The short version: I moved into a condo in June and decided I wanted to add one electrical outlet to one of 3 15-Amp circuits. The new outlet would be right next to where the electrical panel is, so my plan was to remove this branch from the panel, us a short length of 12-3 wire to wire up the new GFI outlet, then reterminate the existing branch wire onto the new outlet. We'll call this Circuit A

No question which hot wire I needed to disconnect, but when it came to find the right white/common wire, my only strategy was to turn off most breakers, loosen a common wire from the bus, then check to see if the Circuit A was depowered (bathroom light gone out).

What I found was that two breakers were served by one common wire. I was using a trouble light on a long extension cord to another branch -- call this Circuit B) to see what I was doing, and when I loosened the bus screw on the common wire that serves Circuit A, I saw my trouble light get awfully bright. I said "Aha! This light is getting 220 volts!".

I think I can *maybe* puzzle out why that Circuit B was getting 220 with a loose common, but it did depower when I removed that common wire completely from the bus.

Furthering the mystery, Circuit B has a 30 Amp single breaker but the only thing on it, so far as I can tell, is a single outlet in a laundry closet. I am thinking that this circuit is either A) an artifact of an old 115 volt supply to a washing machine before they added 220V for washer/dryer. Anyhow, it looks like 14G wire coming off that 30A breaker. Too little wire for that.

What I can't understand is the shared common. Is this ordinary? I don't want to say "common"! Right now, all of the outlets test out OK. Circuit B doesn't really have any way to get loaded -- I just use it for a single lightbulb -- but I was leery of rerouting circuit A through this new outlet if there was something fundamentally dodgy about this.

Dodgy. That word sounds so....British!