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I'm trying to figure out the wiring in my finished garage. It has two ceiling junction boxes, with attached porcelain lights, and three wall outlets.

I'll say up front that all of this was done safely...

I opened up what I think is the "main" ceiling junction box and found four armored cables junctioned here. Okay, I think I guessed right. Three of the cables have just a white wire and a black wire, and one of the cables has a white, a black, and a red traveler.

I separated the four ganged black wires and figured out which is the "power in" with a non-contact voltage tester. We'll call this Hot #1.

I connect each of the other Hots, in turn, to Hot #1 and got these results:

  • Hot #1 to Hot #2 powers the other ceiling light fixture. Btw, this Cable #2 is the one with the red traveler.
  • Hot #1 to Hot #3 powers Outlet A and Outlet B.
  • Hot #1 to Hot #4 powers Outlet C, and surprisingly, also Outlet A.

Huh?! That last one doesn't make sense to me. Can anyone clue me in to what's going on?

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    Need to know what switches you have and what positions/connections they have on them (you haven't necessarily given us all the info). Why did you decide the red is a traveller and not another hot or a switched circuit beyond the "other light". – noybman Jan 11 at 0:31
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    What is the question ? did you have problem before you started, if so what was it ? Did you take notes and how things were wired in the "main" box ? Are trying to get it back to the way it was, did you take apart the wires to try and fix a problem ? How do you want it function ? You can not assume with wiring, you need to know were each wire goes and why. – Alaska Man Jan 11 at 1:28
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    The question is: Please explain how this could be wired to allow the above testing results. House was built 60+ years ago and I want to understand how this is wired before I make changes. – Mike Jan 11 at 1:31
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    Are you only testing with the voltage tester, or are you also plugging in a practical load like a vacuum cleaner? Sometimes testers will read up phantom voltage... – Harper - Reinstate Monica Jan 11 at 3:08
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    Can you post photos of the insides of the boxes involved please (lights, receptacles, switch)? – ThreePhaseEel Jan 11 at 5:01
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Here's the answer.

It turns out there must be another junction box. I made an incorrect assumption that Light/Junction #1 is the main junction. It isn't.

It appears to be wired like this: Breaker feeds some junction that I can't see which then feeds Outlet A and Light/Junction #1.

So anytime the breaker is on, Outlet A is powered no matter what.

Really sucks if this main junction is in the finished garage ceiling and no way to find it without busting up that ceiling (cement/plaster type with lath of heavy wire mesh). Jeez, all they had to do was put the main junction in the unfinished ceiling of the adjacent laundry room where the power cable comes through anyway.

Update: There may not be another junction box afterall. See photos. That's a very tiny access hole that I pushed my phone up against. You can't see the full width of the joist bay. Two cables coming back towards the camera. One is the cable from the breaker and the other actually goes down the wall to Outlet A. The two cables going horizontally to the left is odd, since the only thing over there is Outlet B. More investigation is needed, but it looks like I might be able to feed a new 20 amp (12 gauge) cable through here, which is what I've been wanting to do. Unfortunately, there's no access on the other side for Outlet C and this junction box can't be removed without busting up the ceiling as the cross bar mounting goes through the box.

Update 2: There is indeed another junction box, but it is not hidden. Well, sort of. The furnace duct in the garage is very close to the wall where Outlet A is. The photos I took put the junction much closer to the wall than the known Light/Junction #1. Sure enough, there is another Light/Junction "hidden" behind the duct. No bulb in it, so I never knew it was there. But this is definitely the main box for the garage. Whew!

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  • Glad you were able to get it sorted! Hidden junction boxes sure are a pain in the rump... – ThreePhaseEel Jan 11 at 22:32
  • Did you alreadyconfirm that outlet A isnt whats feeding the light? It seems you are asserting that there is a hidden junction, but if what you are describing in this answer is accurate, theres still not enough information to make this claim unless you have now opened the outlet and confirmed there is only 1 romex/shielded wire coming into it. – noybman Jan 11 at 22:36
  • @noybman: All of the outlets have only 1 cable (armored) coming into them. My answer is based on a test that I must have missed before...When all hots are disconnected in Light/Junction #1 the only thing powered is Outlet A (which has only one cable in it). – Mike Jan 11 at 22:56
  • Thank you for confirming regarding that there is only one cable in each outlet. The last location you have not ruled out to us with the given information is the swtich box itself. The breaker fed cable can be in it, and spltting to lights and outlets. Also, just for completeness, please tell us what the red wire is doing between the lights (presumalby it is a no connection based on what you haven't told us). – noybman Jan 12 at 19:29
  • Well, the switch is at the other end of the garage, over 15 feet away. The only cable entering the garage is exactly where the Outlet A is. So there must be a junction very close by. Too bad the ceiling is constructed like it is. The ceiling junction boxes can't even be pulled out without tearing into the ceiling. If I was a masochist I would be tempted to pull the wood flooring in the room above to get full access. Way too much work. – Mike Jan 12 at 21:39
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It could be that Cable #2 (the 3-conductor cable) from Main light to Other light carries neutral and hot to Other light and to Wall switch. The red wire then brings switched hot back to Other and to Main lights.

The only other mystery is the result that Cable #3 and Cable #4 both powered Outlet A. Supposing Outlet A is a duplex receptacle, are both of its sockets powered in both conditions -- or one socket with Cable #3 and the other socket with Cable #4?

It's also possible that you have something more advanced going on, such as a multi-wire branch circuit. You'd have to bring us photos, and/or measure with a volt meter the voltage between hot-hot and hot-neutral pairings of wires for us to assess that possibility.

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