I'll try to be brief. I'm in Roseburg Oregon. My home was originally built in 1949. We moved in three years ago. The electrical has had several partial upgrades and mods over the years. Much of the wiring is still the old cloth-wrapped rubber-insulated #12 two strand. All my downstairs overhead lights are on one 20amp circuit with this old wire. After three eventless years, a problem has developed with the entry light. It suddenly has 240 volts to it instead of the 120 volts its had otherwise. I wondered why it was blowing bulbs out instantly, so I put the meter on it and found the 240. I then discovered that when I turned off the 20amp circuit breaker, the entry light (but none of the other lights in the circuit) still had 120. Some experimentation determined that the extra 120 was coming from the 15 amp upstairs light circuit (the entry light was cold-no voltage-when I turned that circuit off). In the panel, the two circuit breakers in question are seperated by several others. But wait, there's more. There is a large outdoor flood light, probably a 240 volt unit, mounted to the house and operating on the upstairs light circuit though switched in the downstairs entry with the original porch light. Its mystifying to me. I hope you can help me.
Thank you, Jeff Wheaton
Jeff some where this circuit is wired wrong or was wired wrong before you moved in.When you turn on the upstairs light it may be feeding the white wire in the entry light, Check to see how many wires are in the upstairs switch box and then check to see the lower switch.Make sure the neutrals are not connected to the hot leg for the entrance way switch or lamp.