Multi-wire circuits can sometimes be tricky though. While re-wiring parts of an old house one time, the first thing I did was re-vamp the service and one of the circuits was a multi-wire circuit so I followed suit and continued it as it had been in the past, or so I thought. In the process of redoing some of the older circuits I was to fix all the 3-ways in the two upstairs landings. the first to second floor went well and started on the second floor to the attic. there was no bulb at the top of the stairs and the only one available was a small 40 watt so I put that into the outlet and proceeded to get the circuit working. And by the way the whole house was wired K & T and was not asked to replace it, and the 3-ways were the carter system this is important, after getting the light on I checked the switches from both positions on-off from the bottom and switched the top the light went on and when I tried to shut it off that little 40 watt bulb got bright like it was a 200 watt bulb. After pondering the problem I realized that that 3-wire circuit was never an original design in the original wireing scheme, by following what I had when I revamped the service I now had 240 volts on one of my lighting circuits. needless to say the multi-wire circuit had to come out.
Later I found out the house was originally wired 120 volts and somebody tried for a short-cut which ultimately didnt work.
So my thought on multi-wires is caution and know what you have ahead of time.
-Mark
