What the heck are these wires in my switch box?

I’d really like to add Z-wave or Zigbee switches to my ST setup. I’ve found one switch box that has a bundle of white neutral wires in it, but most of the junction boxes in my apartment don’t. Then there’s this one:

This single gang box has a switch to a ceiling light. I know that the two white wires connected to the switch are line and load (the wire on the left has 120V when the circuit is on, the wire on the right has 0V), and the red wire is a traveler to a switch at the other end of the room that also controls the ceiling light.

Can anyone help me figure out what the two black wires in the back connected by a wire nut might be?

Only one way to find out. Lol. Seriously, you may want to test with a muitimeter. Color code says those are jumpered Hots but anything is possible. Neutrals can also come in all colors although they are supposed to be White. Since you have Line and Load as White already those may be your Neutrals.

If those are hot I’m not sure what they could be connected to…

If those are neutrals, what should I see on the multimeter? 0V on both wires?


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Could be this

It’s a little hard to guess unless you can take a picture of the other switch as well. Also try showing the wires going to the common screws on the switches.

The white wire on the left has 120v when the circuit is on but does it changed when you flip the switch? If it does change then that’s not your line.

99% of the time. Your line and load will be connected to the common screws so look there. Your neutral could be anywhere depending on the electrician. In the mean time. You can use your ground as a neutral for your voltmeter measurement if you are not sure of neutral yet.

Thanks for your suggestions @Navat604. Here is a pic of the switch showing the wires connecting to screw terminals:

The white wire from the left is connected to the common terminal. This wire is hot when the circuit is on, regardless of the switch position. The white wire from the right only has 120V when the switch is flipped. Flipping the other switch will cause the red wire to read 120V.

The mystery black wires in the back read 120V when either switch is flipped in this three-way setup. But when both switches are off there’s 0V running through the black wires.

At the other switch, there is one black, one white and one red wire connected to that switch.

I think @Automated_House might be right. The 3 way setup described in that link seems to fit with the wires in mine. I have no ground wires in my switch boxes (any of them), since my apartment was renovated probably in the early 2000s and I guess including a grounding wire in the boxes wasn’t required by code at the time.

Seems as though those black wires probably aren’t neutrals. Bummer.

In some older wiring the ground is connected only to the metal switch box or metal conduit is used and it acts as the ground.

If you really want to confirm that’s your wiring. Remove the light bulb and check your reading again. You will not get 120v again compare to the previous measurement. Possible you will get some weird voltage but not 120v.
You can use an Aeon micro switch at the light fixture if that’s your wiring.

@Automated_House thanks for clarifying re: grounding. It does look like there’s metal conduit coming in and out of my switch boxes. But most switches don’t have any grounding wire actually connecting to anything. What can I do to make a connection from a grounding screw terminal/pigtail wire on a switch to the box itself?

@Navat604 I’ll try unscrewing the bulbs and remeasuring the voltage. I’ve thought about adding relays to the boxes at the lights themselves, but was hoping to avoid that if possible since that’s a bigger project than working at the switch boxes.

I’ve also considered asking an electrician to look into pulling neutral wires into my switch boxes. But since I live in an apartment (i.e. no attic/crawlspace to access) I’m kind of assuming that would involve busting some holes in the drywall.

You would have to check your breaker panel box and see if it is grounded. I’m not an electrician, but from what i remember reading, the metal conduit and boxes actually act as your “wire”. So by screwing your switch into the metal outlet box, it is then grounded. There would still need to be a wire at your breaker/fuse panel box that then runs to the earth. Since it looks like you have a power into fixture scenario, pull off your fixture and confirm what wires you have in there.

I see. I’m not sure if I want to start messing around in the breaker box itself. But this weekend I will probably try unscrewing the fixture to confirm what’s going on up there. Should there be a grounding wire between the fixture and the breaker?

Just put your negative voltmeter probe to the metal box and positive meter probe to line hot. If you have 120vac then the box is grounded. If not then it’s not grounded.

Haha, that’s what I was doing when checking voltages in the switch boxes anyway so I guess I should have realized. Thanks for clarifying.

If I add a relay to the wiring at the fixture itself, will the relay be able to report to ST that the light is on when I use each of the three-way switches (rather than the relay through the ST app)? i.e. does it sense that the light is on and report that, or does it need to be connected directly to a switch to do that?

Yes, you will get status report when using the 3 ways switches or with the app with a relay module. The Aeon micro also does power report as well if you buy one with that option.

If you don’t connect the relay module to the switches then there is no physical control and I don’t recommend that. No internet = no ST app control.

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OK. Assuming my setup is powered into the light like the diagram above, I’m trying to wrap my head around how I could connect a relay to line and load (which I think I get), and also the three-way switches (not so sure I get).

Here is a drawing with the Aeon micro in place at the light fixture. It’s close to the end of the post.

Got it. Apparently the switch connections on the aeon relay take 18 gauge wire. I would need to get short runs of 18 gauge wire and wirenut them to the thicker wires that run to the switches, correct?

That’s correct but only for the switch side not load, line and neutral terminals. It will fit your 14 gauge. You can get the standard rw90 18 gauge stranded wire for 30 cents a meter for the switch terminals. Don’t cheap out with random wire at home.

I took a look in the box at the fixture. There’s actually more going on up there than I was expecting.

The single black wire going to the fixture is hot. So that should make the white wire going to the white bundle the neutral.

Then there is a bundle of three black wires and one white. This all doesn’t quite match up with the diagram above, so I’m not sure what to do next.

Maybe the bigger issue, assuming I sort out what goes where, is that I don’t think the box is big enough to actually fit the aeon micro switch in addition to all of the wires in there…

So if the black wire going to the light is hot, then you probably have the wiring as diagrammed in the link prjct92eh2 posted.

Basically, you provide a hot and neutral to the light, and the switches are downstream. The neutrals are switched downstream. That technically makes them potentially hot, so they should have a piece of black electrical tape on them, as you see in the diagram that was linked.

You can test this by going to the switches and see if the neutrals are hot in one of the switch positions.