Yup, that's right. Now, maybe this is okay, but it seems terrible to me. I bought this house about 3 months ago, so I don't know the reasoning why, but from what I've seen this guy made some poor choices in his DIY work.
There is a 30 amp 240 breaker wired to two individual 120v circuits through 12 awg wire. All I can figure is they ran one leg to one wall of outlets, and one to the other, then somehow branched extra wires to bring the neutrals and grounds back together. I don't know what the wiring looks in the walls to split this, or to splice the neutrals and grounds.
My question first, is this acceptable, and is the breaker size proper for the wire.
Two, I need space for one more slot in my panel, there is no reason for these circuits to be separate as far as I can tell – all they run is a couple outlets each, all in the same room. I'm sure this idea is BAD too, but can I bring both hot leads into a single 20 amp 120 breaker?
Best Answer
Splitting the hots and sharing the neutrals like that is no big deal; this is called a multi-wire branch circuit (MWBC). We have several answers on that, and Google can help too.
MWBCs are perfectly reasonable and safe. They are a little confusing at first, but they are good at delivering twice the amount of power with only one more wire.
Because if the shared neutral, a few extra installation rules apply. Most especially, make sure the hots are on separate poles. That is to say, there must be 240V between the two hots. Just use a 240V breaker, of the type which takes two breaker spaces, and requires both sides be turned off together.
A 30A breaker on 12 AWG wire is not allowed. You need to downgrade the breaker to 20A.
Any circuit breaker supplying 15A or 20A receptacles must be no larger than 20A, so the 30A breaker would not be allowed even if the wire was thick enough.
If you really, really want to decommission the MWBC and turn it into a single 120V circuit, you can do that. Just disconnect and tape off the red wire in the panel, put the black wire into a 120V breaker of appropriate amperage (i.e. 20A), and move the wires so that everything feeds off the black wire instead of the red. Honestly this seems like a foolish thing to do, it would be smarter and faster to just school up on MWBCs so you feel comfortable with them.
Edit: so you're out of spaces. As I said in the comment, you can join 2 circuits to become 2 branches of 1 circuit, and make that join right in the panel 6" before the breaker (or at the breaker if it's listed for 2 wires).
You can also do that with MWBCs - just land both hots on a single-pole breaker sized for one wire (e.g. for a #12 MWBC, a 20A breaker, not 40.) Yes, you have the dangerous condition of both hots being on the same pole, but the breaker is sized to protect the neutral, so it's not dangerous.
Lastly, you say the MWBC backbone is actually 10AWG. (heh, former dryer circuit). That veritably screams "put a subpanel at the end of me". That would support four receptacle circuits, and easy with 10AWG - no wrestling black snakes. Breaker the main for 30A to protect the 10AWG wire (the subpanel can be anything >=30A). If the MWBC splits there, land each side on a single. Otherwise continue the MWBC off a 2-pole. Now you have headroom in that panel for 2 more circuits. More if you ever upgrade the 10AWG backbone.
I strongly encourage you to be "once bitten twice shy" about running out of breaker spaces, and get the biggest panels you can, always. I joke about 84-space panels, but panels get expensive once they exceed 40/42-space, so just get two 42's :) By the way, industrial quality panels always have multiples of six because they also come in 3-phase -- therefore, 16 or 40-space screams "cheap builder-grade panel type".