My exterior outlet/garage outlets are part of a 15a tandem switch currently that's not GFCI. Looks like this:

enter image description here

One of the hots runs the string of outlets I want to protect, then there's a hot from the other part of the tandem switch, and a single neutral split between them. I don't think I can use a single GFCI 15A circuit and just switch the single circuit over.

Closest thing I can find is this:

enter image description here

But that's two pole, not one pole (and I can tell the difference in the picture, but honestly don't understand quite the difference). My panel takes QO breakers...

Any help much appreciated!

UPDATED IMAGES INSIDE THE PANEL enter image description here

enter image description here

enter image description here

Just so people understand the context this is a new house I moved into. I didn’t ignore rules, I’m just trying to fix an issue that was there before, and do it safely. Your input is very helpful.

The top left of the panel is input to the sub-panel. The panel came with the two breakers on the left, and the breaker all the way on the right. Coming into the box were essentially three sets of wires through one metal conduit, with each set having a piece of electrical tape around them. There’s a black, red, and white bundled together, and another black red and white bundled together (14g). The two black wires go to one breaker, with the white neutral above it, and the two red wires go to the next breaker, one neutral above it. The 3rd set is a pair of black and white 12g wires that go to the 20A rightmost circuit. Then there are two circuits in the middle that have the visible square D symbol that I added. The first one on the left is the GFCI 15A circuit, not hooked up to anything but the neutral and turned off, the failed addition, and a 20A circuit hooked up to a 12g pair of black/white wires that goes out to topmost right position in the sub panel to an outdoor GFCI outlet I added (the only things I added to this panel).

Curious to know if this changes anything. Thanks for your thorough thoughts!

  • 1
    Are you aware that your existing setup is potentially dangerously overloading that shared neutral wire?!?
    – brhans
    Mar 26, 2019 at 22:07
  • From what I can tell it's just a shared tandem switch which is properly wired, but correct me if I'm wrong. (edited the original first image)
    – Azulith
    Mar 26, 2019 at 22:12
  • 2
    NO!!! You can't use TANDEM breakers on shared neutrals!!! You are very confused. There is a nugget of truth behind this dangerously not-even-wrong situation is that you can use shared neutrals on 240V 2-pole circuits, but this is not that. A tandem breaker is thediametric opposite of a 2-pole breaker. Mar 26, 2019 at 22:24
  • 1
    In 1995 there was a horrible boiler explosion on the Gettysburg RR. tourist railway. By stroke of luck, an oddball boiler design blunted the force, and crew survived. The NTSB determined it was low water from misreading the sight glass, which was clogged. Seems the grandfather knew how to"blow down" the glass to prevent clogs, and taught it to the father, who mistaught it to the son, who had been blowing down wrong but earnestly believed he was doing it right. After so many consecutive runs of only him blowing down the glass, it progressivly clogged until it misread. Mar 26, 2019 at 22:42
  • 2
    Hello, and welcome to Home Improvement. I'm the greeter; @Harper's one of the experts. I'd take what he says very seriously. Mar 26, 2019 at 22:43

4 Answers 4


Since my original answer was very generic to shared-neutral/multi-wire branch circut/MWBCs and tandems, and you've added a lot of new info, I exported it to this straw question here. Here are my comments particular to this case.

The two breakers in each MWBC need to be handle-tied both for safety, and to assure they are on opposite poles, and to warn the next maintainer that these circuits are special. 2-pole breakers will also fill the bill.

That said, the classic "single-file" tandem breakers, as seen in your panel, are clearly built to be compatible with handle-ties if two such breakers are installed abreast. This would be an appropriate way to tie the MWBCs. It's possible it had been that way, and one of the breakers failed, and they found "classics" were no longer available and they had to take the "switches abreast" type. If you find two handle-ties in the bottom of your panel, that's what happened.

On a fairly full panel like this, normally you get 2-pole with quadplex breakers. Your QO panel does not have quadplex breakers available.

The other option is simply splice both hots together onto a single breaker (or half a tandem). That single breaker can be a GFCI.

Now it's time for GFCI

There's no such things as a duplex ACFI or GFCI. They take a lot of space. Likewise no such thing as a quadplex AFCI or GFCI; but they do make them in 2-pole.

If you only need to provide GFCI to one of your multi-wire branch circuits, at first glance it seems like you have room. But you don't, because that would "orphan" one of your shared-neutral MWBCs.

Your best bet, then, would be to consolidate the two non-MWBC non-GFCI circuits (the top 2 breakers) onto a single duplex (which, notably, you have two of). That will free up one more space, so you'll be able to give 4 spaces to the 2 shared-neutral circuits, allowing 2-pole GFCI/AFCI. That is your best bet.

There's one more option; you are allowed to fork circuits right at the panel (i.e. have a circuit depart 2 directions out of the panel). You could simply declare both MWBCs to be the same circuit. Both circuit's reds would go to a red pigtail; both circuits whites likewise, and blacks likewise. Then the 3 pigtails would go to the GFCI. This would limit the current on each MWBC, obviously, but you might get away with it.

The best option yet

I'm sorry, but having seen you drop $40 on a GFCI you can't use, I cringe at proposing you spend $160 more. This panel is stupid-small anyway, so why don't we kill two birds with one stone?

Go get one 60A 2-pole GFCI breaker and a 16-space subpanel. Mount the new sub right next to this one. Fit the 60A GFCI here and use it to feed the main lugs of the new sub-sub-panel. Any circuits you install in the new sub-sub will be GFCI protected by inheritance. Install these circuits in the new panel using ordinary 2-pole breakers.

