running power to outside workshop

johnnywSeptember 19, 2011

I have a large barn with a 100 amp electrical service. I also have a work shop (shed) about 150 feet away. I would like to run power from the barn to the work shop to run power tools (bench grinder, small drill press, sawzall, small portable air compressor, etc). I would like to run the wire overhead (about 15 feet in the air, attached to steel aviation cable which is attached snugly from barn to workshop) VS underground. My thoughts are to run 10/2 romex out of a 30 amp breaker from the barn box into a small sub-panel installed in the workshop. However, I want a 30 amp breaker in the subpanel and I'm not sure if I should have two breakers on one line. Also, I'm not sure if romex can be used outside above ground. Any suggestions, as far as the type of wire I should use, the size wire, or any pros or cons with my plans would be greatly appreciated. I am obviously not an electrician, but I have some knowledge of electrical hook ups. Thank you in advance for any help.

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Romex draped over a makeshift messenger wire is far from the correct way to install aerial conductors. I suggest that you reconsider an underground installation, which is generally more "DIY friendly".

If you must have aerial here is a link with several suitable cable types.

Here is a link that might be useful: aerial cables

    Bookmark   September 19, 2011 at 10:09PM
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Ron Natalie

As wayne points out what you propose is far from correct for a number of reasons and in fact is QUITE dangerous. I would recommend you have an electrician install the subpanel at LEAST.

First, NM is not allowed to be used in this situation. It's not rated for wet locations, use outside the house/building, not permitted for messenger supported use, and most likely not rated for sunlight exposure. As Wayne points out you must use either a cable that's allowed for aerial use or really consider running either rated cable or conductors in conduit underground.
The messenger, if used also has to be appropriately supported and grounded.
You have to use an approved lashing or messenger ring system to attach the cable, you can't just wire tie or tape the cable to the messenger.

FYou also probably want to run 240V to your subpanel. That's going to take four conductors (two hots, the neutral, and the ground). If you install a subpanel in the outbuilding (i.e., there is more than one circuit there), you'll need a grounding system (rods) there. You don't technically need a main breaker out at the subpanel if you can kill all the power in less than six handle throws, but I'd recommend it.

You might consider having an electrician put the feeder and subpanel in. It's a lot easier to install your own branch circuits in the subpanel than running the feeder.

    Bookmark   September 20, 2011 at 4:23AM
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With the cost of running an overhead feed being cost prohibitive, I decided to go with the underground idea. How deep does the trench have to be (minimum), and is it acceptable to use 10/2 with ground NM-B orange jacketed romex inside conduit for the underground feed. I ask because I already have about 250' of that romex and if I can use it it would save the cost of buying new wire. Again, I have no need for 240 volts so I am planning on feeding 120 volts off a 30 amp breaker. Any comments or advice would be appreciated. Thank you

    Bookmark   September 25, 2011 at 3:41PM
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The burial depth is generally 18 inches for non-metallic conduit. This distance is measured from ground level to the top of the conduit. I say "generally" because your local jurisdiction may have other rules.

You CANNOT use NM cable inside buried conduit. You must use wire that is rated for wet locations.

Finally, if you put in a 30A circuit, then all of the components need to be rated for 30A. This means the use of a 5-30R receptacle. If you check that out, you'll notice that none of your equipment mentioned earlier will have the correct plug for that receptacle. So if you're not planning on installing a subpanel, you really need to limit your circuit to 20A.

    Bookmark   September 26, 2011 at 9:11AM
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Thank you for the information. I was planning on running the 30amp feed into a subpanel with a few separate breakers. So, if I understand you correctly, I cannot use the NM-B cable outdoors in conduit, either underground or above ground, even in sealed non-metallic conduit? I was under the immpression that if the wire was contained in sealed conduit preventing exposure to the elements, that would be acceptable. Why would that be considered a wet location. When I did my research, the gray UF-B wire says it can be buried without conduit, so which wire can be buried IN conduit?? It's very confusing. And, as far as the subpanel, would I come out of the main panel with a 30 amp breaker and then feed into the subpanel with additional 15/20 amp breakers?, or is that not the way it is done. Please advise. thank you

    Bookmark   September 26, 2011 at 9:46AM
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You are correct, the inside of a "sealed" conduit can fill up with water, even if from nothing other than condensation. Yes, UF is designed for direct burial (at least 24" deep). Inside conduit, you typically install individual wires rated THWN. Cable assemblies are not permitted inside conduit, except for situations where you need some physical protection. Besides, you really wouldn't want to try to pull 10/2 NM cable through a 150' conduit.

You can supply your subpanel with a 30A feeder from the main, although realize that most subpanels are 240V, and require a 4 wire feed from the main panel. Note that you'll also need to install 2 ground rods for your subpanel. Then you can put several 20A branch circuits in the subpanel to supply your workshop.

Personally, I would supply at least 60A to the subpanel. Yes, the wire will cost more than what you need for 30A, but you don't want to have to rewire everything when you buy that new dust collector.

Finally, I'd recommend a bit more research on this topic before beginning your installation. There are lots of other rules that need to be followed that haven't been addressed here. This is not a good "my first electrical project" candidate.

    Bookmark   September 26, 2011 at 10:14AM
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