I'm planning to replace a number of 3-way switches with smart switches and as a necessary first step have been trying to map out the wiring in each of the relevant boxes. This is what I've come up with (it's Powerpoint, try not to make fun ;)):
This shows 3 boxes on two floors. A through I are all box entry points. The colors represented are what is actually used (except gray has been substituted for white). A simple gray box in the diagram is just a connector. The dotted lines are what I believe is going on in the walls, but that's where I would love to get a second opinion.
The tricky one for me is the leftmost box (2nd Floor Middle Stairway Bottom) because it contains wires from two different circuits, as I confirmed using a voltage tester (even when the breaker for "Circuit" was off, the purple and peach wires in that box were hot - which I guess isn't illegal but I was none too pleased to learn).
I assume the orange wire coming out of S1B's common terminal splits at or near light 1, in the 2nd floor ceiling, providing the load for light 1 and also ultimately for light 2, on the 3rd floor ceiling, as the orange wire from D-E is unbroken. I couldn't find this particular configuration as an example of multi-light 3 way wiring, though. I guess the other possibility is that the orange wire goes C-D-E unbroken, and at light 2 splits and goes all the way back down the stairs to light 1 through another conduit in the ceiling, but at the end of the day I'm not sure it matters.
What I'm less sure about is the neutrals. At first I thought the white at C connected to the white at D, but then it occurred to me that that would mean that the neutrals from 2 different circuits were all connected (if we assume the white at A and B are on the same circuit as the purples and peach). That would be a big no no, right? So on second thought, it seems like the white at D must be the return from light 1, and the whites in that 2nd floor box, like the purple and peach, have nothing to do with the circuit I'm interested in. Rather everything on the circuit I care about ultimately starts and returns through F, which I believe connects to an outlet on the other side of the wall.
So, does this seem right or could there be other possibilities I haven't considered? And how could I be more certain?
The reason this matters is that it determines where the smart switches have to go, and what kind I have to use. And also I just want to be sure that when I play with the white wires in the 3rd Floor Stairway Top box that I'm not going to get in trouble with that mystery circuit that shares the 2nd floor box.
Thanks for any insights!