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I am planning to run single phase 220 volt power out to my shed. I'm think 70 amps out there to cover what I need: a few lights, mini fridge, compressor, welder, and a few power tools. (it's only around 200 square feet)

From the main panel to where it exits the house is ~55 feet, and I was thinking of putting a disconnect on the house. From there, it's another 50 feet or so underground to the shed (I'm thinking 1 inch RMC?).

What type of wire should I use for this? I'm thinking maybe #3 or #4 NM from the main panel to the disconnect, and then THHN to the sub panel. What size ground should I use? Do I need a grounding grid?

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  • THHN can't be buried as it's a damp location. Thankfully most modern THHN is actually THWN rated as well.
    – KMJ
    Commented Dec 12, 2023 at 23:42
  • What does the pathway to the shed look like? If it's easy digging and not under a driveway I'm going to end up writing an answer with mobile home feeder cable, because it's usually the easiest way to wire an outbuilding this size.
    – KMJ
    Commented Dec 12, 2023 at 23:43
  • it's a straight shot, the worst parts is that where i'm at, it's all clay Commented Dec 12, 2023 at 23:46
  • Will need two grounding rods at the shed. Have you done the load calculation yet? To see how much amps you can spare. For the distance and size, you will want aluminum instead of copper.
    – crip659
    Commented Dec 13, 2023 at 0:04
  • Are you in Europe? If not, you probably meant 240 volt power. Please revise your post to clarify both points.
    – isherwood
    Commented Dec 13, 2023 at 13:56

2 Answers 2

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This is one of those scenarios where I'm going to recommend MHF mobile home feeder cable. It's aluminum, relatively easy to work with, and rated for direct burial. 2-2-4-6 is used commonly enough that my local electrical distributor stocks it at less per foot that a single strand of #2 or #3 THWN. Between the four strands and conduit, the supplies will be five times as expensive for your proposed solution as for burying MHF.

'But it's aluminum!' you might say. Yes it is. So is the feeder to your house and the lugs in your panels. Aluminum is an awful choice for wiring to individual devices. It's a great choice for wiring big consumers and service entrances.

You can't run MHF indoors unless it's in conduit, so plan for that in the house run, or doing a transition between cable types on the outside of the house like your proposed disconnect. Personally I'd skip the disconnect and just run conduit through the house, but I'm not you.

Your ground can be #6. Unless you have specific huge 120V loads, you can also size your neutral down to #4. Bonus: 2-2-4-6 is rated for 90A so if your load calculation supports it you can put an 80A or 90A breaker on instead of a 70A.

Other notes: get the physically biggest subpanel you're reasonably okay fitting in or on the outside of the shed, keeping access requirements in mind. The cost difference is trivial. Thanks to main panels being a commonly used item, a main panel bundled with some breakers might actually be cheaper than a subpanel. It's far less of a headache to have a dozen breaker spaces you don't need than to find yourself struggling ten years down the road because you put in two more lighting circuits and you're totally out of breaker space.

You don't need an Ufer ground. If the shed isn't yet built, go for it, put one in. Otherwise a pair of ground rods should be plenty. If you want to put one in, of course, go for it.

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Step 1 - Load Calculations

You need to do an official NEC Load Calculation. Actually, you need two Load Calculations.

  • Main Panel/Utility Feed

This is to figure out how much is available. For example, if your utility feed to the main panel is 200A (very common) then depending on the size of your house, type of appliances (gas or electric for hot water, cooking, etc., type of HVAC, etc.) and other factors you might have a load calculation of anywhere from 60A on up to 200A. Really. So you need to find out how much is available before doing anything else.

  • Shed

70A is a definite possibility for how much power you need. But you actually may need a lot less. And it is possible you may need more - e.g., if you want to put in anything but the tiniest tankless electric water heater (TL;DR Don't do that).

If your shed needs 70A and your main feed has at least 70A available, great. If your shed needs 70A and your main feed has only 60A available then you have a problem. There are a lot of ways to adjust loading in both the house and the shed, but we need a lot more details to help with that.

Step 2 - Subpanel Feed Size

Once you know how much power you want to, and are able to, send to the shed, you determine the feeder size. This can be anything equal to or larger than the shed load calculation. The breaker needs to be equal to or smaller than the wire capacity. Take a look at an ampacity table. Ignore copper, because at this distance and this size (really anything more than 30A) aluminum saves a lot of money. Only look at the 75 C column because 90 C is not likely to be possible.

  • 8 AWG = 40A
  • 6 AWG = 50A
  • 4 AWG = 65A - but since there are no 65A breakers, you can put in a 70A breaker with 4 AWG wire, provided your load calculation is 65A or less
  • 3 AWG = 75A - but since there are no 75A breakers, you can put in an 80A breaker with 3 AWG wire, provided your load calculation is 75A or less
  • 2 AWG = 90A - this is a very common wire size, so it may be more cost-effective than some of the smaller sizes
  • 1 AWG = 100A

For example, if you end up with a load calculation of 70A, you can use 3 AWG, 2 AWG or 1 AWG (or larger) wire with a 70A breaker. Larger wire, if it doesn't cost much more than smaller wire, has the advantage of lower voltage drop (but not much of an issue at 110 feet) and the possibility of future expansion without replacing the wires.

Note that if you use NM cable for the inside portion, the size may be different - but all wire/cable sizes need to be large enough for the feed breaker's full capacity. In addition, splicing wires together at large sizes is more expensive (no cheap wire nuts) than at smaller sizes. So conduit and wire the whole distance might make sense.

Disconnect

As I understand it, you need a disconnect at the outbuilding. So even if you install a disconnect at the house, you still need a disconnect at the shed. Fortunately, if you use a "main panel" in the shed (e.g., a 100A or 200A main panel) then you are all set, because the "main breaker" of a panel functions as a disconnect. It does not matter at all that this breaker is much larger than the feed - that's OK because it is only functioning as a manual disconnect. Just make sure ground and neutral are kept separate because this is being used as a subpanel and not as a main panel - at most that will require buying a separate ground bar if it is not included. A "main panel bundle" often makes a lot of sense as the included "extra" breakers will often more than make up for the difference in price between a small subpanel and a large main panel.

Ground

You need to have ground rods (usually 2, unless you have an "ufer ground" in concrete) at the outbuilding. That is in addition to a ground wire going back to the main panel.

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