Thanks, everybody. Yes, I have 220/240 service coming in to feed an electric dryer, stove and baseboard heating.

Jeff said:

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This is common. My kitchen outlets are wired this way. Half are on one leg (phase) and half on the other, all sharing a neutral and ground.

Good to know!

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In this situation the wire used should be three conductor with ground (14-3WG or 12-3WG). The two hot wires (red and black) should connect to a "ganged" (two-pole, 220V) circuit breaker (15 amp for 14 guage wire). This ensures that the two circuits are not on the same phase. If the two circuits are on the same phase you can have 2X the current in the neutral wire (ie. 30 amps, with two seperate 15 amp breakers on the same phase). Not a good situation.

I'm not there right now, but circuit B's hot wire is red, IIRC, but it is definitely on a completely separate breaker.

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I would suggest correcting the problem and adding another breaker for your new outlet.

So in essence, the ganged breaker just knows how to give the right phase to each, or are there 2 separate bars back there for each phase? (the latter would make sense)

Given that they are independednt breakers and one is 30A, this *does* seem dodgy. Plus, this panel does not have a master breaker if it did overload -- I'd be relying on the main breaker in the basement. Thankfully, I don't think I'm overloaded yet.

A colleague pointed out the existence of mini tandem breakers, meaning I can get 2 separate 15 A circuits in the space of one traditional breaker. I hadn't a clue. This is how I think I am going to add the auxiliary outlet, though -- a whole separate (mini) breaker with it's own common/ground.

Back to it this evening!