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Old 05-14-2009, 10:42 AM   #1
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Max sub panel for 10-4 wire


Hello,
I am helping my nephew add some power to his garage and was looking for some knowledge. I have done the work befre, but am just a little hesitant about the circuit breaker sizings.

Right now he has a 10-4 wire run to the garage and hooked into the house panel as 2 -15 amp breakers. They trip when his small air compessor fires up. I suggested pulling those out and running a single circuit to a sub panel in the garage, but want to veryify my idea. what size circuit should I run the 10-4 from, 30A? What size sub panel can I run it to in the garage?

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Old 05-14-2009, 11:01 AM   #2
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Max sub panel for 10-4 wire


How does he have two breakers hooked to a 10/4 wire? (I hope it's a multiwire branch circuit) What type of wire is it and how is it run? If it is a 10/3 with ground he could use a 20A Double pole breaker and have two 20A circuits out there. That should be more than enough for a small compressor.

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Old 05-14-2009, 11:15 AM   #3
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Max sub panel for 10-4 wire


You can use the 10/3 to feed a small sub panel in the garage. And yes 10/3 is good for 30 amp.

But I agree with jerry regarding one 20 amp MWBC. How far away is the garage from the panel? The original installer may have installed #10 wire due to voltage drop, and intended it to be a 15 amp branch circuit or MWBC.

Last edited by J. V.; 05-14-2009 at 11:26 AM.
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Old 05-14-2009, 11:23 AM   #4
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Max sub panel for 10-4 wire


sorry for the confusion. Yes he has 10-3 wire not 10-4? Doing too many things at once here.
He has 10-3 run underground to his garage about 65 feet awy from his house
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Old 05-14-2009, 11:54 AM   #5
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Max sub panel for 10-4 wire


Put it on a 2 pole 30,

Install a small 125A sub panel (8 space?)

*Use a main breakered panel or install a 60 amp "backfed breaker" to act as a main. (with a accessory clip that screws it to the panel)

Install a ground bus in the sub (no jumper from neutral)

Install a ground rod or two at the sub and tie it to the ground bus.

Run you individual circuits from the sub to the receps.



Wait.....that's not code compliant. Detached garage = 50 amp minimum sub panel circuit.


Just change the 1 pole 15's to 1 pole 20's (or a two pole 20)

Last edited by 220/221; 05-14-2009 at 12:20 PM.
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Old 05-14-2009, 12:03 PM   #6
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Max sub panel for 10-4 wire


.....
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Answers based on the 2008 & 2011 NEC.

Last edited by Speedy Petey; 05-14-2009 at 12:08 PM.
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Old 05-14-2009, 12:04 PM   #7
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Max sub panel for 10-4 wire


Quote:
Originally Posted by 220/221 View Post
Put it on a 2 pole 30,

Install a small 125A sub panel (8 space?)

Install a ground bus in the sub (no jumper from neutral)

Install a ground rod or two at the sub and tie it to the ground bus.

Run you individual circuits from the sub to the receps.



Wait.....that's not code compliant. Detached garage = 50 amp minimum sub panel circuit.


Just change the 1 pole 15's to 1 pole 20's (or a two pole 20)
Are two 20's still code compliant? I thought 2008 mandated a common trip for MWBCs.
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Old 05-14-2009, 12:04 PM   #8
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Max sub panel for 10-4 wire


I get confused. This stuff is complicated
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Old 05-14-2009, 12:05 PM   #9
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Max sub panel for 10-4 wire


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Originally Posted by 220/221 View Post


Wait.....that's not code compliant. Detached garage = 50 amp minimum sub panel circuit.
The circuit can be any size, but the remote disconnect must be rated for at least 60 A. So, what you suggested should be fine, as long as the subpanel has a 60 A main, or maybe even a 60 A A/C pullout.
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Old 05-14-2009, 12:08 PM   #10
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Max sub panel for 10-4 wire


Those air compressors are notorious for tripping 15 amp breakers when using extension cords. If I plug my compressor directly into the wall outlet it runs fine. If I use a 14 gauge extension cord it will trip. So if he's using an extension cord have him try plugging directly into the outlet or getting a heavy gauge extension cord.

I'll leave it up to the real electricians on here to explain why the extension cord causes trips, I just know it does it.
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Old 05-14-2009, 12:14 PM   #11
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Max sub panel for 10-4 wire


Quote:
The circuit can be any size, but the remote disconnect must be rated for at least 60 A
Ah, thanks. I'll edit my first reply to account for the sub main.


And thanks to you too Petey .......



Quote:
Are two 20's still code compliant? I thought 2008 mandated a common trip for MWBCs.

We are still using 05 in AZ.
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Old 05-14-2009, 01:01 PM   #12
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Max sub panel for 10-4 wire


What, a guy cant change his mind?
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Answers based on the 2008 & 2011 NEC.
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Old 05-15-2009, 12:12 PM   #13
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Max sub panel for 10-4 wire


Heres the skinny.
Use the 10/3 to a minimum 60 amp main breaker sub panel. Remove the two 15 amp singles and put in one 30 amp double pole in the main. Keep the neutrals and grounds separated in the sub panel. Drive two 8' ground rods within 6 feet of each other. Connect the ground rods to the ground terminal strip inside the panel.
Note: The 60 amp main breaker (in the sub) is only a switch. The sub panel and wire (feeder) is protected by the 30 amp breaker in the main.
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Old 05-15-2009, 12:45 PM   #14
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Max sub panel for 10-4 wire


Quote:
Drive two 8' ground rods within 6 feet of each other
What he meant to say was......
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Old 05-16-2009, 10:44 AM   #15
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Max sub panel for 10-4 wire


Quote:
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What he meant to say was......
Sorry. No less than six feet apart. I feel better already.

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