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connecting electrical to new outbuilding

Trevor Lambert | Posted in General Questions on

I’m figuring out how best to connect electrical to a new, multi-purpose outbuilding. Currently our electrical service entrance is at a drive shed about 300′ from the house. It’s 200A service, with 250MCM direct bury cable at a depth of 3′ from the drive shed to the house. The new building will be between the house and the drive shed, about 20-30′ from where the underground cable passes.

I’m toying with two ideas.

1. Split the cable about where it passes the new building, and splice in another cable leading to the new building. Then run another length of cable from a splitter box in the new building and splice it to the other end leading to the house. Downside of this is I think it might require the splice to be accessible, which I wouldn’t be too thrilled about.

2. Same as above, except cut the cable at a point far enough past the new building that it can be pulled up and reach the new building without a splice, then connect a new length of cable from the splitter box to the house. This would be significantly more work because of the landscape around the house; the wire goes under a compacted gravel parking pad and landscaping gravel around the house. It’s probably 4′ deep right at the house. It’s a slab on grade and the cable enters from underneath the slab, not the more commonly seen conduit running up the side of the wall.

3. Another option, which I’ve almost completely discounted, is to run new service from another power pole to the house, then run a cable from the house to the new building. This would be a massive amount of money due to the involvement of the power company and the length of cable they would be running. It would also probably result in moving the panel in the house to accommodate a different entrance location, another expensive and logistical nightmare.

Am I missing any other options? I’ve ruled out running another cable from the house to the new building because it has all the downsides of option 2, plus having to locate a splitter box inside the house and reroute some interior wiring for it. I’m also expecting the new building to use at least as much electricity as the house in the future (e-vehicle charging, big woodworking equipment, etc.), so it’s not great from a voltage drop perspective).

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Option #2 would be more elegant, since it uses fewer splices, and splices are always considered to be weak spots, but option #1 is entirely workable too. You need the correct splices, installed the correct way, and the correct insulating methods. The good news for you is that underground splices like this made the correct way do not need to be accessible, you can bury them completely.

    The "correct splices" would be compression butt splices suitable for use on underground cable of the conductor type you have (copper or aluminum). The aluminum type will have a grayish antioxidant paste pre-applied inside. You need to get the correct butt splices for the 250MCM wire, which you'll probably need to order. Some manufacturers of these connectors are Panduit and Burndy. DO NOT use mechanical connectors that use set screws here.

    "Made the correct way" means installing the splices with the correct crimp tool. This tool will probably be a Burndy Hypress Y750-2 or similar tool, along with the correct U die for the barrel splice you'll be using (they are color coded, and also say what die "index" they need). You can buy these tools used for several hundred dollars, plus another $50-100 for a U die. You might be able to rent one, or just hire a commercial electrical contractor who will probably own one. Proper installation of the connector requires a "hit" (crimp) between EVERY set of colored bands on the connector. This usually means 2-3 crimps per lug, or 4-6 crimps for a butt splice. Some splices need more than that.

    The "correct insulating method" can mean tape, but I would't do it that way since it will require multiple layers of several types of tape. It's much easier, and far more reliable, to use the 3M "Cold Shrink" products. Look at page 318 of their catalog here: https://multimedia.3m.com/mws/media/511022O/emd-catalog-splices-product-pages.pdf
    To find the correct cold shrink insulating boot for the size connection you're making. Be sure to clean the existing cable before installing the cold shrink kit -- the cable needs to be completely clean to ensure a good seal. Usually water and some paper towels is enough since dirt doesn't usually stick to the XHHW jacket very well.

    If you're going with direct bury cable, you probably want to use type XHHW. This stuff is durable, and is what the utilities commonly use. It's readily available from commerical supply houses, but you probably won't find it in the box stores -- at least not in the size you need. You must use the same gauge as your existing cable, and I'd match the same type of conductor (copper or aluminum) too. I would not attempt to switch conductor types. When splicing these cables, be sure to lay them straight in the trench after splicing, don't let them curl up or form loops. Tight bends in the area of the splices tends to make weak spots that can cause the insulation to fail over time. You want the wire run to lay straight, with the splices in a straight line with the wire. Basically if the final installation looks straight and clean, you've done a good job and you're ready to cover the trench.

    You're probably going to need a seperate ground rod at the new building due to the distance. it is from the other structures That's not a big deal, and is actually a good thing for you since it means you won't need a seperate ground bond wire between buildings. At this point, all you need to is make the electrical connections, which means tying the "in" and "out" sides of the existing cable together in the new building, tapping off them, and putting in a new panel with a new main breaker to serve that building. I personally would specify a proper tap box for this, especially if you're using aluminum cable, along with compression lugs. This will give you secure, bolted connections that won't ever loosen or fail. You can also use mechanical 3 port insulated taps such as the Burndy Unitaps, just get one for each conductor (you'll probably need three), and make sure they are rated for the size of conductor you're using. You can house these in a large pull box. You typically need to keep your cable run from the main line to the main breaker in the building under 6 feet or so just like you do between an electric meter and a panel. If you go farther than this, you need a main breaker in it's own box near the main run, with a feeder cable from that main breaker to a remote main lug type breaker panel.

    This is not typical wiring for residential work, but it's pretty common commerically, so if you want to contract it out, any commerical electrical contractor should be pretty familiar with what needs to be done.

    Bill

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