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I'm replacing the two-pronged outlets in my basement with 3-pronged grounded outlets in the United States (Illinois). All the wiring is in metal conduit with metal conduit boxes, which are grounded to the breaker box (I checked with a multimeter from the hot slot to the metal box). My goal is to wire all the 3-pronged outlets grounded to the metal conduit boxes, with a GFCI at the outlet closes to the breaker box (which I believe is proper code for a basement).

The metal conduit has three wires running through it: a red and a black, which are both hot, and a white, which is neutral - I tested all these with a multimeter at different outlets. These outlets are all daisy-chained together, sharing a neutral white wire, but powered by the different red or black wires.

To further clarify, the red and black wires are not in a series daisy chain (to my eyes and testing). When I opened the circuit breaker box, both the red and the black wires had their own breaker switches, but the wires ran through the same conduit.

At first I thought this was just weird, but chocked it up to odd 1960's construction, or my own lack of knowledge.

But, it is causing issues: when I plug something in downstream of the GFCI (powered by black) in an outlet powered by red, the GFCI trips, so I can't use those outlets.

I was using a used GFCI outlet (bought at a secondhand construction materials store) - I bought a new one in case it is the outlet's fault, but haven't yet tried to install it.

Also note that the breaker isn't set up for 240V supply; there are no appliances in the house running 240V (we have natural gas).

I drew a picture which I hope helps illustrate my description better. There is either a red or a black box around the outlets to show which color hot wire they are being powered by.

Diagram of wiring

Thanks for looking! I am an electrical layman but could provide more info/investigate further if needed!

EDIT Thanks for the advice, everyone.

To clarify my wiring, here's a picture inside one of the outlet boxes: Outlet Box Wiring

Here is a picture of the breaker box. I outlined which switches are connected to the hots, and typed in the amperage since the picture isn't great (20 amp circuit breakers): Breaker Box

Checking the neutral wire at one of the outlets on this branch is 14 ga, which has a 15-amp rating - is that typical?

Finally, here's a picture of an old circuit breaker that was sitting unplugged on top of the box: Circuit Breaker

It is a Square-D brand breaker, type QO I believe.

So, based on everyone's advice and the research I did, I think I need something like this: QO Qwik-Gard 20 Amp 2-Pole GFCI Breaker GFCI Breaker

Could I get some feedback on that assessment?

Finally, after reviewing the electrical code, it appears GFCI circuits are required in just unfinished basements, due to potential wet conditions. My basement is semi-finished (drywall , but exposed concrete & open ceilings), and not really wet. Could I get away with a non-GFCI 20-amp circuit breaker? It would save me about $70. I'd rather not die, though.

Thanks again!

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    The GFCI tripping when something is plugged into a red outlet seems like correct behavior; it is properly detecting an imbalance in the live and neutral. No clue why it's wired like that, though. – user3757614 Nov 1 at 5:56
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    It's a multi-wire vbranch circuit. Nothing wrong with them, but they require special handling when it comes to GFCI. Also for safety reasons the two hots must have 240V between them. – Harper - Reinstate Monica Nov 1 at 12:24
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    Nicely done, especially for your first question! – FreeMan Nov 1 at 17:43
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    @Harper: why is that? – whatsisname Nov 1 at 19:25
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    @whatsisname Sketch out how the current will flow if the loading is 15A / 14A. If the hots are 240V apart, 14A of one hot will return through the other with 1A on neutral. If they are 0V apart, all 29A returns via the same neutral wire. – Harper - Reinstate Monica Nov 1 at 19:46
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MultiWire Branch Circuit

What you have is a MultiWire Branch Circuit, or MWBC. That is a neat trick that lets you get double the total power, with only 50% more wire (3 wires instead of 2). If you look at your outlets, you should see that the tab on the hot side which normally connects the top and bottom receptacles has been removed. That separates them into two separate circuits as far as the hot wire, but they share the neutral on the other side. In fact, the neutral only carries the difference between the two hots instead of the sum of the two hots, provided the two breakers involved are on separate poles. You can verify that the two hots are on separate poles by checking the voltage between them. You should have a nominal voltage of 240V between the hots with 120V between each hot and neutral. If you get 0V between the two hots and 120V between each hot and neutral then you have a BIG problem.

