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I’m running a 20A multi-wire branch circuit (MWBC) to a shed. Can I use this with a double-pole GFCI breaker in the panel (or combined GFCI+AFCI)? Uses will be for power tools, battery chargers, interior lights, and exterior lights. Will I have problems with frequent tripping?

The code prevents running more than one circuit to an exterior structure (if not using a sub-panel), so MWBC is my only option.

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    How long is the run to the shed?
    – ThreePhaseEel
    Commented Oct 26 at 17:29
  • How old or new is the power tools you will be using?
    – Retired Master Electrician
    Commented Oct 26 at 18:36
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    I removed the concept of "single throw": that's a switch thing, not a circuit breaker thing.
    – Triplefault
    Commented Oct 27 at 1:13
  • Thanks for the replies. The run is about 50’. I also have to use a switch at the shed to be able to cutoff power, so planning to use a double-pole single-throw switch for that. Power tools are all newer.
    – John DiBaggio
    Commented Oct 27 at 5:50

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I'd arrange this differently if I were you

There are two issues with putting the GFCI at the upstream end of the feeder in your setup:

  1. A GFCI trip will kill the lighting, which can plunge you into the dark in a hazardous situation (say, due to a still-spinning-down saw blade).
  2. That extra 50' of run essentially sensitizes the GFCI because of capacitive leakage currents to ground along the run.

So, what I'd do instead is have a deadfront GFCI on the shed receptacle circuit, located inside the shed. This way, you can use a very cheap non-fused air conditioner disconnect box for the disconnecting means at the shed, and don't have to worry about the lights going out if the GFCI trips. (It does mean you have to bury your cable deeper if you're using a direct buried cable, but you're better off running a fat conduit and pulling individual wires through it anyway.)

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  • Thanks for all the explanations and suggestions. My conduit is buried the 18”, inspector already checked that (did it last spring before planting grass). And I’m planning to run THHN. So I know I don’t need the feeder to be GCFI-protected, but was thinking it would be nice peace of mind anyways. I’m probably not so worried about safety concerns if both lighting and outlet circuits go out at the same time—I won’t be using a table-saw or anything I foresee being super dangerous if I can’t see.
    – John DiBaggio
    Commented Oct 27 at 17:29
  • I’ll need to learn more about the disconnect box you mentioned with a GFCI receptacle. I was planning on just a double pole 20A switch inside the shed for the cutoff.
    – John DiBaggio
    Commented Oct 27 at 17:38
  • Could I do something like this for the MWBC: - Use a double-pole 20A basic switch inside the shed for the disconnect - On one side, put a GFCI receptacle with the load connected to indoor receptacles (so they’re protected) - On the other side, put a GFCI receptacle, with a pigtail on the line for indoor lights (so it’s not protected), and with the load connected to an outdoor light and outdoor receptacles (so they’re protected) Would that work? Would it avoid nuisance tripping on either/both circuits?
    – John DiBaggio
    Commented Oct 27 at 19:44
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    That would work as well @JohnDiBaggio, although the air conditioner disconnect box will be cheaper than any double-pole wall switch or safety switch you can find out there
    – ThreePhaseEel
    Commented Oct 27 at 19:46
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    It’ll be the shed powering it; otherwise, I need another disconnect for that. The inspector said they’d want to flip one switch at the shed and see anything powered shut off in/on the shed.
    – John DiBaggio
    Commented Oct 27 at 23:11
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This is perfectly fine and any GFCI tripping should be minimal. However, there is one catch: if you have a problem with one half of the circuit (e.g., a power tool gets wet and results in a GFCI trip) then the other half of the circuit (the lights over your head where you were running a saw that now stopped due to the GFCI trip but is still quite dangerous) will also go off.

One possible alternative that I have seen in some previous similar questions is to use a separate circuit for lighting by way of a 3-way switch, with one switch at the house and the other switch at the shed. That actually makes sense if some outdoor lights are needed and if you put at least one light inside the shed on that same switched circuit then you solve that safety issue, as that light will stay on even if the receptacle circuit goes out due to a GFCI trip.

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