Outdoor Shed with 220 Vac and GFCI


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Old 05-21-15, 12:27 PM
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Outdoor Shed with 220 Vac and GFCI

I need to run 220Vac to a garden shed about 50' from the house. The shed has a dirt floor. There will be a two horse power air compressor in the shed as well as one 120 Vac outlet and an overhead light.
My idea is to run four #10 AWG THHN in a 1" conduit 18" underground. Red, Black, White and Green. The plug for the compressor needs red, black and green.
Here is my question:
I plan to use a 220Vac GFCI rated at 20 amps in the panel. This should be fine for the air compressor. Can I then use one of the hot feeds to the white wire neutral for the 120 Vac outlet and the overhead light load?
 
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Old 05-21-15, 12:31 PM
J
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Why would want a GFI on a compressor?
If it was mine I'd just protect any outlets.
 
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Old 05-21-15, 12:40 PM
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Yes you can do it -- this is a variation on the multiwire branch circuit. The only component you need that isn't mentioned is a "building disconnect" at the garden shed. A simple double-pole switch or an air-conditioner style pull-out disconnect box will suffice. It should be located on the exterior, or at the nearest point of entrance to the building.

I would recommend that you use a regular 20A double pole breaker, no GFCI protection on the 6-15 compressor receptacle and a 15A GFCI receptacle for the general-purpose outlet.
 
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Old 05-21-15, 02:35 PM
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I already have the 20 amp GFCI 220 Vac breaker. I was hoping to be able to finally use it for a project.
Assuming I use it, will taking a 120 Vac load off of the feed, as I had described above, cause the breaker to trip? I thought a GFCI sensed stray current flowing from a hot lead to ground. In a 120 Vac GFCI, all the current that flows in the hot lead is expcted to be flowing in the neutral return lead. With the 220 Vac GFCI I would think that it senses all the current that flows in one hot lead is expected to come back in the other hot lead. If I take a load from a hot to the neutral, would it trip the breaker? I have never used a 220 Vac, so I am a bit confused.
 
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Old 05-22-15, 05:09 AM
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Hot to neutral loads will not trip the 240 volt ground fault interrupter breaker.

The 240 volt GFCI breaker will provide protection for any 120 volt (hot-neutral) or 240 volt (hot-hot) load on that (multiwire) branch circuit. It monitors the currents on all three lines of the MWBC -- two hots and the neutral.

Be sure that the neutral for that MWBC is connected to the GFCI terminal provided for that purpose and not directly to the panel neutral bus. The GFCI breaker comes with its own white lead to connect to the panel neutral bus.

I would have gone with #8 wires underground to the shed instead of #10, to allow up to 40 amps @ 240 volts in the future. (The green wire can still be #10.) For starters this can still be hooked up to the 20 amp GFCI breaker and no additional parts not already mentioned except possibly some lugs or small connectors are needed. In the distant future, a subpanel would be needed out at the shed when more than 20 amps @ 240 volts (two allotments of 20 A @ 120 V) are needed and the 20 amp GFCI breaker could be moved out there.
 

Last edited by AllanJ; 05-22-15 at 05:33 AM.
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Old 05-22-15, 05:55 AM
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Thank you for taking the time to explain the 220 Vac GFCI operation. All of your ideas have great insight into the future. Since I have not procurred the wire, I will do the larger conductors, just in case.
Thanks again.
 
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Old 05-22-15, 07:11 AM
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Is the 20 amp breaker larger enough to allow the compressor to start?
 
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Old 05-22-15, 08:26 AM
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Yes. The compressor is currently in the garage on a 20 amp circuit. Never had a problem.
 
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Old 05-22-15, 08:31 AM
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It wouldn't be a bad idea to try it on the GFCI breaker before you get the system all set up. While it "should" work, industrial types of motors and motor starters like compressors aren't always fully compatible with GFCIs.
 
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Old 05-22-15, 09:14 AM
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Good idea. It is easy to change out the standard 20 and try the GFCI.
 
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Old 05-22-15, 10:19 AM
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240 volt circuits do not require gfi protection.
 
 

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