CORRECTION Rereading your description, it appears that instead of a split in each duplex receptacle (tab removed) the split here is some duplex receptacles on one half of the MWBC and some on the other. The principle is the same and the GFCI issue is the same.

The trick to making an MWBC work is landing the two hots on opposite poles. A typical circuit breaker panel will alternate between the two poles. This allows for 240V breakers for hot water heaters, dryers and other large appliances. It also allows for an MWBC. Note that if the two hots are placed on two breakers that happen to be on the same pole (e.g., regular breakers but not next to each other or 2 1/2-size breakers that replaced one regular breaker) then you will end up with:

  • 0V between the hots (not in and of itself a problem, but a symptom - and an easy way to check that an MWBC is correctly wired)
  • Current on neutral is sum instead of difference - this is the real problem as you could have 40A going on a 20A rated wire.

In addition, an MWBC must have common shutoff so that you can't turn off (in your example) the black wire to work on a receptacle without also turning off the red wire. When you turn off a breaker to work on a circuit, you normally are trying to get everything you might touch at the time to be dead - so turning off the pair of breakers at the same time is critical. Using a typical 240V breaker - which is really a pair of 120V breakers but designed to trip at the same time - is the easiest way to do that.

GFCI vs. MWBC

The problem with GFCI on MWBC is that GFCI depends on "all hot" == "all neutral". Since most ordinary 15A and 20A receptacles are NOT on MWBC, most (possible all) combination GFCI/receptacles do not support MWBC. One reason is that a typical duplex receptacle has room for 4 wires (not counting ground): incoming hot, incoming neutral, outgoing hot, outgoing neutral. But those same 4 connections can instead be used for incoming/outgoing hot 1, incoming/outgoing hot 2, incoming neutral, outgoing neutral - and you have a setup similar to that. That works because the incoming and outgoing wires (e.g., 2 wires for hot 1) can be connected together.

A GFCI can not have the incoming and outgoing wires connected together. A GFCI needs to compare the outgoing (usually called "LOAD" on a GFCI) hot and neutral. Mixing in the incoming power (from the panel) messes that up. So there just aren't enough connections to make an MWBC work.

However, there is a simple solution: Use a breaker instead. Replace the existing double-breaker (it should be a double-breaker, but it might be two separate breakers which is unsafe as noted above) with a double-breaker with GFCI. That is designed to handle an MWBC or any other 240V load and provide GFCI protection. It is a little different to hook up as it connects to neutral as well as the 2 hots, unlike a regular breaker that only connects to the hot wires. You must use the right type of breaker - post a picture of the existing breakers and the pros will figure out what you need.

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    And note that a MWBC doesn't use as much labor as two circuits that deliver the same power. Many years ago I needed an additional outlet, the electrician suggested the MWBC approach because it would increase the cost by only a few dollars but would provide more power if needed. – Loren Pechtel Nov 2 at 2:01
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Nothing easier. Add a gray neutral to the conduit.

Gray is the other allowed color for neutrals, and it's ideal in situations like this to disambiguate between two neutrals. Now you have two pair: black/white and red/gray.

For the outlets served by black hots, leave those alone. For the outlets served by red hots, remove the white neutrals entirely - cap them to each other so they go "through". Then attach the gray neutrals to those outlets.

With that done, find the first black outlet past the panel and fit a GFCI receptacle there. Find the first red outlet past the panel and fit a GFCI receptacle there.

Oh wait. There are already 4 circuits in one of the conduits.

