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My house is 60+ years old, with original cloth wrapped 2-wire 12 ga wires throughout the house, and an updated 200A breaker box. I wish to replace a 3-way switch setup on a ceiling fan/light with a double 3-way switch setup - a 3-way on the light and a 3-way on the fan. I am looking at using two Leviton 5640 in the existing single-gang spots, while replacing the old boxes with old work.

I planned on wiring it up based on some homeowner's how-to books, like this diagram shows. My question is what wire should I use to replace the existing 3-way setup, while adding for the double switches? I've wired a fan/light on a double switch before, using 12/3 with a ground, but I didn't know what wire I should need for the double 3-way setup.

From best I can tell, I need a hot and ground going to the first switch, 4 feeder wires going to second switch, and a 2 hots and a ground coming from second switch to fan/light.

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If you're going to use wiring similar to the first diagram from that thread, and you're planning to use 12 AWG wiring. You're going to need five conductors (plus ground) between the switch boxes, so I'd probably use two separate cables.

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In the case that the power originates at the fan/light, I'd make use of some 12/2/2. Using a length between the fan/light, and first group of switches. Then another length between the two groups of switches.

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NOTES:

  • Grounding conductors have been excluded from the diagram, to make it a bit clearer. Make sure all grounding conductors are properly installed.
  • Notice that the white wire that's being used as a traveller between the switches, has been reidentified at both ends using a bit of black tape/marker/paint.
  • As @Wolf Harper points out, make sure the two cables between the boxes stay in close proximity with each other. Or use conduit with single conductors pulled through it.
  • Thanks @Tester101, that was actually my first thought. However, I will tie into the hot/neutral that are already in place at the fan junction box, so a 12/2 to the first switch is not necessary. What do you think? I should also note that the feeders from switch 1 to switch 2 will cross the source and fan junction box as well. – Barbarossa Feb 20 '16 at 3:14
  • @Barbarossa Did you put that information in the question? I must have missed it. – Tester101 Feb 20 '16 at 3:20
  • @Barbarossa I've updated my answer. – Tester101 Feb 20 '16 at 3:45
  • In your cable routing, keep those multiple cables together, don't send them opposite directions around an obstruction. You will have current flowing up one cable and back the other, which code generally frowns upon. Because where they are not right next to each other, it will create magnetic fields (high EMF) and eddy currents (high heat). One way to dodge this problem is lay conduit anywhere you'd need 2 cables, and use single conductor wire (THHN) through the conduit. – Harper Feb 20 '16 at 15:36
  • @WolfHarper Thanks, noted. I drew them separated simply for clarity. – Tester101 Feb 20 '16 at 16:03

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