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I'm running electricity to my shed by extending an outdoor GFCI receptacle. I just finished my rough-in inspection and got approval to cover. The inspector had a few comments, but spoke fast, and since I'm a beginner, I didn't fully understand what he was saying until after he left. He mentioned that I had two circuits, but that it was fine because "multi branch circuits are allowed, I would just need to share a common neutral and use a two pole breaker and a two pole switch". All of that is over my head, and more complex than I intended. I didn't realize until after he left, that I had accidentally run my wires differently than I had originally planned which resulted in two circuits instead of one.

Here are a few photos to visualize the situation. I have a conduit feeding the shed with 12/2 THHN and two NM-B wires coming out from inside the shed.

box with wires

Inside the shed, one NM-B wire goes to two receptacles, and the other wire goes up through the top plate to some ceiling lights and a switch.

two receptacles inside shed

I only intended for one circuit and I think I only need one circuit. Instead of running both NM-B wires out to the box, can I simply join those two wires, and run an extra wire to the first receptacle to make it a single circuit? Or am I missing something? Like this:

one extra wire

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  • So the inspector saw that you had 2 separate pieces of nmb or romex going to the shed? If he did and then told you to tie the neutrals and use a handle tied or double pole breaker to have the 2 circuits I would go with that. This would normally not be code compliant being 2 separate cables. The inspector Is the “authority having jurisdiction”. After the cover inspection this will be an advantage as you really still have 2 separate circuits just the breakers are handletied. Do what the inspector told you everything will work correctly you just have a little extra copper feeding your shed.
    – Ed Beal
    Jun 25, 2021 at 21:41
  • @EdBeal To be honest, I still don't fully understand what that means or how to wire it up correctly. Yes, he saw the two NM-B wires, and mentioned I needed a double pole switch so I could shut off both from the shed. He didn't mention anything about adding another wire, and I don't think he realized I was planning to extend from the outdoor GFCI. Am I wrong in thinking the goal is to use the wire in the conduit as two hots and no neutral in order to create two circuits using one 12/2 wire? If so, I'm confused why he said to share a common neutral and how that would be wired up...
    – Andrew
    Jun 25, 2021 at 21:57
  • He also mentioned "they like to have two circuits, one for receptacles, and another for lights...people often put in a 500 watt heater." but I didn't catch whether he was saying that in support of the multi branch circuit, or trying to say it was okay even though I wasn't planning on having two circuits.
    – Andrew
    Jun 25, 2021 at 22:05
  • A multiwire branch circuits is 2 hots, 1 neutral , 1 ground. He was in favor of them because small “milk house” heaters are 1200-1500 watts. Putting this on a separate circuit from the lights is a good thing because a single circuit is more likely to trip. (Normally the wires are required to be all in the same sheath) since he suggested this you could have 2 circuits with the cables you have pulled. Normally a 12 or 14 awg cable with 4 wires is required. So I would go with what he was going to allow a double pole to kill both hots, then neutrals tied together. He approved cover use 2 circuit
    – Ed Beal
    Jun 25, 2021 at 22:23
  • If you don’t feel comfortable with the MWBC just run 1 cable to the shed. Or don’t enter the shed if the second one is already there.
    – Ed Beal
    Jun 25, 2021 at 22:25

1 Answer 1

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You lack adequate wires for a MWBC (multi-wire-branch circuit) but you could easily add a wire, since you are in conduit.

If you DON'T want to have a MWBC, you have all the wires you need, you just need the appropriate disconnecting means (a 20A switch) between your feed and the (single) circuit in the shed. For one more wire and the breaker/switch upgrade, you get twice the power available, but that's your call. At least you used conduit, so you can upgrade later if you find a need.

I'd personally use a subpanel for the switch function in the shed, even if only doing 120V for now (which means every other breaker space won't be hot, for a typical panel) as it would be one less thing to change out later if/when upgrading. But that's opinion and long range budgeting, not "a need" - it might save you money in the long run, it might not, if you never upgrade.

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  • You said: "You could easily add a wire", but that sounds hard. It's 1/2" conduit with two 90° and two 45°. I already pulled three wires through, using fish tape, and it was kind of hard to pull. I'm not sure how easily I could send the fish tape back through to pull a single wire, or if I would need to pull it all out and do it all again with all 4 wires.
    – Andrew
    Jun 25, 2021 at 23:18
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    @Andrew Pull it all out and pull 4 wires.
    – JACK
    Jun 25, 2021 at 23:23
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    Pull it out, pull in 4 wires (the same 3 plus one more non-white, non-green, non-gray - can be a second black if you have left-over black, but if buying wire for it save your sanity by using a different hot color) and a pull string. Buy a quart of lube if you found it hard to pull the first time. Use stranded wire the next time, it pulls way easier around corners than solid does. you'll get back the extra couple of cents (more than solid) in time & frustration saved pulling it. Fish tape is also a bit stiff for pulling through conduit easily - rope is much nicer, and can be vacuumed in.
    – Ecnerwal
    Jun 26, 2021 at 0:36
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    ...or pulled in when you pull out the wire, in this case.
    – Ecnerwal
    Jun 26, 2021 at 0:51

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