0

I'm wiring up a three way switch and have followed the attached diagram. However, I have three lights between the two switches. Can someone help me expand on this diagram to show how I wire up three lights.

Thanks

enter image description here

  • That drawing is wrong and violates NEC. Not to mention quite terrible in other ways. . We've seen it before. – Harper Sep 18 '17 at 0:47
2

Mty friend ThreePhasEel is dead square right about the Code violation. I don't know what's wrong with those homesteader sites, but they're no place to get code advice, that's for sure.

That said I would do this in quite a different way. I would wire it like this.

        Cab1        Cab2                   Cab3
 Power ----- SW1 ======== lamp1 ========================== SW2
                            |             
                            |cab4
                            |
                          Lamp2 --------- Lamp3
                                  Cab5

In this diagram === is /3 cable and --- is /2 cable. Since you have to run extra cable anyway, this uses the more readily-available /3 and avoids cable3 going through Lamp2 and Lamp3's boxes at all. Which simplifies their wiring greatly, meaning less mistakes, and keeps box fill down. Presumably cab3 will parallel cab4 and cab5 and skirt around the boxes for lamp2 and 3, since entering would only confuse things.

Now for sanity's sake, let's color the wires. I want you to get one of those $3 5-packs of colored electrical tape. If wire is wrapped with colored tape, treat it like it is that color. Always mark both ends of each wire the same, at the same time so you don't forget. The color codes will be

  • neutral - white
  • always-hot - black
  • switched-hot - red
  • messengers - yellow

Cab1 ---- leave white and black

Cab2 ---- tape the black wire yellow. Tape the red wire yellow. White stays.

Cab3 ---- tape white wire yellow. Tape black wire yellow. Red stays.

Cab4 ---- tape black wire red.

Cab5 ----- tape black wire red.

Lamps 1,2 and 3: if they have brown and blue, tape the blue wire white. Tape the brown or black wire red.

Now it's stupid easy.

  • In each junction box join all the same color wires with a nut.
  • except in lamp1 box, join 1 messenger to 1 messenger, so 2 splices. It doesn't really matter which messenger goes to which. They are interchangeable. That's why they're both yellow.
  • on each 3-way switch, the brass messenger screws get the 2 yellow wires. The black screw gets the one that remains (black in SW1 or red in SW2).
  • That works too -- I'd personally do it the way you describe actually if I was in a new build, but I was presuming that there was some constraint that'd require putting the light boxes between the switch boxes, topologically speaking – ThreePhaseEel Sep 18 '17 at 2:12
  • Thanks! Let me go over this and try to understand it. I'm sure I'll have a few questions. – slock Sep 18 '17 at 3:57
2

First off, that diagram is probably a Code violation

See how they run the travelers in one cable and the switched-hot/neutral in the other cable between the lamps? That can be considered a no-no under NEC 300.3(B)/310.10(H) due to issues with steel boxes and other steel parts (such as a steel stud that both cables run through) getting toasty from inductive heating due to circulating loop currents, as well as excessive 60Hz magnetic fields that can cause EMI.

The fix is simple -- run a single /4 cable (a /2/2 cable can be used instead with one of the neutrals retagged if you can't get /4 in the right size) between the lights, instead of the pair of /2 cables the diagram calls for.

From there, it's easy

Your third box goes between the left and right light boxes, and is wired as follows:

  • Traveller 1 in the cable from the left light box connects to Traveller 1 in the cable from the right light box, likewise with Traveller 2
  • All the switched-hots in the box (one from the left box, one from the right box, and one that's the black wire from the fixture) connect together
  • All the neutrals in the box (again, one from the left box, one from the right box, and one that's the white wire from the fixture) connect together
  • All the grounds in the box connect together

Tagging wires according to function makes it easier

Tape-tagging wires by function just makes this easier yet. For instance: tagging all hots black, all switched hots red, and all travelers say yellow for one traveller and blue for the other in this scenario means that you can simply connect like-color-tagged wires together and have everything work.

  • Thank you very much for the explanation. Is the following diagram also wrong (Option #1) buildmyowncabin.com/electrical/wiring-diagrams-3way-7.html Since posting this question I've been reading up on three way wiring and am starting to understand it more. I do see where the original diagram they switched the hot and neutral. If that was not done would it have been okay? I've currently have 2 sets of 12/2 running between the lights as showed in both diagrams. – slock Sep 18 '17 at 1:27
  • @slock -- we thank people with upvotes and answer accepts around here ;) – ThreePhaseEel Sep 18 '17 at 1:30
  • @slock It's wrong too. People are dumb and lazy and don't care about code, or they think because /4 cable is hard to find they can just do whatever. I'm sorry you wasted wire... just run a /3 cable from lamp box 1 to switch 2 as I describe in my answer, and remove or just cap off the extra /2. – Harper Sep 18 '17 at 2:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.