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I'm converting a former laundry area into a bar/coffee area. There's a relatively new 240V, 40A circuit that is now unused, so I'd like to repurpose it for 120V service.

I'm hoping to run 1/2 of it to a new 20A GFCI, with the load side then going down to another 20A receptacle below the new counter (as shown on the photo).

The other leg I'd like to run from the GFCI box (i.e., using the GFCI box as a junction, there's plenty of room) to an adjacent room where I also need a new pair of 20A receptacles (a work bench).

I know I'll need to replace the 40A ganged breaker with two 20A breakers.

So I have two questions, both in terms of code and general sanity:

  1. Can I use the GFCI box like this as a junction for the other side of the 240V circuit that will be run to the other room? The junction would be accessible and visible, just not obvious because it would be behind the GFCI rather than in a dedicated junction box.

  2. Is it ok that the two new 20A circuits share a common neutral and ground wires from the junction back to the breaker panel? The wire was intended for 40A service, so I'm not afraid of the load, just concerns that there may be a rule that you can't do this.

Photo: 240V receptacle in the center (to be removed), its feed line is visible (orange wire behind plumbing). New GFCI on the right top, hole for new receptacle for load side of GFCI below (yellow wire is coming from the GFCI, white wire unused).

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  • Are you planning on tapping to the original wire with 12 AWG?
    – ChiefTwoPencils
    Commented Jan 20, 2015 at 8:52

2 Answers 2

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Code may vary where you live

Yes. You are going to create a "Shared Neutral" circuit. This isn't difficult or dangerous, but you should do some labeling in the panel and the box itself. First, it looks like the receptacle is a 30A/220V grounded receptacle (the wire itself may also be 30A -typical for an electric dryer- but that doesn't matter since you are using 20A breakers and receptacles). You are going to share the neutral and ground wires for these (2) new circuits by wire-nutting the two circuits in your first location (the junction inside the GFCI).

Shut off power to the 40 amp double pole breaker.

Assuming you have a red, black, white, and green/bare wire: The red is one hot. The black is the second hot. The white is the neutral for both. The green/bare is the ground for both.

Assuming that the "red" circuit will be the GFCI at this location, connect the Red to the hot\line side of the GFCI, make a pigtailed white wire to do the same. On the load side of the GFCI, run the continuation of the circuit. Using the Black wire and another pigtailed neutral, junction these to the line that is going to the other room. To be a good citizen: Label the inside of the box "Shared Neutral". Extra points if you "tie" the two hots together with a piece of white electrical tape with some extra length so it looks like a little "flag" that says "Shared Neutral". Double extra points if you label the cover plate so that someone sees it before they make any future modifications.

Go to your panel. If you are not comfortable working in the panel, shut off the main power and probably have a friend hold 1 or 2 flashlights for you.

Disconnect the (2) hot wires from the 40 amp double pole breaker (I assume these will be red and black). LEAVE the neutral and ground in place. Remove the breaker and simply replace it with (2) 20 Amp breakers. Keep the same position in the panel OR make sure that the breakers are on OPPOSITE poles -as if they were a single 220 volt circuit. IT IS IMPERATIVE THAT THEY ARE NOT BOTH ON THE SAME POLE OR YOU RUN THE RISK OF SENDING 40 AMPS THOUGH THE NEUTRAL ON THE SAME POLE (If you need further explanation, please ask -this is the most important safety step). Some people will disagree and will say to use a 20 Amp double pole breaker. The problem with this is that it inaccurately appears to be (1) 220 volt circuit instead of (2) 110 volt circuits with a shared neutral. There is an exception to this rule below.*

LABEL The panel cover by bracketing the (2) circuit labels and write "Shared Neutral". Extra points: inside the panel, tie the (2) hots together (the red & black) with a piece of white electrical tape that says "Shared Neutral".

NOTE: You will NOT be able to split the circuit after the GFCI ...so don't think you can give GFCI protection to both circuits with (1) GFCI receptacle. If you want the other circuit to be GFCI protected, you must use another GFCI receptacle in the first location of the second (black) circuit. You also will not be able to use a single pole GFCI breaker on either circuit. You must either use (2) GFCI receptacles as above. Alternatively, you may use (1) double-pole (220volt) GFCI circuit breaker -but you should still label the circuit as "Shared Neutral".

