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Old 05-06-2010, 01:24 PM   #1
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Power to the garage


Hey all,

I know there have been a good few threads on this before, I just can't seem to find exactly what I'm looking for.

I'm planning to run power to my detached garage, about 45 feet away. The plan so far is to underground with PVC conduit from the back of the house.

I was planning on a 60A feeder to the garage, but I've just realized I only have a 100A service at the house. So now I'm thinking 40A or so is probably the way to go. What wire should I go with for 40A? It's probably about a 60 foot run from panel to panel. I'm thinking #8 THWN based on a chart I saw somewhere, but I want to be sure.

Does anyone have a rough idea of price for #8 THWN? Also, can I run an undersized (#10?) EGC in this instance? I seem to recall that this is allowed in some cases.

As far as ground rods go, I'm not sure if I need to drive one or two. Or for that matter, what size wire I need from the sub-panel to the rods?

Last question, I swear:
I'm thinking I should probably run an extra conduit for future use, maybe phone, network, or security at some point. Any problems code-wise with dropping this in the same trench?

Thanks much, I love this site
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Old 05-06-2010, 02:00 PM   #2
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Your box might be for 100A total, but chances are you aren't using close to that at any given time. If your box allows, a 60 amp would work. If you can, of course run 220V. Even with 40A, that would give you four fully independant 20A 120V circuits. But just like your house (count the amps on all the breakers and you will have way more than the 100A total rating) a subpanel in the garage will give you more circuits.

I think #10 is only good for 30A. You can never run a wire undersized for the breaker monitoring it.
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Old 05-06-2010, 02:07 PM   #3
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Quote:
Originally Posted by kindrox View Post
Your box might be for 100A total, but chances are you aren't using close to that at any given time. If your box allows, a 60 amp would work. If you can, of course run 220V. Even with 40A, that would give you four fully independant 20A 120V circuits. But just like your house (count the amps on all the breakers and you will have way more than the 100A total rating) a subpanel in the garage will give you more circuits.

I think #10 is only good for 30A. You can never run a wire undersized for the breaker monitoring it.
Thanks for the reply. I'm pretty sure at this point I'm just going to stick with the 40A feeder to save money on the wire. I'm running a 240V feed for sure, but I don't expect to be pulling much current out there. I've got no major power tools.

As far as the #10 wire goes, I was referring only to the ground wire with the feeder. My understanding is that in some cases you can run a ground that's a size smaller than your hots & neutral. Just not sure if that applies in this case.

I should probably have also mentioned in my original post that I'm in Minnesota, under the 2008 NEC. Yay for tamper resistant receptacles in the garage...
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Old 05-06-2010, 02:11 PM   #4
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Go ahead and run #6ga Cu (you'll need three) THWN along with either a #10 or a #8 green. You can always downsize the breaker if it presents a problem. Use 1" PVC for your conduit. No problem with dropping another conduit in the trench. Plan on driving two grounding electrodes, connected to the subpanel with #6 or #4 copper.
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Old 05-06-2010, 02:34 PM   #5
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Quote:
Originally Posted by McSteve View Post
I'm thinking I should probably run an extra conduit for future use, maybe phone, network, or security at some point. Any problems code-wise with dropping this in the same trench?
Even if there aren't any code problems, you should make practical considerations. I did a google on "electrical trench seperation" and ran accross this: http://www.aps.com/files/_files/pdf/ESPServices/600.pdf

It's a document from a power utility that basically says primary power feeds should be in a trench 36" deep, secondary power should be in a trench 24" deep, that other utilities should remain 12" away from power, and for things in sleves, the seperation could be reduced to 6".

Obviously this document wasn't written for your situation, but I'd use it as a general reference and likely put my electrical 24" deep with another piece of conduit for phone/computer/etc about 12" deep.
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Old 05-06-2010, 02:43 PM   #6
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Stay with 60 amp for the garage. You might later want to add a tool such as a welder, a kiln, or more commonly an air compressor.