Now if you want the non-GFCI sub to be 16-space and the GFCI sub to be 6-space, swap them. Either physically by tearing this panel out of the wall and terminating all its conduit connections to the new panel; or electrically by extending the feed wires to the adjacent 16 and putting the GFCI there to power this one.

  • I think that the MWBC hots are actually on two poles in this case, just in an unusual configuration. Notice that the bottom tandem breaker has two black wires connected, and the tandem above has two red wires -- assuming conventional NM/3 color codes, these are probably two MWBCs with one hot on each of the two poles. The breakers for each half of the MWBC are not next to each other, and so cannot be handle tied, but the neutral probably isn't actually overloaded as currently installed.
    – Nate S.
    Mar 28, 2019 at 16:24
  • @Nate Ah, OP posted some pix. This is in conduit which means colors don't mean anything. However an effort was made to mark the grouping of the wires, so if that is reliable, then yes - they are on opposite poles and thus less unsafe. Proper installation requires handle-ties, so the neat and simple answer here is a quadplex breaker (if QO makes those). If not, 2-pole breakers are needed. Mar 28, 2019 at 16:47
  • True, with conduit we can't know for sure, but I think it's pretty likely. Agreed handle ties are needed -- I'm guessing that originally the bottom breaker was that old offset tandem type as well, and was handle tied to the one above, which would have been a code-compliant installation. When the bottom breaker was replaced with the new style, the handle ties didn't fit so were discarded.
    – Nate S.
    Mar 28, 2019 at 16:59
  • @NateStrickland yup, we're thinking the same thing. I just said pretty much that in an edit. Mar 28, 2019 at 17:14

That is a square D tandem breaker they are on the same leg of your main power it is a code violation and a fire hazard on top of a electrical safety issue. Some working on one of the circuits even though turned off can get shocked because the other circuit is live. A double pole breaker using 2 slots would be required to use a common neutral (called a multi wire branch circuit) this takes power off of both legs of the main power and each leg is 180 degrees out of phase so when 1 leg is at peak voltage the other is at minimum this is why the neutral can handle a MWBC. 2 breakers can be used if handle ties are used but it still takes 2 slots. If you get those circuits on separate legs you can use the existing wiring with a double pole breaker or 2 breakers that have handle ties or even better would to use the double pole GFCI breaker then both circuits would be protected. If you want to save some $ use a non GFCI breaker(s) and install a GFCI outlet on the first one in the string, then using the load terminals on the GFCI feed the other outlets. Note with MWBC the neutrals need to be pigtailed removing the outlet can not open the neutral. Hope this helps you understand what's needed to do it safely & in a code compliant way.


Close, but no banana!

Amazingly enough, the existing wiring is almost right -- the two blacks are on one leg, and the two reds are on the other leg, as they should be, so you actually don't have to worry about burning your house down with this!

However, there is still a problem here, and that is that there's no common maintenance shutoff for either multi-wire branch circuit originating in this panel. Code requires this so that somebody doesn't shut one half of the circuit off, then get bit by the neutral because the other half is still allowing the neutral to be "live".

At this point, we then run into trouble. Normally, I'd say "go slap a quadruplex breaker in there and call it a day", but the closest thing to a quadruplex QO is a pair of QO tandems with their inner poles handle tied using a QOTHT. However, that only provides common maintenance shutoff for the inner pair of poles, so its not an option.

Instead, what I would do, since you can pull the single pole GFCI from the failed addition out and have a space spare atop that, is get two QO215GFI breakers and install them in place of the failed addition and the two tandem breakers, thus displacing the 20A breaker up a space. Each black/red/white bundle lands on the load lugs of its corresponding circuit breaker (black/red to the hots, white to the load neutral), and the line neutral pigtails go into holes on the panel neutral bar. You'll have to remove the remaining twistout from the panel deadfront before you put it back on, as well. This also takes care of all your GFCI needs for those circuits as well, of course.

  • Technically if OP could find another of those weird old QO tandems with the handles offset instead of next to each other, both sets of those could be handle tied, and the circuits would be up to code. I'm guessing that's how it was installed originally, and then the bottom breaker got replaced with the new style, and the handle ties no longer fit so they were removed.
    – Nate S.
    Mar 28, 2019 at 16:45
  • Can't you just stack three QO tandems with two QOTHTs between them? Mar 28, 2019 at 17:27
  • @Harper -- you could, but since the GFI breaker he added but can't use can just be pulled back out again, we don't need to do that sort of stunt :) Mar 28, 2019 at 22:37

Aside from the fact that you must fix this "shared neutral" issue, whatever it really is (I'm not convinced you described it right):

  1. You can move some other circuit that is using a single pole breaker to one of the poles of the twin, then put a GFCI breaker into that slot and wire your garage outlets to that breaker.
  2. You can leave it like it is (other than the neutral issue) and install a GFCI receptacle on the FIRST in that line of receptacles, so that all of the ones down stream are fed FROM that one. Perfectly legit way to do this, no need to change the breakers at all.
  • Now that I can see the photo, I count 6 poles of circuit breakers and 6 separate neutral wires (plus the curly for that AFCI/GFCI breaker). So I think I was right, you described it as a "shared neutral" but that's not what you have there. Not sure what you meant by that but this is just standard stuff. So my suggestion of just swapping breakers would be the way to do this.
    – JRaef
    Mar 28, 2019 at 19:13

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