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Old 05-11-2013, 09:56 AM   #1
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MWBC to outbuilding?


I know this is everyone's favorite topic... I am installing a storage building and would like to have a couple of 20A circuits for lighting and battery maintainers for lawn equipment. Originally, I was going run a single circuit, to avoid the expense of installing a subpanel and grounding rods at the shed, but I've heard that an MWBC can be used to supply an outbuilding without using a subpanel and grounding system. Is this true?

Here's an overview of the set up:
  • 10'x14' building at the back of the yard. Will be used for general storage and lawn equipment.
  • Wire run from subpanel in garage to the shed location is about 90'. Run will be in buried conduit.
  • I want to install lighting in the shed, as well as some convenience receptacles in and outside the shed, and an outdoor floodlight for the backyard.
I think a single 20A circuit would handle my needs, but a couple of circuits sounds appealing if the additional cost is not prohibitive.

I understand that the buried wire should be GFCI protected. My initial plan was to use a blank-face GFCI in the garage to accomplish this, but I don't think this would work on a MWBC.

Finally, is 12 AWG sufficient for a 90' run, or do I need to move up to 10 AWG, to account for voltage drop?

Is this doable, or am I headed down the wrong path here?

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Old 05-11-2013, 10:00 AM   #2
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MWBC to outbuilding?


If the wire is buried in conduit, and at a proper depth, you do not need GFCI protection on the feeder.

However, you will need GFCI protection on any convenience outlets located in the shed.

Personally, I would not worry about voltage drop on such a short run.

Also, be advised that you will need a "disconnecting means" located in or at the shed. This can be accomplished by using something as simple as a double-pole wall switch, rated at 20 Amps or more.

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Old 05-11-2013, 10:41 AM   #3
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MWBC to outbuilding?


At 90', you are pushing the envelope for considering voltage drop. I think you are just fine, but you are getting close. I would run nothing less then #10 wire even on the 20 amp circuit. This gives you the ability to install a small 30 amp load center down the road should you need it. I would also pull 4 wires.

Its amazing how quickly we can take a 20 amp circuit and realize we need more power. When I am trenching, back filling and buying conduit, I always plan for expansion.
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Old 05-11-2013, 11:32 AM   #4
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MWBC to outbuilding?


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Originally Posted by J. V. View Post
At 90', you are pushing the envelope for considering voltage drop. I think you are just fine, but you are getting close. I would run nothing less then #10 wire even on the 20 amp circuit. This gives you the ability to install a small 30 amp load center down the road should you need it. I would also pull 4 wires.

Its amazing how quickly we can take a 20 amp circuit and realize we need more power. When I am trenching, back filling and buying conduit, I always plan for expansion.
Let me ask this... is the MWBC permitted? If I pull 4 10 AWG wires to support a 20A MWBC, wouldn't that essentially give me the same capacity as a small load center? I'm not opposed to the load center idea, but if I can get the same capacity with an MWBC, it seems like a more cost-effective solution.

The purpose of the shed is to get all the crap out of my garage, which will be workspace (and is already wired with a few circuits and contains a subpanel, so expanding service into the work area is relatively easy).
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