot (white, by law, but also requires black tape or sleeving)
  • Traveler (black; I recommend yellow tape or sleeving)
  • Traveler (red; I recommend yellow tape or sleeving)

The 3-way's job is to connect always-hot to one or the other travelers. If there are any 4-ways in the line, they will swap the travelers (or not), alternating which one has always-hot.

The always-hot must be white because neutral is not present, so white is being reused for a hot, and if always-hot is present, it must be the white so it is more clear that it's not neutral. It also requires the sleeving.

Cable 4

Cable 4 carries the other fork of the 3-way circuit. Down cable 4, there will be a 3-way switch, as well as any number of 4-way switches; and then at one of those switches, a branch will come off going to the lamp. The three wires are:

  • Neutral (white, by law)
  • Traveler (black; I recommend yellow tape or sleeving)
  • Traveler (red; I recommend yellow tape or sleeving)

So that pretty much "wraps up with a bow" the cable 2-4-5 relationship. But now, we've got a problem.

Cables 1 and 3 seem unrelated to the others

If cables 1 and 3 were both downline circuits that got their "hots" from cable 2, I'd be like "Okay, that's normal enough". But what you seem to be saying is that their hots only connect to each other, not cable 2. If that's the case, the neutrals mustn't connect, either.

If cables 1 and 3 are shut off by a different circuit breaker than cable 2, then it's absolutely vital they be separated. You cannot mix neutrals of different circuits!

Granted, you're supposed to clump together all grounds. But because of this, people often think they can do the same with neutrals. No. Nothing prevents both circuits' current from returning on the same neutral (e.g. if one wire broke). This can cause a 200% overload on the active neutral wire, and what will detect that? Nothing. Neutrals don't have breakers. Also, it will cause trouble if a GFCI or AFCI breaker is fit to either circuit, because they monitor differential current between wires.

If separation is called for, you might want to just break this out into a totally separate junction box, to reduce further chance of mistake.

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  • As always, Harper, you nailed it. After thinking about it a little more, and going home on lunch to snap a new picture, I agree that cable 2 is the power supply, not cable 5. And yes, cables 1 and 3 are unrelated to the others. They have been separated and capped individually. I'm glad I decided to investigate this random box with 2 switches that didn't seem to control anything. I'm not surprised by anything I find in this house anymore. – clwhoops44 Mar 4 '20 at 19:21
  • @ Harper after putting a little more thought in I still have one question that remains about cable 4.... if the white wire in cable 4 is a neutral, how can there be a 3 way switch downstream of that? – clwhoops44 Mar 5 '20 at 13:58
  • @clwhoops44 The downline 3-way selects one of the 2 travelers and sends it to the light as switched-hot, and sends neutral to the light. – Harper - Reinstate Monica Mar 5 '20 at 22:56
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When you find a white connected to a black it is usually a switch leg not a neutral. If you check you will probably find a switch that white goes to that powers a light , white should be reidentified as a hot with tape or paint at both ends the black at the switch will be hot when turned on , this method has the white always hot so it will not be misinterpreted as a neutral.

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