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Discussion Starter #1
A recessed light on a dimmer switch stopped working so I opened up the switch box and am trying to understand the wiring. This switch box also contains a 3-way dimmer switch for a table light (with matching switch located across the room). In the diagram attached, the left side is the switch box containing the 3-way dimmer switch (which is working) and regular dimmer switch (not working), and the right side is the 3-way switch across the room. The table light has a single 14-2 romex cable connecting to the light fixture wires. One other note, the 3-way dimmer circuit is on a separate breaker from the regular dimmer circuit.

My observations/questions:

1. I am guessing that the white wire from #4 should not be "nutted" to the black wire from #3.
2. Should the white wire from #1 (from Breaker A) be "nutted" with the white wire from #3 (from Breaker B)?
3. Why am I not seeing any three-wire romex cables for the 3-way dimmer switch circuit?

Thanks.
 

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Licensed electrician
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Before you go making changes when you probably don't need to, change the non-working dimmer out for a regular switch. If the light now works get a new dimmer.
 

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Discussion Starter #3
Thanks Jim. I will give that a try. One reason I am thinking the wiring might be the issue is that we recently renovated our kitchen which is on the opposite side of the wall this switch is on. There was a kitchen outlet now removed) on that backside which tied through this switch box so someone had to be in there to unwire that outlet. I also just want to make sure there is nothing "unsafe" with the current wiring scheme ("nutting" neutrals wires from different breakers, etc.).
 

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If the three way is on a different breaker then it should have its white connected with #3 cable. You have cross the neutral from two different circuits.

Disconnect #1 white and cap. Connect #2 and #4 whites.
 

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Discussion Starter #6
So here is an update.

I swapped out the non-working dimmer for a switch, connected white #4 to white #3, and capped off white #1 by itself. The switch now works (so I am guessing it was a faulty dimmer), but now the 3-way light no longer works (which I am guessing is due to unconnecting #1 white from #3 white and capping it off).

So, I am suspecting if I reconnect #1 white to #3/#4 whites, the three-way would work again (which would essentially be the wiring I had originally except for "nutting" #4 white with #1 and #3 now). But, is this okay to do?
 

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Scared Electrician
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the lack of 3 wire cable indicated that at minimum this is a sub par installation (it is legal to use two 2 wire cables in place of one 3 wire but is a poor choice of methods)

You are not supposed to have neutrals from one circuit touch another (parallel paths- a no no)


This looks like they might have been using the neutral from another circuit /switching the neutral to make this bootleg 3 way work.
 

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Discussion Starter #8
That is my take as well. What if it was the same breaker? That is, what if it was #1 white with #3 white, where both #1 and #3 were on the same breaker? Clearly not the intended way for a 3-way to be wired, but it would appear to avoid the crossed neutral issue.
 

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Since there are two breakers involved the only way to make this safe and somewhat code compliant is if breaker A and B are on different legs of the service. Measure the voltage and see if you get 240 volts between the two breakers. If not then it is possible have 30 amps on that neutral. The two breakers should be side by side with handle tie.

This would make what is called a multi wire branch circuit MWBC.
 

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Discussion Starter #10
Thanks again Joed. I will check my panel is see how these two circuits are placed on the legs. There are a few other places with three-way switches in our house (which was built in the 60's) and would be original to the house. It appears they are also wired without 14-3 cable. So I am guessing they used the same approach (piggy-back onto another neutral). Maybe the original electricians knew what they were doing and used MWBCs at each point.
 

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Discussion Starter #11
And here is the next update. This is not a multiwire branch circuit. Both breakers have their own neutrals. I do have a better understanding now of how the circuits are set up.

Referring to the diagram above, the electricity from Breaker A arrives at the 3-way light circuit in wire #5 (right-side of diagram) and passes it on through wire #6 on its way to lights in other rooms. Note that the neutral wire from Breaker A continues on into #6 and not into the 3-way circuit. Wires #7 and #2 are the same wire, and is the "traveller" wire between the 3-way switch/dimmer. Wire #1 carries the electricity to the light fixture, with the return neutral being dumped into Wire #3 (to Breaker B).

So, it appears that the neutral wire for Breaker A is used for all elements "downstream" of the 3-way circuit, but that the 3-way circuit portion is returned through Breaker B's neutral wire.

Breaker B used to supply electricity to items in our old kitchen. Since our renovation, it appears to only be supplying electricity to the single dimmer switch in the above diagram. So, though not to code, it probably can handle the extra return from the 3-way circuit until I figure out how to get it wired properly.
 

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If it only supplies the dimmer then another fix would be join the two hots as they leave the panel thus making it all one circuit. It would leave you with a free breaker for future use.
 

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Discussion Starter #13
That is a great idea as I am wanting to add another breaker anyway. Kills two birds with one stone. Thanks.
 
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