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This has been a great site over the years, thank you. Now it’s my turn to post a question.

I did see some similar questions as mine; however, s/he used a different switch so I’m not sure if the answer is the same.

I have two lights operated by separate single pole switches. I’d like to replace them with dimmer switches. There’s not a lot of room in the box, so I found a switch that has two light dimmers on it (not a fan/light slider). It is Lider Electric LDDS-1P-WWP Dual LED dimmer switch singe pole… The box says singe; however, I assume it’s single. (The type-o on the box doesn’t add to confidence about the product.)

As you see in the picture of the dimmer back, there is one screw for the hot line (two backstabs in the back), two load screws, and of course ground.

As you see in the picture with the wires, each set has a black, white, and ground. They appear to go directly to the lights. If I follow the instructions and connect the white wires together, I still have one too many black wires, and no load wires.

I was going to try wiring it different ways, but thought I’d post the “what do I do?” question here. To be honest, it did try wiring it with one black (capped the other with a nut), and the two white wires. Both switches operated one light.

Also, I don’t know if this is related or maybe is a separate post: it’s regarding the dual toggle switch in the picture. With the power on and the toggles off, my non contact tester beeps up a storm when near the white wires and not the black wires. I would think it should be the other way around? When the toggles are on, both white and black beep up a storm.

(Note: I see the screw on the side of the box… is that at risk of arcing and be relocated?)

Thanks.

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SkierDude
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    You probably have a pre-NEC 2011 switch loop - if you took before pictures (with the wires that connected to the old switches still connected) that would clarify the situation considerably. If the white wires were connected to the old switches, (as the hooks imply) that's almost certainly what you had. The grounds are not properly connected here, either. – Ecnerwal Mar 06 '24 at 16:43
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    You have old switch loop. The whites are supposed to be hot(and marked black) while the blacks are switched hot. You need to find out if both lights are on the same single breaker. Someone else will be by on how to connect. – crip659 Mar 06 '24 at 16:44
  • Thanks for the fast replies! @Ecnerwal: yes, the white and black wires were connected to the old switches. The white ones were on the brass screws. The grounds are attached to the switch. – SkierDude Mar 06 '24 at 17:12
  • The issue with the grounds is that the one loosely twisted around the other is NOT a suitable connection. The same configuration can be made properly with a "greenie" grounding wirenut that has a hole for the longer wire to pass out the end, which can then be tighted down to make a proper compression connection beween the shorter and longer wires. – Ecnerwal Mar 06 '24 at 17:15
  • @crip659: yes, they are on the same single breaker. This is in a room addition built in 2011 or so. The room is on the same breaker (3 receptacles, two lights). And, if the whites are supposed to be marked black, I can do that. – SkierDude Mar 06 '24 at 17:16
  • @Ecnerwal: Easy fix, thanks. Thoughts on the drywall screw in the side of the box? I'm starting to lose confidence in the builder (was in 2011, can't call him back). – SkierDude Mar 06 '24 at 17:18
  • As I think about this, I wonder if I use the double toggle for the other two light switches (not shown), and get separate sliders for these. However, one of the other not shown lights is a 3-way, I'm guessing there's a double toggle for that. – SkierDude Mar 06 '24 at 17:20
  • Go ahead and screw down the drywall screws. I'm confused at this point; could you succinctly restate what the remaining problem is and what you know (what works, what doesn't) A wiring sketch keyed to this new description would help. Show where the wires go, their colors, number them and use the numbers in the description. – MadMonty Mar 06 '24 at 17:45
  • @MadMonty: I'll work on that after work/dinner. But in short, the problem hasn't changed significantly. We've identified the lack of neutral, and the white wires are hot (should be marked black). My guess is they're the line and the black are the load. I'll do a diagram later for you.c – SkierDude Mar 06 '24 at 18:21

1 Answers1

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You can't turn a pair of switch loops into a double-switch (dimmer or otherwise) that only has one "line" connection. Why? Because in each cable the current needs to be balanced. In a switch loop that means "hot" (or "line") and "switched hot" (or "load") need to be in the same cable. That works fine for a single switch loop. But if you have two switch loops in separate cables then if you combine the "hot" wires onto one double-switch then there is no guarantee that each hot will have the same current as its matching switched hot. So that just doesn't work.

The alternatives are either to find a single-gang double-dimmer switch that has separate "hot" connections (I don't know if any are readily available) or, in your particular situation, a double-switch that includes a single switch and a 3-way switch with entirely separate connections.

