I think I know the answer to this but I'm looking for confirmation or correction from people here who I can trust.
I'm getting ready to add a gable fan in my attic and found something suspicious. There is a grounded 12/3 Romex run in the general area. I've traced this back to two 15A breakers in the panel. One circuit (with the black wire) is in use. The second circuit (with the red wire) is capped off and the breaker has always been off. Presumably it has been this way since the previous owners lived here, since myself and an air conditioning contractor are the only ones who have been inside my panel in the couple of years I've lived here.
The suspicious part is that the breakers are two tandem breakers sharing the same spot in the panel. So this means they are sharing the same pole coming from the power company... correct? I assume the intention was to someday use the red wire for something else and share the neutral and ground.
My belief is that this would be a very dangerous idea. Am I right?
Now, if I were to move the red wire to a new breaker, making sure it was on the opposite pole, could I use the red wire for my new circuit and still share the neutral and ground?
Finally, if that answer is YES... do both breakers need to be the same amperage rating? And is there anything I need to do to mark or indicate this is the case?