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?


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".

| improve this answer | |
  • I was able to wrap my head around how and why a MWBC is used. I think that this wire actually turned out to be 10awg, but as stated, if the outlets are only rated at 20 I needed to downgrade anyway. The only thing I don't like about this, is you're now restricted to a 240 amp breaker, which takes up 2 slots in my already cramped panel. I might consider rewiring all of the outlets to a single hot lead so I can run it off a single 120 breaker, but only to free up space in the panel. I agree that it isn't the best solution. I need ONE more slot, and I don't feel like installing a new panel. – slambeth Jan 24 '18 at 16:05
  • You know a tree topology is allowed in wiring, yes? You can split a branch off a circuit. That can even happen literally 6" from the breaker. I do it all the time. You can have wirenuts inside service panels. Just mark things clearly. The upshot is that two underused "circuits" can share a breaker, becoming one circuit with 2 branches. – Harper - Reinstate Monica Jan 24 '18 at 16:56
  • Thanks for the info, I didn't realize that. If I understand you correctly, then I can basically splice wires with wirenuts, but inside a junction box somewhere? Can I even just use an outlet box with a blank? That is cool, but the circuits I need to add are 240 50 amps, and I need two of them so I need to clear up 3 more slots. I suppose I could combine a circuit or two to one breaker by branching to free up slots. – slambeth Feb 8 '18 at 15:26
  • You would laugh if you saw photos of my panels, they are surrounded by blank plate junction boxes! (Thieves stole my breakers and cut all the wires short). I would do a survey of each of your circuits and figure out if you have some underused ones. Or you could add a subpanel near the main panel (now you will need 5 spaces) and move as many circuits as you like (i.e. More than 5) to the new subpanel, if necessary with the junction box trick. – Harper - Reinstate Monica Feb 8 '18 at 17:51

Going to a standard (15 or 20A) outlet, it should be a 20 amp breaker max (also, with 12 awg wire, the max breaker is 20 amp). Using a double pole breaker with 2 hot 1 neutral and 1 ground is called a multi wire branch circuit. With 12 awg wire on a 20 amp breaker this would be code compliant and is used regularly to save on wire cost. The neutral and ground splits and goes with each hot.

| improve this answer | |
  • Okay, so I get why it works, it just seems strange. Somewhere, he spliced, or just continued to run the same neutral and just stop using one leg of the 240, and start using the other. The point is, both circuits share neutral to bring in 120. Could I run this all on a single 120 breaker ( if I found a fixture that allowed two wires on a single breaker) – slambeth Nov 19 '16 at 13:09
  • 1
    If you have a breaker rated for 2 wires both can be put on 1 20 amp breaker with no other changes in wiring. The only possible negative is if the 1 circuit is loaded you may trip the breaker but if you need the space it might be worth a try since the breaker needs to be replaced anyway. – Ed Beal Nov 20 '16 at 8:00

Several years ago a professional electrician installed something like this for where I needed some extra power. My first thought was Huh?? in that I was getting 40 amps of 120 power off wires that could only carry 20. However, when you think about it, nothing is overloaded.

It feeds one box of 4 outlets. The hot side of two outlets is connected to hot wire A. The hot side of the other two outlets is connected to hot wire B. The neutral side of all outlets is connected to the neutral wire.

I have a 20A widget. I plug it in on the left. Hot wire A, 20 amps. Neutral, 20 amps. Fine. I take another 20A widget. I plug it in on the right. Hot wire A, 20 amps. Neutral, zero amps. Hot wire B, 20 amps. Fine.

The additional cost of giving me 40A instead of 20A was less than 2% of the job price. I didn't actually need 40A of additional power but having it this way lets me at least partially isolate the surge load from the main load.

I'm not sure exactly what you're saying about bringing both hot wires into the same fixture. As I described above, this is fine. Running two hots to the same outlet is going to give you a 240V outlet or else a short.

There's also the little detail that you have a 30A breaker on 20A wire and 20A outlets. That's a no-no, the breaker size must never exceed the capacity of the circuit it's protecting.

As for why have them in the same room--note that you said "garage". My first thought is heavy power tools.

| improve this answer | |
  • This was a small room built inside the garage. The original builder was an electrician, and put 30 amp 240 circuits to all corners ( smart idea). What likely happened, is the second owner built this small room inside the garage, and repurposed the 240 wire already there to run 120 outlets. – slambeth Nov 19 '16 at 13:18
  • But why would this electrician have used double pole 30-A breakers with #12 wire? – Jim Stewart Nov 21 '16 at 23:11
  • Good point. Either way I need to run a 20 amp breaker. I think I'm going to be stripping this whole room down on the inside anyway, so I'll get a good chance examine the wiring then, because i don't trust the clown that did all this work before I bought the house. While the MWBC seems neat, and saves wire it limits you to using a two pole 240V breaker, and taking up two slots in your sub panel. If this were simply two individual circuits I could gain some space by combining this a tandem 120 breaker. – slambeth Dec 6 '16 at 16:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.