3 Way switch with feed at light

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Joshua5438

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Hello, I am installing a 3 way switch where the feed comes into the light instead of the first switch. I will have a switch at two doors, 50 feet apart with 4 light fixtures between them. I have found a diagram online and it makes since but wanted to get opinions on if this is the best way for my layout.

two-lights-between-3-way-switches-power-via-light.jpg
 
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wwhitney

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So the layout of boxes is going to be: switch A -- Light 1 -- Light 2 -- Light 3 -- Light 4 -- switch B? And you only want to pull wires between neighboring boxes, and the Light 1 box has constant power?

Then the diagram is almost correct, it doesn't comply with the somewhat recent requirement that a neutral conductor be provided at one of the two switch boxes. [You won't actually need the neutral conductor for standard 3-way switches, so you just leave the end capped in that switch box.] So you'll need to have 4 conductors between adjacent boxes, except Light 4 -- switch B can be only 3 conductors.

Using 12/4 or 14/4 NM cable would be nice, with black, white, red, and blue, but it's more than twice as expensive as /2 NM cable. So the diagram shows the use of a pair of /2 cables to get 4 conductors. If you do that, it's best practice to run those two cables on top of each other, so they follow exactly the same path. And one cable should have both of the conductors reidentified to 2 new colors, e.g. blue and red; those are the travelers between switches. [The black dot on some of the white wires in the diagram represents reidentifying them. Reidentifying the black traveler isn't strictly necessary.]

BTW, Light 1 box is going to have 10 conductors entering it, so it will need to be 22 in^3 if using #14 NM, and 24.75 in^3 if using #12 NM. The other ceiling boxes will only need to be 18 or 20.25 in^3. If the lighting fixtures use a canopy with a volume labeled on it, you can add that volume to what the label on your ceiling boxes says.

Cheers, Wayne
 

Joshua5438

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You are correct on the box layout.

Can you explain the purpose of the neutral wire to the switch box? I have heard that recently but have not been given a true explanation as to what it is for.

I had planned to mark the black traveler as RED to match the red wire in the 12/3 group. I will also mark all white travelers as BLUE as that makes since and to me, makes it easier to identify as a traveler and not just a switched hot.

Thanks for mentioning that about box size. All of my boxes are 30.3 cu in.
 

wwhitney

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Can you explain the purpose of the neutral wire to the switch box? I have heard that recently but have not been given a true explanation as to what it is for.
"Smart" switches that require a neutral to run some internal electronics are becoming more prevalent. So the neutral is to allow a switch like that to be easily installed in the future. The idea is that it would be enough to install a smart switch in one location on a 3-way circuit, and the smart switch could use the travelers to communicate with a companion switch in the other location, so the neutral isn't required in both places. But you could easily provide a neutral in both switch boxes by using 4 conductors between each pair of boxes.

This change in the NEC requirements occurred in coordination with a change in the UL listing for the devices. Previously some of these smart switches were using the EGC for a return path for their electronics, i.e. intentionally putting current on the EGC, which was allowed by the UL listing for up to 0.5 ma of current. Among the various reasons this was not the best idea was that if you had a GFCI protected circuit, and several of these smart switches on it, the aggregate current could be enough to trip the GFCI.

I ran into this change recently when I had some old stock and new stock of a vacancy control sensor with a plastic case. They were identical, except the old stock had a green lead, and the new stock had a white lead. Actually I think the new stock had a white lead covered with a green sheath, and the instructions tell you to use a neutral if available, removing the green sheath; or use the EGC if there's no neutral, leaving the green sheath.

Cheers, Wayne
 

Joshua5438

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Thanks for explaining that to me. It makes since. Guess I will be running (2) 12/2 to my 1st switch so I have that extra wire for when I do not need it. haha. In all honesty, with the way everything is advancing, I will probably install a smart switch at some point.
 

Jadnashua

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Lutron Maestro 3-way (actually, can have up to 8 switches) works with just a three-wire connection to each location. I have a few in my place. They also have dimming capability from each location, and theater off, where the lamps do a slow turn off. It's also a soft on, so bulbs tend to last longer, but those with filaments are not as common today. Essentially, you run power and neutral and a single, common traveler between each switch. It's a little more complex than that, but not much.
 
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