Detached Garage service from house

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Old 09-07-07, 02:22 PM
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Detached Garage service from house

Hello,

I have a detached garage that is about 20 feet from my house and I would like to run electric service to it from my house panel.

I have 200 Amp service in the house and would like to install a 100 Amp panel in the garage. Can I use a 100 amp 2-pole breaker in the house to run power to my garage? What size wire should I use? Should I use direct burial or conduit? Is a ground rod required? If so, how deep should it go?

The largest current draw that I have in the garage currently is a 5hp air compressor, so 100 amp service should be plenty. The rest is fluourescent lighting and a few electrical outlets. No other large appliances are currently planned.

Any information would be greatly appreciated.

Thanks for your time.
Rick
 
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  #2  
Old 09-07-07, 02:39 PM
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> Can I use a 100 amp 2-pole breaker in the house to run power to
> my garage?

Yes.

> What size wire should I use?

It depends on whether your local code allows NEC table 310.15(B)(6) for subpanels. If they do allow this table, you need #2 aluminum or #4 copper for the hots and neutral. If they do not allow this table, you need #1 aluminum or #3 copper for the hots and neutral. The ground can be #8 copper or #6 aluminum.

> Should I use direct burial or conduit?

It's really up to you. PVC conduit with THWN conductors is more work, but only needs 18" burial depth whereas direct burial cable (USE, SE-U, URD) is easier to install, but requires 24" of cover. If you have rocky soil, I would lean toward conduit for added protection. I often recommend aluminum mobile home feeder cable; it's fairly cheap, and it's rated for direct burial.

> Is a ground rod required? If so, how deep should it go?

A 5/8" x 8' copper-clad ground rod is required driven flush to the earth. The rod should connect to the subpanel ground bus with an acorn clamp and bare #6 copper.

> a few electrical outlets.

Remember, these need to be GFCI protected.
 
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Old 09-07-07, 04:23 PM
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Thanks for the quick response Ben.

The soil is sandy, no rocks at all. I think I'll go with direct burial cable and rather than worry about whether or not local code allows the smaller guage wire, I'll just go with the #1 Aluminum or #3 Copper (whichever I can find the cheapest!).

Thanks again!
 
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Old 09-11-07, 06:18 AM
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OK...so, I have a 100 amp sub-panel in the garage and a 100 amp double pole breaker for the house. I have driven a ground rod and connected it to the ground bar in the sub panel.

Since I have a ground rod, do I only need three wires from the house to the sub panel? I have done a lot of reading and the term "bonding" is something I'm unsure about. My understanding is that the only place that neutral and ground should be bonded is in my main panel and that I need to keep neutral and ground separate at the sub panel. If this is the case, then should I run three wires - two hots and one neutral from the main to the sub? I'm thinking of using 2/2/2 aluminum cable, marked black on the hots and black with a yellow line on the neutral.

Does this all sound about right?

Thanks,
Rick
 
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Old 09-11-07, 06:29 AM
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You should run four wires to the garage, in my opinion.

If you have any other metallic paths between the buildings (water line, telephone line, coax line, etc.) then the fourth wire (ground) is required.

If you do not have any other metallic paths then you are allowed to run three wires (no ground). However, you will not be able to add any metallic paths in the future until the ground is added. If you don;t use conduit then adding the fourth wire at a later date is much more work.

If you run four wires to the garage, then the ground and the neutral remain separated in the garage panel.

If you run three wires to the garage (no ground) then
the ground and the neutral must be bonded together in the garage panel.

Rick, please consider buying and reading several books on home wiring. The questions you are asking are basic questions. While it is good you are asking them, the books would have explained them. My concern is that you aren't asking numerous other questions that are not as basic, that the books will also explain.

Electricity can and does kill people. Please respect it and make sure that you do this job (any any electric job) properly.
 
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Old 09-11-07, 06:47 AM
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I fully understand your concerns. I know the basics, thus no questions on the basics. I have been reading a lot and I appreciate the inherent dangers of working with electricity. Being within the guidelines set forth by the NEC is my main concern here.

I'll be running four wires, as you suggested. I think I'll have to buy a couple of lugs for the ground bar in each panel so I can connect the ground wire.

You answered my questions perfectly. Thank you.
 
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Old 09-11-07, 11:59 AM
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You must install a grounding rod at a detached structure whether you run three wires or four. These are two different, mostly-unrelated types of grounding. The grounding rod provides protection from external events (e.g., lightning, downed power lines). The grounding wire between structures provides protection from internal events (e.g., shorts). It is essential for anyone working with electricity to understand the difference between the two grounding systems, the grounding electrode conductor and the equipment grounding conductor. All ground wires do not serve the same purpose.
 
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Old 09-11-07, 05:16 PM
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"I know the basics, thus no questions on the basics. "

This statement does not coincide with the original question.
 
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Old 09-11-07, 05:43 PM
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ok, to rephrase then... I know electricity principles and safety, but not NEC codes and requirements.

I really do appreciate the input that has been provided thus far. You guys are awesome
 
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