drhender wrote:
Recently, I was changing a light fixture on my back porch and
discovered that I had to shut off two breakers to cut the power to the
circuit. I take that to indicate that somewhere, somehow the two
circuits have effectively been crossed together such that the circuit
can draw power from either breaker.

I'd like to fix this... is there a simple way of determining where the
two circuits are conjoined?

Thanks.

It is not that uncommon to have to turn 2 or more breakers off in order
to completely shut off the power inside a single switch box. This is
especially true of 2 or 3+ gang switch boxes that say lead to the garage
from the kitchen etc.
If you are saying that it takes 2 breakers to shut off a single light
then that is a crossover. The first place to look is in the switch box.
If not that then I would map out your entire system by systematically
turning the 2 breaker on and off. Then find all the outlets or lights or
appliances that come on with both beakers. Then open each box one by one
until you find the problem.

There are other possible situations that could cause this kind of
behavior such as an open neutral. But you would notice other problems
like brighter and/or dimmer than normal lights elsewhere.

Sometimes neutrals from 2 different circuits are connected together
somewhere and when you go to disconnect things you get an arc/shock as
you are disconnecting the neutrals at the fixture box even though the
power is off on the circuit you are working on BTDT. In this case the
circuits are not really joined but you should go and try to separate the
neutrals if you can. Most likely this was done in a 2+ gang switch box
fed by 2+ circuits.

An Edison type (shared neutral) circuit where a duplex outlet is fed by
2 circuits someone replaced an outlet and forgot to break the joining
tabs between the upper and lower outlet. If this were the case however
the circuit would have a second problem where the 2 circuits were
improperly put on the same 'leg' in the panel.

Kevin