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I would like to add a circuit at the far end of a 3-way switched circuit. I choose to wire the first circuit this way:

Switch diagram

I have the source coming in from the breaker to a light switch with 14/2, then continue with 14/3 to the light and then continue with 14/3 to the far switch. What I am now trying to do is power up 2 additional circuits from the far light switch, but because the common wire is switched at the first switch I don't see a way to have the far circuits work independently of the first 3 way switch. Please advise. Thank you.

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    You can’t. You need one more conductor from the light to the far end switch. You would need to replace the 14/3 with 14/4. – Tyson Sep 5 '18 at 14:40
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You are correct -- there is no way to add independent circuits off the second switch. Here is another way to look at the switch configuration using three-way switches (the black wire being the 'white wire taped black' in your diagram, the red wire to the light being black, and the neutral being white).

Three-way switching scheme

As you can see, the right-hand switch -- where you want to branch from -- does not receive the hot connection independently of the left-hand switch. You'll have to run an additional, independent 14/2 through the conduit, or replace your 14/3 with 14/4 as Tyson suggests.

  • That drawing does not match his wiring. OP includes a diagram of his wiring. – Harper - Reinstate Monica Sep 5 '18 at 14:54
  • @Harper It sure does -- I updated the image to note the color discrepancies between the two diagrams. The image in my answer is an equivalent wiring diagram to show how the switches work. – calcium3000 Sep 5 '18 at 15:05
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Here's what your wiring actually is, based on your description. Note that I've gone ahead and color-marked the wires for their function: Black/white for supply hot/neutral, yellow for travelers, and red for switched-hot. I do that for real with colored tape; so can you.

enter image description here

You want to power additional loads, and they will require supply hot/neutral (black/white). You want this from the rightmost switch.

As you can plainly see, neither hot nor neutral is present there.

If you go back to the lamp, you can pick up neutral, but that's no help. You have to go all the way to the left switch to obtain hot/neutral.

I recommend running a /2 cable from the left switch. It isn't really practicable to "bring this along" in the other cable; you'd need special /4 cable to advance hot from the left switch to the lamp, and unobtanium /5 cable to bring it to the right switch along with the other circuits you do need.

Hail Mary play: smart switches

Another option is to leave the 3-wire cabling in place, but junk all the electromechanical 3-way switching and use smart switches and smart modules to control the lamp. These communicate with each other via radio waves or powerline signaling, greatly reducing the number of wires needed. However each smart switch needs power 24x7, which necessitates bringing supply hot/neutral to each switch location, including the right one. You can usually get this done within the 3 wires already in the wall; they kinda design for that.

Powering from multiple breakers

This is very dangerous. The most essential part is you must keep all neutrals matched to partner hots. So you cannot simply clump all neutrals together, you must only clump together neutrals that are served from the same "hot". This requires you be mind-bogglingly fastidious about marking every single wire in some way with tape. It's really a giant pain. I do it in THHN/conduit, but that's why I stock gray neutral wire and 10 colors of tape. If you're not into 'fastidious', don't feed the same box from 2 breakers because you will lose your mind trying to keep track of all that, and create an unsafe situation that trips GFCIs.

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You are one conductor short* to do what you want to with regular mechanical switches. However, it's easy enough to accomplish with wireless switches such as the Lutron Caseta.

With your existing wiring, you'd need to install a wireless dimmer / switch at one location, and a Pico remote mounted in the cover at the other.


*To do this with four conductors between boxes ... let's say you have

Switch Box 1 - black source hot, white source neutral, three way switch

Ceiling box - light

Switch Box 2 - black downstream hot, white downstream neutral, three way switch

and a black-red-blue-white cable from switch box 1 to the ceiling box to switch box 2.

Make these connections...

Switch box 1 - source hot and black pigtailed to one traveler terminal; blue to common; red to other traveler terminal; white to white.

Ceiling box - tap red to fixture hot, tap white to fixture neutral.

Switch box 2 - downstream hot and black pigtailed to one traveler terminal; blue to common; red to other traveler terminal; white to white

This is a California three way with the device in the middle. You have constant hot, neutral, and switched hot in all three locations with four wires between.

  • Two conductors short, by my count, and the problem is that pushes you into /5 cable, which is unobtanium. – Harper - Reinstate Monica Sep 19 '18 at 0:18
  • @Harper - see my updated answer ... I think it's doable with 4 wires. – batsplatsterson Sep 19 '18 at 1:18
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    Huh. Yeah. That would work. And CA-3ways are Code. – Harper - Reinstate Monica Sep 19 '18 at 1:26
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You can accomplish this using a SPDT relay. Basically, you control the coil with one of the travelers, which would feed your NO contact. Then your other traveler would feed your NC. Then the common will be always hot, with your switch side interlocked, as to not affect your 3-way switched lights.

Keep in mind, relays typically aren't rated for much current, so if you need to run more than a few amps you would need a bigger contactor. I can't speak to how legal this is pertaining to the NEC, but as long as everything is inside UL listed enclosures, and everything is grounded well, it would work. Might have even done it at my own house. :)

  • This is a workable approach, using something like a RIBU1C as the relay that is prepackaged and fully UL listed... – ThreePhaseEel Jan 23 '19 at 4:57

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