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I have a single gang electrical box on an outside wall of my home. I need to expand it so it can house two GFCI outlets on separate circuits. I have been looking for an extender with an opening in the back the size of a single gang box but have not found one. Does anyone know where I can buy one? If not, can I cut a hole in the back of a two-gang weatherproof box and mount that to the single gain box? Any suggestions would be appreciated!

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  • Everything below is good information, but I didn’t see the simple ‘run a new circuit into a new box beside the old one’. Is that an option? Jun 16 at 0:59

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so it can house two GFCI outlets on separate circuits

Frame challenge. If you literally need two receptacles, one on each of two GFCI protected circuits, there are two options that do not involve expanding the box at all. It may or may not be practical, but that is a separate issue:

Multi-Wire Branch Circuit (MWBC) with GFCI Breaker

  • Install a double-breaker GFCI. (Note: This may not be possible in an older panel.)
  • Run a /3 cable (12/3 for 20A, 14/3 or 12/3 for 15A) or two hots + neutral in conduit from the breaker to the outside box.
  • Install a weather-proof standard (not GFCI) duplex receptacle in the outside box.
  • Remove the tab on the hot side.
  • Connect the two hots to the hot side (one on top screw, one on bottom screw) and the neutral to the neutral side.

MWBC with Two GFCI Receptacles

  • Install a plain double-breaker.
  • Run a /3 cable (12/3 for 20A, 14/3 or 12/3 for 15A) or two hots + neutral in conduit from the breaker to an inside double-gang box.
  • Install two GFCI Duplex Receptacles in the box. Pigtail the neutral to feed the LINE side neutral and use a different hot wire for each LINE side hot.
  • Run 4 wires (plus ground if not metal conduit) to the outside box. This can be two cables or four individual wires in conduit. If it is four wires in conduit, make sure that the hots are clearly marked (different colors or colored tape near each end) and the neutrals are clearly marked (since most non-professionals won't bother getting gray wire, mark one of the white neutrals with colored tape).
  • Connect one pair of wires to each LOAD side.
  • Install a weather-proof standard (not GFCI) duplex receptacle in the outside box.
  • Remove the tabs on both the hot side and the neutral side.
  • Connect the two hots to the hot side (one on top screw, one on bottom screw) and the two neutrals to the neutral side.
  • Wires must be paired properly - i.e., LOAD side of one GFCI to top of outside, LOAD side of the other GFCI to bottom of outside.

There is a third option - run two separate circuits to two separate GFCI/receptacles and then bring LOAD from both of them to the outside box. The trick there is that even though they don't need to be a double-breaker (as necessary for a GFCI/breaker or makes sense for a new MWBC), they do need to be next to each other and handle-tied. In all three cases, double-breaker or handle-tie is critical to make sure that the next owner (who may not realize there are two circuits or an MWBC) always turns off power to everything in that outside box before doing any work on it.

In all of these cases, the outside receptacle must be marked GFCI Protected, and if it isn't really obvious, labeling the location of the GFCI device would be helpful for when there is a GFCI trip.

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  • @Harper-ReinstateUkraine Yeah, I thought of that option too. The trick there is the handle-tie - that gets a bit more confusing than a simple double-breaker. But if OP already has two separate circuits in mind and wired to near the destination then that makes sense. In all of these the GFCI will be inside and sticker needed. Jun 15 at 22:27
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    Sorry I deleted that because I thought you already covered it. I was proposing your second case but with non-MWBC from panel to GFCIs, so each GFCI would be fed its own hot/neutral on its own /2. The breakers would need to be tied since they both serve the same receptacle. Jun 15 at 22:29

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