  • EXCEPTION TO THE (2) 20 AMP CIRCUIT BREAKER RULE: If you want these circuits to be AFCI protected you must use (1) double pole (220 volt) AFCI circuit breaker. You should still label the circuits as "Shared Neutral".

...That was fun to write! :D

Peace, Greg

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  • Diagrams might be helpful for this answer, as your description could be quite confusing for an amateur. GFCIs on multi-wire branch circuits can be difficult to wire properly, so a diagram would help with that as well.
    – Tester101
    Commented Jan 18, 2015 at 0:36
  • Wonderful answer, Greg, exactly what I was looking for! Leaving notes inside the box and at the panel are great ideas, I'll be sure to do that. But I do have a follow-up question -- since the 240V wiring was rated for 40A, what's the issue with the shared neutral carrying that load if the two branches were in phase? (I plan to use the same adjacent breakers as before, so it's not an issue for me, I'm just curious.)
    – richardtallent
    Commented Jan 18, 2015 at 3:31
  • As long as the two circuits come off a double-ganged breaker as used for 240 volts, the neutrals cannot be in phase.
    – wallyk
    Commented Jan 18, 2015 at 6:38
  • Is labeling the neutrals code in your area? I suppose the labeling of the neutrals gives some additional information but to me it seems it's mostly implied you'd be sharing neutrals whenever possible.
    – ChiefTwoPencils
    Commented Jan 20, 2015 at 8:47
  • @chief, my city adopts NEC 2008 with some miscellaneous amendments, none applicable to this question. since the neutral is shared, I'm thinking I should still use a double-pole breaker (but 20A rather than 40A) so there's no question about the neutral having current moving through it. Maybe that's overkill, but I don't want anyone in the future working on one side of the circuit to become a shorter path to ground for the other side. I would guess that this is a good reason to label the shared neutral.
    – richardtallent
    Commented Jan 20, 2015 at 14:34
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Yes, it is legit to have a “shared neutral” circuit, to be more specific, a “multiwire branch circuit” or MWBC. The Code forces you to engage in certain best practices, because even if you get it perfect, it's very easy for the next guy to inadvertently create a hazard.

First, 2011 code absolutely requires a 20A ganged breaker (or approved handle ties). Aside from the reason you recognize, it also assures you are on opposite poles of the 240v, which avoids overloading the neutral.

All neutrals on an MWBC must be “pigtailed” - google it, i.e. you are not allowed to daisychain through outlets by using the extra 2 terminals. (Well, OK, you can daisychain the “hot”.) This is to prevent “lost neutral” damage downstream if an outlet is removed or has a problem.

Exception: Your GFCI outlet. By nature, they require a dedicated separate neutral to any downstream outlets, so naturally you must daisychain both hot and neutral for those. To make that clear, mark that neutral in a distinctive way so the next guy doesn't grab it thinking it's the MWBC neutral.

To answer your followup, opposite poles matter. While your homerun is 40A rated to the old plug, you certainly are extending the MWBC beyond it, and using 20A wire (e.g. 12 gauge) to do so. In the future, others will add to that MWBC. What happens when the next homeowner runs the other "hot" out to the workbench? Due to time/billing/skill limitations the "next guy" can't reverse engineer the full scope of every circuit they touch - they must be able to assume it's correct with no hidden gotchas, which is a big part of why there's a code. That is especially important with MWBCs because they are so touchy.

If the restrictions or safety margin of MWBCs bother you, slap a sub-panel at the site of the old plug, and run simple circuits off it.

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  • 1
    If space allows for a sub-panel, that would allow use of four 20A circuits, rather than two. Otherwise, I'd suggest adding some tagging and labeling to the large-gauge wires at the breaker box indicating the existence of smaller-gauge downstream wiring without additional circuit protection.
    – supercat
    Commented Jan 14, 2016 at 19:16

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