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Old 11-10-2015, 10:55 AM   #1
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Cable size to bury for outbuilding subpanel?


Hello all. I am buying a rural property with a cabin on it and plan to convert a separate building about 40 feet away into another house (basically like converting a detached garage into an apartment). The main cable has a 200 amp main panel, and I want to run 240/120 volt, 100 amp service from the main panel to a subpanel (with a main breaker) in the detached building. I'll be wiring the detached building myself to turn it into a house, but I am debating whether to hire someone to install the subpanel or do it myself and pay an electrician to look over my work. It doesn't seem too complicated if I can figure out the details, but perhaps I'm being naive.

My question is... I want to bury the cable from the main house to the detached structure, so what size and type of cable do I need to get? (Even if I hire someone, I'll try to save money by bargain-hunting the cable.) Do I need a ground rod at the detached building, or would it ground back to the main panel?

Also, any idea what would be reasonable to pay someone to do this job?

Thanks!
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Old 11-10-2015, 03:16 PM   #2
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Use #1 copper or 1/0 aluminum. XHHW is a good choice for wire type, but there are several other options too. You need four conductors: two hots, neutral, and ground. Run 1-1/2" or 2" PVC conduit buried at least 18" deep. Even though you must run a separate grounding conductor to the outbuilding and use separated neutral and ground buses in the subpanel, you still need to use two ground rods at the outbuilding (or otherwise comply with the NEC's grounding electrode requirements), bond the plumbing, and do all the other tings required when wiring a house.
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Old 11-10-2015, 03:24 PM   #3
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I will try to get the ball rolling...

type XHHW wire. You will probably need 1/0 for the hot wires (x2) and a #2 for the neutral (both aluminum) You will also need an insulated (green) ground wire. Use #4 aluminum or #6 copper for the ground. You will need to have 2 ground rods 8 feet deep and at least 6 feet apart.

Wires need to be in conduit.

Check all this with your local permit dept and certainly wait for a pro here. Im a novice but thought id chime in so you had something to start researching
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Old 11-10-2015, 03:33 PM   #4
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As far as cost. It all depends on the job, location. Figure several thouands of dollars though
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Old 11-10-2015, 03:36 PM   #5
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Lastly. And im just throwing it out there. If its zoned as a house, would a subpanel even be allowed? Would you need a seperate run and meter?
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Old 11-10-2015, 03:42 PM   #6
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Quote:
Originally Posted by Trucon01 View Post
I will try to get the ball rolling...
That matches perfectly with your avatar. lol!
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Old 11-10-2015, 04:04 PM   #7
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Are you kidding about several thousand dollars? I hope so.
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Old 11-10-2015, 04:12 PM   #8
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I talked to the inspector and a subpanel is allowed. It's not going to be a legally separate residence with a separate address and all. It will basically be an in-law apartment.

Oh, it's in rural North Carolina, so costs will probably be lower than in some places.
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Old 11-10-2015, 05:41 PM   #9
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No. Wasnt kidding about prices your prices will prob be lower, true. But you have the cost of materials, i hope a permit and time of an electrician. Digging a trench, assembling conduit, hammering in ground rods, etc. It adds up.
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Old 11-10-2015, 05:42 PM   #10
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Do you have 2 spaces available in your main panel?
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Old 11-10-2015, 05:57 PM   #11
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Yes, there are spaces available in the main panel. The old cabin is really rustic and only has a few circuits. I wonder if it would be cheaper to get the electric company to provide new service to the detached building.
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Old 11-10-2015, 06:11 PM   #12
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The old building is already wired, then. How was it getting power in the past? If you can get by with two 20-amp circuits, you can skip the subpanel and run a MWBC to the old building.
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Old 11-10-2015, 06:33 PM   #13
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It's complicated. So the deal is, there is an old 1920s cabin that has electrical service and the current main panel. In the 1980s a separate building about 30-40 feet away was constructed to serve as an office and storage. Three 15-amp circuits were buried and run from the cabin main panel to power the office lights and outlets. I now want to convert the office into a house/apartment. The old cabin is old and run-down and tiny, so for loan purposes I need to do the conversion of the detached building above-board and have the county (and banks) deem it living space. The newer, detached building is well built and when converted will hold far more value than the old cabin, which is why I am converting it. My parents will use the old cabin.

I am under the impression that to provide power for a detached dwelling like a garage in-law suite, it needs a separate subpanel. I could be wrong about that. Capacity-wise, we would be totally fine running a few more circuits from the existing panel at the cabin to the newer conversion building. I just need to do it so the county will consider my conversion living space and reflect that on the property record.

Now that I think about it, I have heard that people can get around this issue by building a breezeway between the two dwellings so that aren't technically separate. This is a rustic mountain place, so I'll do it however it cheapest while still safe.
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Old 11-10-2015, 06:36 PM   #14
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If you haven't figure it out, this is a family deal involving property in hillbilly heaven in the mountains. I will be the first person ever up there to mess with a building permit. Haha.
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Old 11-10-2015, 06:50 PM   #15
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To help you visualize, here is a Google Earth image of the property. The building at the bottom is the old cabin with the current electrical service and main panel. The bigger building at the top is the building I am converting. I think I need to figure out the best (cheapest) way to get a 100-amp subpanel in the top building. Or build a breezeway so they can be considered one building.
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