As it turns out, combination 3-way/regular switches are available, such as this Leviton from Home Depot:

3-way/regular

Note that you can also use a double-3-way switch, as long as it has entirely separate connections for the two switches. That's because a 3-way switch works perfectly fine as a single switch if wired correctly.

manassehkatz-Moving 2 Codidact
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    Thanks. First part makes sense. Since there are two backstab holes in the back, I'm guessing that doesn't mean I can attach both hot lines into those and tighten with the screw.

    Regarding the 3-way switch for the other side of the box (not shown), only one of the switches needs to be a 3-way. I but I know the other one can be wired as a single.

     – SkierDude Mar 06 '24 at 21:58
  • Why would they sell a dual dimmer switch that operates two separate lights but with one hot line terminal if the issue could be unmatched current going to both? – SkierDude Mar 06 '24 at 22:04
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    Because (a) dual dimmers a relatively small market compared to general double switches (i.e., fewer uses), (b) old-style switch loops are gradually going away (none in new construction in most places for many years now), (c) if you are designing a new two-switches with lights setup today with neutral required everywhere, etc. using switch loops might actually use more wire than panel->2 switches->2 separate lights. Your setup is relatively unusual. – manassehkatz-Moving 2 Codidact Mar 07 '24 at 00:20
  • Coming back to this answer. It said “only one line connection”. However, there are two two spots for hot in the back. Can I use those? – SkierDude Mar 07 '24 at 03:05
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    Yes, from the picture and from the diagram in the installation manual, it looks like each of those holes (2 hot, 2 load 1, 2 load 2, 1 ground) can take a wire and that they are screw-to-clamp and not "back stab". But that doesn't solve the switch-loop problem. The reason for two "hot" wires is normally for one incoming (from the panel) and one going onward to the next switch or light or receptacle or whatever. Doesn't help here because of the balance each cable issue. – manassehkatz-Moving 2 Codidact Mar 07 '24 at 03:22
  • I ended up buying two separate sliders. But just out of curiosity, I tried putting the two switched white wires (now marked black) into the hot holes in the back and screwed the clamp. I then put the switched black wires into the load terminals. I attached the ground.The switches work fine. – SkierDude Mar 08 '24 at 00:20
  • I absolutely 100% expected that the switches would work fine. The problem is that it violates code because there is no guarantee of balanced currents in the cables. With modern low-power lighting (i.e., light up your living room, not a big warehouse) it would probably run fine for years. But there is a non-zero possibility of a problem that would cause overheating wires that in turn could cause a fire, all of which is prevented with two separate switches. – manassehkatz-Moving 2 Codidact Mar 08 '24 at 00:21
  • I just don’t understand how it wouldn’t be balanced. They’re completely separate sets of wires. But… I’ve been around the block a few times and learned to recognize to know what I don’t know and listen to people who do know.

    I will use the separate sliders.

     – SkierDude Mar 08 '24 at 00:29
  • Circuit 1 - 1A of lighting - 1A down the hot, 1A up the switched hot. Circuit 2 - 2A of lighting - 2A down the hot, 2A up the switched hot. Now you combine the hot wires. The electrons don't "know" to split up 1/3 on this wire and 2/3 on the other wire. In fact, if the wires are identical (and they usually will be, but longer wires will have more total resistance) then the current will split 50/50. So now you 1.5A vs. 1A and 1.5A vs. 2A for the two cables. But it gets worse. What about if you only have the 2A on? Now it is 1A vs. 0A and 1A vs. 2A. Etc. – manassehkatz-Moving 2 Codidact Mar 08 '24 at 00:33
  • That explains it… thanks! – SkierDude Mar 08 '24 at 00:44
  • Because this is how my projects roll: the dual switch I bought is actually dual 3-ways even though I showed the person what I needed (duplex with one 3-way and one single pole). And, the switch is 15 amp… forgot that the single pole circuit is 20 amp. The switch is for an outside light that isn’t used (solar powered) and not drawing power so I’m not concerned. – SkierDude Mar 08 '24 at 04:33
  • The dual 3-ways is actually NOT a problem. You wire up one wire to the "common" screw and the other to one of the "traveler" screws and it will work just fine as if it were a regular (not 3-way) switch. – manassehkatz-Moving 2 Codidact Mar 08 '24 at 04:51
  • As far as 15A switch on 20A circuit, my understanding is that it is generally permitted much the same way that 15A receptacles (as long as at least 2) are permitted on 20A circuits. There are more issues if the switch is controlling a receptacle (for reasons) but controlling one or more light fixtures (provided total current for all fixtures connected to that switch is <= 12A) you should be fine. – manassehkatz-Moving 2 Codidact Mar 08 '24 at 04:57