fansswitch
My old switch does not have a white wire. Should I add one and attach it to the grommet inside the box?
Also, in most diagrams, the black wire enters the box from the top an leaves from the bottom. In this case they both are on top. How do I know which wire should be connected to RD?
Thanks
You don't have a neutral at the switch. White is a traveler, and should be marked in some way to indicate that it is not a neutral (but is likely not marked). Red is the other traveler, and black is common (either attached to power or the light, depending on which switch it is). Multiway Switching can be confusing, and is the subject of many questions on this site.
This is likely what the circuit looks like
You're describing the wiring at switch 2, where the black wire is only "hot" when both switches are in the proper state (both up, or both down. Unless one is installed upside down). When the switches are toggled, one of the travelers between them will be energized.
The power feed to this all comes in at box 1 (vanity light) and feeds the /2 switch loop to box 3 as well as the /2 feed to box 2. Box 2 and Box 3 are connected by a /3 cable, apparently intended to be a switch loop.
With all this, you'd think you'd be able to:
and have it work. In fact, it will work if you do this; however, there's a catch! This arrangement takes one switch loop feed (from the vanity light) and returns it back down two different cables (to the vanity light and to the overhead light), which can be seen as a Code violation (of the 300.3(B)/310.10(H)/300.20 complex) as it creates a large loop area that puts out stray magnetic fields in your bathroom (good for ruining the picture on that old TV you have mounted in the wall in there ;).
A more conservative approach would be to utilize a dual pole switch to break the two hot feeds simultaneously while sending the return paths back the way they came, as follows:
This arrangement also works, and keeps the two light switch loops neatly separated. However, Legrand does not make a two pole Adorne switch, and I do not believe there are faceplates available for the combination of an Adorne device side by side with a standard switch in a two gang box. You'd have to get a standard-form-factor (i.e. not Adorne) fan controller to do this.
Best Answer
Yes, run a short length of new white wire to the white terminal of the switch and add it to the wirenut (grommet) of the two white wires.
One of the black wires goes to the black terminal on the switch, the other black wire goes to the red terminal on the switch.
It won't damage the switch if you mix up the red and black wires, it simply won't work and you'll just have to swap them around.
You could restore power and use a non-contact voltage detector or a multimeter to find the hot (black terminal) and switched hot (red terminal), but that's just more work requiring tools you make not have.
Mark the switched hot (red) wire with some electrical tape to help remember which is which in the future. Red electrical tape would be perfect, but any color is better than nothing!