Forgive me if this a repeat, but I'd like to be sure I know exactly what's going on. I am installing 2 bathroom fan/light fixtures directly over the tub in 2 adjacent bathrooms on the same circuit. These need to be protected, so I bought a 15A single pole 120V GE breaker to protect the circuit. When I went to the panel box, I have the black wire going to one 15A breaker (both bathrooms, master bedroom) and red wire going to a 2nd 15A circuit breaker below the first (living room) coming from the same cable. The white and ground go to the neutral bar. From my reading here, I know that this is a multi-wire branched circuit. Also from my reading, it looks like to properly protect the 2 bathrooms, I have two options:

1) Install a 2-pole 15A GFCI 240V breaker ($90, but easier) 2) Install 15A GFCI outlets on all outlets ($25-$40, but more involved)

Are these my only options? Aside from the $ and time involved, are there any differences between my 2 options?

If I go with 1)...my panel uses the 1/2" slim breakers. The 2 breakers on the MWBC are stacked, does it matter WHERE in the panel I put the bigger 2-pole GFCI breaker? I have a few empty spots.

If I go with 2) Do ALL outlets need to be converted to GFCI (including those in the living room) or do ONLY the outlets on the bathroom circuit need to be changed out? Is there any special wiring I need to adhere to?

  • I edited your question to remove the "bonus" question you asked, so as to keep the question a bit more focused and less likely to be closed. If you still want the answer to it, ask it as a separate question.
    – mmathis
    Sep 20, 2017 at 20:14
  • In what order are the outlets on the circuit wired? Is it master bedroom first then bathrooms, or bathrooms first then master bedroom, or master bedroom in the middle? What other bathroom loads are on the circuit, for that matter? Sep 20, 2017 at 22:25

1 Answer 1


Land all your MWBCs on a 2-pole breakers NOW.

First, big safety problem. If an MWBC has both its legs on the same pole, it will overload the neutral.

It's important to search your entire panel for multi-wire branch circuits and replace all their breakers with 2-pole breakers. Not two adjacent breakers. A 2-pole breaker specifically.

Why? GE Qline does not have the concept of a double-stuff breaker. It has extra nubs on each of the busbars, to which half-width breakers can clip. Absolutely nothing in the Qline system forces adjacent half-width breakers onto opposite poles. As a result, simply being adjacent does not assure the breakers are on opposite poles.

However, Qline 2-pole half-width breakers are keyed so they can only land on opposite poles. So you must put MWBCs on those. They're not expensive.

What is a GFCI?

A GFCI is a black-box which inputs hot(s) and neutral on its "line" side ... and then it outputs protected hot and neutral on its "load" side.

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That's it. That's all there is to a GFCI device. To signify the protected side, I use brown and gray wire in conduit work, but you can do the same by tagging wires with brown and gray electrical tape. Now if that's hard to visualize, look at this.

enter image description here

This is a GFCI device in its purest form: a "deadface" GFCI. There aren't any sockets to confuse the matter. It's very clear that you use this one by putting it somewhere it can serve downstream loads, and connecting two line and two load wires, exactly like the diagram.

Back to MWBCs, you can put a single-phase GFCI device anywhere past a split point. It will protect everything downline.

I don't really care if you get one that also happens to have a couple convenience outlets wired to the LOAD side. Just as long as you know what's really happening.


A GFCI compares current flows on hot(s) and its partner neutral. So in an MWBC, a 1-pole GFCI can only work where the MWBC has forked off a hot and neutral, and neither one will interact again with the other hot.

It's fairly typical in residential installations for an MWBC to fork pretty much at its first junction. Red and white go one way, and black and (same) white go the other way, and they never meet again.

enter image description here

You can install a simple 1-pole GFCI anywhere past the split point. For instance in this drawing, the receptacle (the 8 is a receptacle) is right at the break, and could be nudged a bit so it is after the break, by feeding the right-side "circuit 1" off its LOAD terminals.

enter image description here

Your case...

So your objective is to figure out how your wiring is laid out, and where your branch points are. And then figure out where GFCI's can go in this topology. If you find that part of the circuit goes off in a 2-wire branch (and never rejoins), you can put a GFCI device at the foot of that branch.

So it really boils down to whether you can find a point in the right place in the topology: after a split, but still upstream of the tub lanp/fan. Or whether you can create such a point, and fit a deadface GFCI there. Or a liveface. I don't care ;)

Bathroom receptacle circuits

Most (male) electricians don't get this... but a bathroom shouldn't be plunged into darkness from an overcurrent or GFCI trip, while you're holding a rather hot hair curler. That is simply not safe.

As such it is typical for bathroom lights to be on a different circuit than their receptacles. There is also a rule that receptacles can't share a circuit with anything except loads in that bathroom; or alternately other receptacles in any bathroom.

  • mwbc...... sigh. But +1 for the right answer :D
    – noybman
    Sep 21, 2017 at 3:59

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