New subpanel in detached garage

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  #1  
Old 12-30-07, 10:28 PM
L
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New subpanel in detached garage

New subpanel in detached garage

I am installing a new 70A panel in a detached garage connected to my 250A main panel. The garage is located about 30 feet away from my house.
My needs are minimal in the garage: 3 outlets, 2 light fixtures and a door opener.
When my house was built a few years ago, the electrician planned for such a future extension and burried a 10/2 UF-B Cable under the house (spec in http://www.colemancable.com/CatalogP...roductsID=229). It is connected to a 30A breaker in the main panel and ends on the outside wall of my house.

Here is my plan:
1/ Get a junction box on the outside wall of my house.
2/ Connect the 10/2 UF-B to a new 10 gauge wire in the junction box.
3/ Burry the new wire 10 gauge wire in a 3/4" PVC conduit 6" below grade all the way to the garage.
4/ Get a 70A SquareD subpanel in the garage and have 2 circuits on 20A breakers each.


My questions:
1/ Am I overlooking anything?
2/ I was told that using a single 10 gauge wire is not recommanded when running more than 10 feet. Instead, I was told I should run each conductor separately to avoid overheating. Is it true?
3/ I have been reading that I need a 4-wire circuit going to my garage (2 hots, 1 neutral and 1 ground). Is that true? If so, it would mean that I am out of luck with the 10/2 UF-B cable on my outside wall. What is the 2nd hot used for in the first place?
4/ Unlike in my main panel, I understand that in my subpanel the ground and the neutral will be separated. Is it correct?
5/ Since there is more than 1 circuit in the garage, I will need supplementary ground. The builders have left a ground rod sticking out of the roof. Is it what I am supposed to use and connect to the green screw in the subpanel?
6/ Do I need a subpanel in the first place? Can't I run teh 10 gauge wire to the garage as a single circuit and put my outlets and door opener on it?

Any help appreciated. Thanks in advance.
-Laurent.
 
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  #2  
Old 12-31-07, 04:28 AM
R
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1. 10 gage wire is only good for 30 amps. 10-2 wire is only good for 120 volts, not 240 volts. The 30 amp breaker in the main panel must only be a 120 volt breaker, and will limit the circuit to 30 amps.

2. If you are running UF cable, it's okay to use conduit for protection, even for long distances, as long as the conduit is large enough. If you run conduit the entire length, I would use individual conductors, as it is usually cheaper. It is certainly easier to upgrade in the future.

3. As I above, 10-2 will onlygive you 120 volts. The extra hot wire provides the other leg of the 240. If you are installing a sub panel in the garage, and if you have other metallic paths between the buildings (water pipes, telephone line, etc.) then you must run four wires for the sub panel. however if no metallic paths exist between the buildings then you can run a sub panel on three wires, but not the three you have. The one you l;eave out is the ground, not a hot.

4. Yes, if you run four wires to the panel. Or if you run three wires and make it a 120 volt sub panel.

5. A ground rod would be sticking out of the ground, not the roof. Please clarify your statement and what you have. Yes, a sub panel at an outbuilding needs a grounding method.

6. You can run the 10 gage wire as a single 20 amp circuit. This avoids a sub panel. Better yet, you could run a properly installed 10-3 (or 12-3) cable as a multi-wire circuit and get what amounts to two 20 amp circuits at the garage.
 
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Old 12-31-07, 08:27 AM
L
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Thanks for your answers. Ideally, I would get rid of the existing 10/2 wire and replace it with 10/3 (or 8/3). However, I cannot do this because I have no way to come out of the house with a new wire. The 10/2 is buried under cement.

I will probably go the route of a 120V subpanel in the garage since I do not have use for 240V and will connect 3 individual conductors in the junction box on my outside wall (hot/neutral/ground).

I am unclear about your comment on other metallic paths. There are none currently but I might add some in the future. Does it make any difference? In order to connect the new subpanel, I am planning to:
-pigtail 2 black lines from the 1 hot to provide power to both breakers
-connect the white neutral to the neutral bar.
-connect the groud to a ground bar, not connected to the neutral one and instead to the metal rod. By the way, this metal rod is sticking out of the roof currently. This is because it is a concrete garage. When the guys pored it, they sticked that rod in the ground and said it will work for my electrical.

Does it all make sense?
 
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Old 12-31-07, 08:59 AM
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I am unclear about your comment on other metallic paths. There are none currently but I might add some in the future. Does it make any difference?
Not to you, since you already have separate grounding and neutral conductors.

I still don't understand a rod sticking out of the roof. Is this a 40-foot grounding rod?
 
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Old 12-31-07, 09:10 AM
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I understand this can be confusing. To show what I am talking about, I placed a picture in http://laurentmartin.homeserver.com/rod.jpg.
I took that picture a couple of weeks ago when they were building the garage. The structure is done now.

The part I am worried about is the pigtail of the 1 hot to power the 2 breakers. Is it legal?
 
  #6  
Old 12-31-07, 11:00 AM
J
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This is a "Ufer Ground" (look for it on Wikipedia or Google). It's fine.
 
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