Two breaks for one circuit

Recently, I was replacing a light fixture in my home. When I attempted to cut the breaker to that fixture, I discovered that I had to turn off two breakers to kill the circuit that includes the light fixture. My assumption then, is that the two circuits (one for each breaker) that should be separate have somehow been crossed together such that they both breakers effectively power a single circuit.
I'd like to fix the problem. Is there a simple way to determine where the two circuits have been connected?
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I wish there was. It would save me a lot of time trying to find the interconnection. I find this situation occasionally in older houses that have had wiring changes done by homeowners and handymen. It is not uncommon to see this in homes with knob and tube wiring that has been refed with a new circuit. Start at the circuit breaker panel and physically follow the respective cables as they make their way through the house. This is a time consuming task. When you encounter a junction box disconnect the feed from the circuit breaker panel and see if it is still hot. If so then trace the wires from that junction box and so on. In the box containing the wiring for the light fixture that you just changed was there a considerable number of wires there? That might be a clue. You should also open all outlets, switches, and light fixtures on this circuit and separate the conductors to see if you have two hots.
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On Mon, 27 Oct 2008 11:52:30 -0700 (PDT), drhender

There is not an easy way, but there is a straight forward way.
Identify all the devices on the two breakers. May as well start with the lamp you are working on. It should have an in and out. When you break it at that point, half of the stuff should go out with one breaker.
You should work it back until you get to the point where the two are joined.
If you want to save yourself a headache, you should take a picture of the box before you break any taps.
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drhender wrote:

There are two approaches as I see it: 1. Follow the wiring and split it somewhere in the middle, as others have suggested. OR 2. Pick one breaker, disconnect it, and remove the wiring back to the first junction box. An alternate to removing the wire, re-route it to a box and install a plug so you don't have a bare live wire dangling.
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