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I am in need of some help choosing the correct wire for my detached garage.

My plan is to create a run from my main panel in my basement to the detached garage.

The total length of the run is about 80 feet.

I would like to have 2 - 15 amp circuits and 2 - 20 amp circuits in the garage. My thought is to use a 70 amp breaker in my box in the basement and 70 amp main in the detached garage. If this is wrong please let me know.

My preference would be to use UF-B direct burial wire if possible.

Links to specific wire and/or breakers/sub-panels would be greatly appreciated.

  • Why use direct bury wire instead of THWN in (Sch 80 PVC) conduit? – ThreePhaseEel Dec 28 '17 at 23:14
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    Way cheaper to run 240 v (double pole breaker in the main) to a sub panel in the garage. This will cut the amperage and the voltage drop allowing for smaller wire running a 70 amp 120 service would still require a panel so it would be best to go with 2 legs. Neutral and ground. 40 amps would meet your needs using #8 copper and the voltage drop would only be 1.85%. Note there would be a total of 80 amps of 120 available 40 on each leg. – Ed Beal Dec 29 '17 at 0:40
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Conduit is your friend

Conduit is cheap, trenching is expensive, and putting fat conduit in now can save you more trenching later if your wires prove too small, as well as shielding the wire from mild cases of excavation damage which'd otherwise force you to dig up the cable and replace it. So, I'd put fat (2"+) schedule 80 PVC in the ground now, using prefab wide radius sweeps for the horizontal to vertical transitions.

Fat aluminum is also your friend

All the bad things that you may have heard about aluminum building-wire do not apply to what you are doing right now. The terminations you're working with (pressure setscrew lugs) are much better at handling aluminum wire than the wrap-type terminal screws or backstabs on receptacles and switches, the aluminum wire that you buy now is made from a different alloy with better properties for building wiring than the aluminum wire that gave aluminum its bad rap, and the busbars and lugs themselves on your breakers and panels are likely made from plated aluminum, too.

So, that said, I'd use 2AWG, wet-location-rated (THWN or XHHW-2) aluminum wire for both hots and the neutral with an 8AWG or 6AWG (8AWG works, 6AWG might be more useful for grounding electrode conductors though) bare copper ground wire. That will give you 70A at the garage without coming close to filling a 2" conduit.

Big (slots-wise) subpanels are your friend, too

Furthermore, you'll need a subpanel at the garage for this, and again, it's penny-wise and pound-foolish to skimp now due to the labor costs of redoing things down the road. I'd at a bare minimum put a small 20 or 24 slot, 100A panel in as the garage subpanel -- if you can afford something with more slots in it, do so, as often you can get kits with the panel, a main breaker, and some 15 and 20A branch breakers. Don't worry about the ampacity of the main breaker in the subpanel, by the way, as it's only serving as a disconnect -- the 70A feeder breaker in the main panel provides the overcurrent protection.

Last but not least

You'll want to use an inch-pound torque wrench or torque screwdriver (depending on the specified torque) to torque all the breaker and panel lugs to specification when doing this -- the 2017 NEC actually requires the use of calibrated torque tools in 110.14(D), and it's a good idea in any case to make a reliable connection, especially on aluminum wires.

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Running wire for exterior buildings is always a problem especially when no one has an idea of what you intend to do. So let's say you want to connect to the four receptacles you mentioned above at full load. so adding them up you want to run 70A worth of power, but generally when you run 70A feeders you usually run two phases (four wires, two hot one neutral and one ground). That cut the connected load in two to 35A per phase. If that is all you are powering up, at a 3% drop you could run a #8 about 150'. But a #8 would mean you would have to install a 40A breaker for overcurrent protection. If you still want to run a 70A feeder off of a 70A breaker you would have to use #4 copper conductors or an aluminum equivalent, that would be a #2.

Personally I always like to run to a subpanel for this type of construction.

Hope this helps, good luck.

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Check out this post from a few years ago.

Rewiring Garage, building grounds

I ran overhead SEC cable to a sub panel. I had a professional run the SEC cable because I didn't want anyone asking questions about permits if it looked weird. From there it's pretty straight forward. Note that regardless of how you do it, if you cannot pigtail your grounds to your neutral. You must either run a dedicated ground (SER cable) or give your garage a dedicated grounding rod and a separate bus in the sub panel for the grounds. I did the latter.

  • You need to run both a dedicated ground and a ground rod, AND have the separate bus – ThreePhaseEel Dec 29 '17 at 2:42
  • That is not true. I'm rereading my original post and trying to determine if you misunderstood something or your really have no idea.If what you were suggesting was correct then your main panel would require a ground running back to the distribution. The reason for splitting busses is to prevent power over ground. It is completely acceptable to install a dedicated grounding rod at the site of the sub panel. – mreff555 Dec 31 '17 at 14:00
  • First off, services are special as the neutral-ground bond is located at the service entrance, so your statement about services is wrong. Second, using a dedicated grounding rod and bus alone without the ground wire to the main panel is a good way to get zapped as earth is too high-resistance to allow breakers to trip in case of a hard ground fault. – ThreePhaseEel Dec 31 '17 at 15:04
  • You are correct about the first part. Article 230 of the NEC describes 5 exceptions where a single service drop can be used for multiple service entrances. Exception 3 is a detached garage. As for your second statement, are you implying that the resistance of the soil under my garage has a lower conductivity than the soil under my house? I fail to see how a 100 foot run to the main panel before it goes into the same earth will improve conductivity. – mreff555 Dec 31 '17 at 17:44
  • If your garage is a separate drop, then it's a different story as it would have its own N-G bond in that case. If that's not the case though (which it probably isn't, since you said "subpanel"), then your problem is that you're thinking that electricity wants to go to ground. Hint: it doesn't -- it's trying to get back to where it came from instead (i.e. the utility's pole-pig), and the path through a ground wire, the service N-G bond, and the utility neutral wire is vastly lower resistance than going through a ground electrode and the earth to the utility's ground electrode. – ThreePhaseEel Dec 31 '17 at 18:05

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