One big reason multi-wire branch circuits are used is because of conduit circuit derate rules - which require reduction in capacity for multiple circuits in the same conduit. With 10-30A circuits, you are allowed four circuits per conduit in a 120V/240V split-phase system. A multi-wire branch circuit counts as one.

Converting a MWBC into two single circuits adds a circuit. So we must make sure we are not screwing up our derating. If you put 5-10 circuits in a conduit, you must bump a wire size (#12 for 15A, #10 for 20A) for every circuit in the pipe. This is best avoided!!!

To be technical, the maximum is 9 wires. However in a 120V/240V split-phase system, every circuit counts as 2 wires... so in practice, 4 circuits.

  • if a plain 120V circuit has hot and neutral, that counts as 2 wires obviously.
  • However if an MWBC has 2 hots and 1 neutral, the neutral only carries differential current so it doesn't count.
  • The same is true of a 120/240V circuit (2 hots+neutral).
  • A straight 240V circuit (2 hots only) counts as 2 wires obviously.

Now, when the circuit has <=9 wires, a #14/#12 wire derates to 17.5/21 amps. But those are limited to 15/20A anyway, so the <=9 derate is no inconvenience. #10 derates to 28A, but you get to round up to a 30A breaker, so again, no inconvenience.

So, we have to go through each segment of our pipe and count the number of circuits in it.

However you may find it is challenging to run a second neutral back to the panel, because some parts of the pipe can't handle one more circuit. In that case, leave it as an MWBC, make the fixes manassehkatz recommends, and do this:

Branch the MWBC at the first outlet

I bet we have a crowded pipe before the first outlet. If so, the best plan will be to start laying the gray wire at the first receptacle in the chain (which is black-fed).

Here, Supply white pigtails to onward gray and the black GFCI's LINE neutral. Supply black goes to black GFCI's LINE hot obviously. In the onward pipe, black/white are GFCI protected zone (black), and red/gray are unprotected.

At the very first red box, black/white are still GFCI(black) and pass through unused. A GFCI recep is fit here, and red/gray feed its LINE terminals. Its LOAD terminals feed protected red/gray to the rest of the circuit.

Every receptacle downline is either connected to red/gray, or to black/white. There is no criss-crossing.

This thing is still an MWBC, so it still needs a 2-pole or handle-tied breaker, as manassehkatz discusses.

enter image description here

Neutrals must be pigtailed in a multi-wire branch circuit

That is so a device can be removed without breaking the other half of the circuit. It sounds like this was not done on your MWBC. You need to do it for all MWBCs.

However, in the case I describe above, the pairs are totally separate after the "split" at the first box. Because they are not sharing a neutral, you do not need to pigtail them.

  • What about using black/white/red wires, but having half of the double-outlet-assemblies be GFCIs with line terminals connected to black/white (load terminals unconnected) and the other half be GFCIs with line terminals connected to red/white (load terminals unconnected). That would require more GFCIs, but would avoid the need to string new wires. – supercat Nov 1 at 20:23
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    @supercat That would work; leaving the tape on the LOAD terminals is always a reliable play. But I'm prejudiced by my direct knowledge of how easy it is to add wires to conduit. – Harper - Reinstate Monica Nov 1 at 20:35
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I would suspect you are using the load terminals of one GFCI to feed another? which is not a problem unless you use just ONE terminal!

a GFCI device measures the current flowing through BOTH conductors ... if it is not balanced, it trips ... period.

so if you use the load terminals of one device ... YOU MUST USE THEM BOTH ON THE NEXT DEVICE.

** EDIT ** additionally ... what may not be apparent to a novice ... the neutral (white wire) is NOT common across the GFCI device. meaning white "in" (line) and white "out" (load), MUST remain isolated.

**** EDIT **** one more thing, since you are using a shared neutral with "separate" circuit breakers, a toggle/handle tie IS required by NEC. it allows a single circuit to "trip", but will require both to be powered off for service.

****** EDIT ****** yes! ... you need one GFCI device per branch (breaker).

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