Earlier I commented how I am learning how a real electrician does things by examining their work. Here’s an example. This is a picture of the outside of the half bath. Besides putting in a new ceiling light and switch, the electrician has moved the bathroom’s light switch to inside the bathroom; this was a 10-minute job since the walls here are open.
As it turns out, I was just not thinking like an electrician. The new location of the overhead light in this area (with the bare bulb hanging from it) is close enough that the old wires reached. So they just ran the old wires to there, and ran new wires to the new switch. Thus, that light’s box has two unrelated circuits in it, one of which is just passing through. It’s obvious when you think about it, but not something that an average handy homeowner would necessarily think to do.
There’s another example of this in the box for one of the recessed ceiling lights: it houses the junctions from the old ceiling fixture’s box where it provided power for the upstairs bathroom, even though the new kitchen’s lighting will be on its own circuit.
In fact, most of the counter-level boxes in the new kitchen will have 2 circuits in them: one 20A for small appliances, one 15A for lighting. This is definitely something to keep in mind if I ever do any work in those boxes.