I'm upgrading the Three Way Switch in my diagram (which controls entryway recessed lights) with a dimmer (Legrand Adorne SofTap) and I'm pretty confused by the wiring going into the existing switch (2 reds and a white). I put together this diagram of how it was originally. I'm not understanding why there would be two 3-wire cables going into the box to begin with. It would lead me to believe that one of the wires coming from the fixture doesn't actually do anything -- but which? My guess at each cable is this (and I could be completely wrong):

1 - Source Cable

2 - To Front Porch Light (with Single Pole Switch)

3 - To Cable 5 (Other Three-Way)

4 - To Recessed Light Fixtures

5 - To Cable 3

If it makes any difference, the dimmer instructions tell you to connect the neutral wire to the dimmer (in addition to the common and two traveler wires), but I'm not entirely sure if that's necessary.

I tried running the short black wire coming off the single-pole switch to my dimmer's common terminal and left the red wire on the 4th cable (let's call it "4R") disconnected, thereby using the 3R and 3W as the travelers, but that didn't work. That tells me that 4R is active in some way.

Any advice would be much appreciated. This is driving me nuts. Thanks!

 

Welcome to the forums.

Awesome diagram. Very clear and your description is good.
Blue wire of cables 1, 2, 4 is neutral. Blue = white wires which are neutral.


Cable 4 red goes to HOT of dimmer.
Cable 3 red to 1-POLE on dimmer.
Cable 3 white to 3-WAY on dimmer.
A piece of #14 white needs to be added to the cable 1, 2, 4 white/neutral splice and go to WHITE on dimmer.

 

Top shows the circuit with the light off

Middle show the completed circuit (light on) if you switch either switch the light goes off.

Bottom shows the addition of a four way switch for a third (or more) added switch(s) to control the same lights from multiple positions. You need a three way switch on either end and as many four way switches in between to complete or disconnect the two power conductors (substitute either four way diagram in to envision the two possible positions of the four way)

Four way switch is different than a double pole switch (also has four contacts)

Cheers!

 

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So a Lutron electronic dimmer, a manual dimmer etc, they should be able to be wired in as shown.

Between our two diagrams you should be able to see that the two wires in your diagram [Blue and red #3 to blue and red #5] are the parallel conductors in my diagram (commonly black and red)

On your diagram, the black (common pole) 3way switch screw either goes to the lights [the load] or the (+) source [the line]

I recently installed a pair of electronic dimmers that you could control and adjust the brightness from either location. They "spoke" to one another over the parallel wiring. Even the intensity light display on the switch adjusted from the opposite location.

Once someone showed me that diagram and explained how it works, I was always able to figure it out.
Cheers!
Let us know how it turns out or if you have any additional questions,
Work safe!

Cheers.

 

Thanks for all the help @PJmax and @fabricgator! Turns out all I needed to do was run the pigtail off the neutral and into my dimmer and that did it. Finished product looks great.

 

Good job. Thanks for letting us know how you made out.