You are not my electrician.
January 31, 2014 10:13 PM   Subscribe

I need some help figuring out the wiring in my switched ceiling junction box in order to connect a basic light fixture. In the box I have three pairs of black/white wires and one yellow wire. My fixture has one pair of black and white wires.

Initially, all the white wires were connected and all the black wires were connected. My three-bulb light fixture has a pair of clear wires: one smooth and one ribbed. Treating smooth as black and ribbed as white, I have followed the procedure basically outlined here, ignoring the yellow wire. I was able to identify the pair carrying power, but I haven't been able to identify which pair goes to the switch.

Let's say my the wire pairs in the box are A, B, and C. When I connect up my fixture to A only, all the light bulbs turn on, so A is my power source. One of the other pairs should go to the switch, and the other should go to the next fixture in series (wall outlets in this case). Following the procedure linked above, testing whether B leads to the switch yields no circuit whether the switch is on or off. Testing whether C leads to the circuit causes only one of three bulbs in the fixture to flicker whether the switch is on or off — the other two bulbs stay off. (The bulbs are CFLs, but this is concerning.)

I have studied electronics and feel like this should be a simple problem to solve. I have diagrammed the possible connections several times. I have experience installing about a dozen ceiling lights in the past with no issues, including some that required diagramming and testing like this. I am using proper safety precautions and switching off the breaker while working at the junction box. I don't believe polarity should be an issue with the CFLs since they are AC two-terminal devices, and diagramming the connections shows that the polarity shouldn't change from when I isolate live-pair A and get the fixture to light up. The wiring in the ceiling was installed in the last year as part of a significant apartment building renovation, so presumably they it is all properly installed.

Difficulty level: I don't have access to a voltmeter or even a simple line tester right now.

What am I getting wrong? Is the single yellow wire important?
posted by stopgap to Home & Garden (10 answers total)
And when I said "the next fixture in series," I know that the final wiring arrangement will actually be parallel with my light and switch. I used that phrase without intending to imply its specific electrical meaning.
posted by stopgap at 10:21 PM on January 31, 2014

Finally, it seems to me that with all the blacks in the box wired together and all the whites wired together (as I found the box) flipping the switch should cause a short, so something else must be going on with the wiring. Ugh, I hate thread sitting. Sorry.
posted by stopgap at 10:30 PM on January 31, 2014

This isn't something you should be trying to do for yourself. Hire an electrician.

Yeah, it's expensive. But it costs less than burning your home down, or dying of electrocution.
posted by Chocolate Pickle at 11:10 PM on January 31, 2014 [1 favorite]

Before you messed with anything was the fixture working correctly?

Are you 100% positive that the 3 blacks were tied together and three whites were tied together and there were no other wires (except for bonds) in the light fixture junction box?

If so my best guess is you have two other devices besides the light fixture controlled by your wall switch (often a split plug where one half is switched and the other half isn't). This would be an unusual setup but I can't think of anything else that would be wired that way. If you hook up your fixture to A only and it turns off and on with the switch and the intial setup was correct and as described then that is indeed the case. If it doesn't then something is messed up and without a meter you'll need to call someone.

It is entirely possible that someone in the past has messed up the wiring in this box.

Is your wire running in metal pipe? Yellow would be an extremely rare colour in a cable and actually it is a pretty bizarre colour in any kind of residential wiring in Canada/USA. Either way it's unlikely to be there just for fun; at a minimum it would have been ran to allow you to switch two different loads at your junction box even if you only had a single load at install (think fan separate from the built in light).
posted by Mitheral at 1:22 AM on February 1, 2014 [1 favorite]

Pictures would be extraordinarily helpful and you can get a voltmeter cheap at an auto parts store or Home Depot, if that's at all convenient. Is it possible the "yellow" wire is the shielding around a 3-wire cable, containing another set of black/white and bare copper?
posted by Pantengliopoli at 1:31 AM on February 1, 2014

My general rule for doing this kind of thing is to immediately stop the moment I no longer know exactly what's going on, and it sounds like you're at that point.

This isn't the kind of situation where just applying a logical, diagnostic approach will eventually prevail over a lack of knowledge--you need actual experience in order to prevent dangerous mistakes. Call an electrician.
posted by RonButNotStupid at 6:48 AM on February 1, 2014 [2 favorites]

Difficulty level: I don't have access to a voltmeter or even a simple line tester right now.

Then you shouldn't be doing anything. Period.
posted by RonButNotStupid at 7:00 AM on February 1, 2014 [2 favorites]

Okay, Mitheral put me on the right track—one of the pairs led to a switched outlet that is on a different switch. I checked the wiring inside the switches to confirm, drew up a little diagram, and everything worked right away. The yellow wire leads to the switch, so I needed to wire my fixture white to the other whites, leave the three blacks alone, and wire my fixture black to the yellow wire. I had never seen yellow either, but this is new construction, so I doubt it's non-code. This is US wiring by the way.
posted by stopgap at 7:30 AM on February 1, 2014

Also, wiring the fixture to A only made the light be always on—the switch wasn't connected to anything at that point.
posted by stopgap at 7:32 AM on February 1, 2014

You don't need any more advice to not do this on your own; you're evidently going to. What you do need is the advice to get this book before you finalize the job:

Wiring Simplified

Wiring Simplified is the nearly universally-available guide to the type of wiring found in the home, and it is updated frequently to include the latest code (you may find out about yellow wires, for example.) It's not dumbed-down, but it is fairly easy for the layman with rudimentary aptitude to understand. You'll find it both locally and online. You need it.
posted by dinger at 9:56 AM on February 1, 2014

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