Don't make your project a pain later on down the road because you wanted to save $100 on wire today!
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Old 05-06-2010, 03:15 PM   #7
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Detached structure needs 2 ground rods as noted
Also - some inspectors seem to think you must have a 60a feed instead of just a 60a disconnect
So 1st step check with your Inspector on what local code may require

Myself I'd run the 60a feed to a 20 space 100a main breaker panel
This is what I did to my pool cabana
I may never need 60a 240v out there.....but it wasn't really that much more

There isn't really a problem with having the 60a VS the 40a
Having it available is not a problem
The only problem is if you use too much power, then the breaker just kicks off

#8 TWHN will get you to 50a, might as well just go to 60a



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Old 05-06-2010, 03:19 PM   #8
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For a 40 foot run of 3 - #6 verses 3 - #8 there is only a difference of less than $20

My wholesale cost for #6 including tax is $ 0.42 per foot and #8 is $ 0.28 per foot. At Home Depot or Lowes you will pay more than that but the difference should remain about the same

40 feet x 3-#6 = 120' x $.42 = $50.40
40 feet x 3-#8 = 120' x $.28 = $33.60

Difference = $16.80 hardly worth thinking about!

You still need a #10 ground either way so 40 x $ .17 =$ 6.80
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Old 05-06-2010, 03:45 PM   #9
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And I've yet to see any new equipment not rated for 75c
Or an Inspector that asked to see ratings of anything 60c VS 75c etc
I've always gone by the NEC charts in 310 without a problem



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Old 05-06-2010, 05:56 PM   #10
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Quote:
Originally Posted by sparks1up View Post
I guess getting away with something makes it right?
Getting away with something ?
I don't know a single person that uses the 60c for THWN



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Old 05-06-2010, 06:20 PM   #11
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Quote:
Originally Posted by Scuba_Dave View Post
And I've yet to see any new equipment not rated for 75c
Or an Inspector that asked to see ratings of anything 60c VS 75c etc
I've always gone by the NEC charts in 310 without a problem
Agreed. I have checked out the temperature ratings on my equipment. The breaker I'll be using for the feeder from the service panel is 75C rated, as is the main in the panel I will be installing in the garage.
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Old 05-06-2010, 07:11 PM   #12
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Quote:
Originally Posted by sparks1up View Post
Would you use #8 fused at 50-amps for a 50-amp stove outlet?
He is connecting a Sub-panel....not a stove
But, from what electricians on this site & others have stated, yes #8 THHN/THWN for 50a is fine for a stove
And stove outlets are rated at up to 75c
Zoom to see 75c rating:
http://www.homedepot.com/webapp/wcs/...50&marketID=39
.
Square D info on breaker terminations:
Quote:
Terminations
The 10–30 A circuit breakers have pressure plate terminals suitable for single or two-wire terminations.
Copper or aluminum conductors may be used as outlined in Table 2. QO-GFI 15–30 A and QO-AFI circuit breakers have pressure plate terminals suitable for single-wire terminations. These circuit breakers are suitable for use with 60C or 75C conductors.

The QO 35–200 A and all QO-PL and QOT tandem circuit breakers have box-type lugs suitable for single-wire terminations. These circuit breakers are suitable for use with 75C conductors.

So the termination points are rated for 60/75c
.



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Old 05-10-2010, 11:01 AM   #13
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Does your detached garage have covered walls, or are the studs exposed? If the studs are exposed, what kind of wire/conduit are you going to use to run to the individual outlets, switches, and other fixtures? Are you going to run and bend conduit? Or are you going to use that spiral conduit with the wire already inside it? It seems a little cheaper and easier to use (on the inside only), that way you don't have to cut and bend small lengths of pipe and use all sorts of extra boxes and connectors for tight spaces.

This is an interesting and useful thread, I'm thinking of doing the same exact thing for a detached shed that's about 50 feet away from my house. It had originally been wired, but it was a hack job. Instead of running conduit to the house and adding a new breaker, there was a "male plug" hanging out of the side of the shed. Whenever they wanted power out there, they just ran an extension cord from the house. Before I bought the house, the inspector made me pull out all the wiring because it was regular in-wall wiring with no conduit, it was all exposed. Now I have to do it the right way.

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