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I have an outdoor outlet that is on a 20A circuit shared with several indoor outlets. It's the old kind with a flap door that protects the outlets from rain when not in use. There's no GFCI.

I want to plug in two outdoor appliances that need about 15A each so I will upgrade the outlet.

I want to replace the existing outlet with two new outlets that have GFCI and each with its own new circuit from the load center. I’ll run a new dedicated 12/3 cable from the load center to the location for an MWBC configuration.

I can think of two approaches, each with its own issues. I’d like help deciding, or perhaps thinking of a better way.

Method 1: Install a 2-pole GFCI breaker, and a single gang wet-in-use box, a regular (not GFCI) outlet with the two sockets detached (break the tab) each on a different breaker. The benefit of this method is that it can use the existing flush-mount hole in the outside wall. The problem with this method is the breaker costs $120. And I’m not sure all aspects of this are to code, i’d appreciate advice on that. A side effect is that both circuits are tripped if either encounters a ground fault at the plugged-in device. That's probably ok, neither good nor bad. Is there anything about this that is not to code? If my location matters, it's New Jersey, but let's say I want to comply with the most stringent current requirements nationally.

Method 2: Install a 2-gang wet-in-use box, use a regular (not GFCI) 2-pole breaker, and two regular GFCI outlets. This saves about $100 on the breaker, but the disadvantage is putting a two-gang box where there is currently a flush-mount one-gang box. I either have to seal up the hole in the wall and install the whole thing on the surface, or I have to cut out a bigger hole which would be a huge, huge pain. There are 120 years of layers of siding on the house including a mineral siding as the outermost one. I really don’t want to expand the cutout, and I'd rather not make it surface mount just because that's ugly.

Finally, the location is diagonally opposite side of the house from the load center which is why I really want to stick to MWBC and not run two new cables.

Thoughts? Ideas?

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You can get it done for under half the cost of a 2-pole GFCI breaker.

Install a 4-11/16" steel box right next to the panel. Connect it to the panel with a short bit of metal conduit (which handles ground for you).

Install a double Decora domed cover on the 4-11/16 box. The reason for the large size is to provide enough room for 2 GFCI receptacles to fit comfortably.

Power the two GFCI receptacles off either a 2-pole breaker, or two single-pole breakers with a listed handle tie. Use the conduit passage to bring over 2 hots and 2 neutrals.

This is best done with THHN wire, which most places cheerfully sell by-the-foot.

From the "Load" terminals of the GFCI receptacles, run two 12/2 cables (or one 12/2/2) to the outdoor outlet.

Split both hot and neutral on the outlet. Hook one hot/neutral pair to the top socket, and one hot/neutral pair to the bottom socket.

The handle ties are required because the two circuits share a yoke.

These circuits will be independent for GFCI purposes. For overload, they'll common trip if a 2-pole breaker is used, and might common trip if 2 handle-tied breakers are used.

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    Good idea. What about putting the new box (to house the GFCIs) indoors near the outlet rather than near the panel? Then using 12/3 NM to get there from the panel, and 12/2/2 from there to the outlet? Reasons: A) I need a 100 foot roll to get to the location, and 12/3 is cheaper, and B) the area around the panel is crowded. Every time I add something I'm a step closer to needing to expand the plywood field that's mounted to the solid stone basement wall.
    – jay613
    Jul 22 at 21:51
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    @jay613 sure, that's legit. In the first draft I wrote to use a MWBC for the short run from panel to GFCIs... then deleted it since 2 feet of THHN is too cheap to care about. But it would be fine. As far as hauling power 100’ like that across a house, remember - subpanels are cheap, and 2-2-2-4 aluminum feeder (90A) is the same price as 10/3 copper (30A). Jul 22 at 22:11

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