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Old 05-22-2007, 06:26 AM   #1
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Garage Subpanel


I just built a dettached garage about 50ft from the main service panel on my house. The garage is roughed in for 7 wall outlets and 5 outlets for lighting. Also there is a play room upstairs wired for 4 lights and 8 wall outlets along with with a single circuit for a small (10000 BTU) window unit A/C. I will be using the garage to run power tools occasionaly but not a heavy duty work shop with lots of high draw tools. My question is what size and how many wires to run from the main panel to the subpanel. I plan to go under ground through PVC conduit.
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Last edited by Squid56; 05-22-2007 at 06:33 AM.
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Old 05-22-2007, 06:34 AM   #2
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Garage Subpanel


Personally, I'd do a 20A circuit downstairs for tools, a 15a upstairs for the receptacles, a 20A for the AC, and a 15a for lighting up and down.

So, maybe a 50A feeder, with #8 THHN.

I think that would give you plenty of capacity. I'll let the pros give you a more calculated answer.

As to how many wires, I'd run 4. With living space upstairs you may end up putting a phone line out there or something, and that would require a separate ground. So 2 hots, a neutral and a ground. (I believe the neutral and ground can be #10 Cu, to save a little $$). Keep in mind that you'll have to drive a ground rod close to the subpanel outside the garage. Connect it to the ground bus in the sub, and keep the neutral bus isolated from the box. Keep all green/bare wires on the ground bus, and all whites on the neutral bus.

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Old 05-22-2007, 03:40 PM   #3
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Garage Subpanel


Thanks for the info. Can I do 50 amps with #8 wire. I thought I was limited to 40 amps?
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Old 05-22-2007, 04:38 PM   #4
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Garage Subpanel


#8 Cu Thhn/Thwn is good for 55 amps according to Table 310.16
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Old 05-23-2007, 09:10 AM   #5
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#8 Cu Thhn/Thwn is good for 55 amps according to Table 310.16
If the terminations are rated for 75C, the #8 THHN can be protected up to 50A. You will not find any 90C rated terminations in residential, so you cannot use the 55A rating. If it is #8 NM, the rating drops to 40A.
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Old 05-30-2007, 10:22 PM   #6
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Garage Subpanel


Here is what I am going to do. In main distribution panel in the house I am putting in a 40 amp double pole breaker to connect a 220 feed to the 100 amp, 6 breaker sub panel in the garage. In the garage I am putting a double pole 40 amp breaker to accept connect to the sub. I am using 3 seperate #6 stranded wire, white ,black, red, 55amp rated. plus a #10 green stranded box to box ground all run throug a conduit. I am using double single pole breakers in the garage to provide as many circuits as I can provide. If I want to, later I can add 220 outlet to plug in a welder.
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Old 05-31-2007, 08:03 AM   #7
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Garage Subpanel


Why do you need to put a second 40 amp double-pole in the sub?
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Old 05-31-2007, 08:38 AM   #8
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Why do you need to put a second 40 amp double-pole in the sub?
The garage where the subpanel is going is detached. A detached structure requires its own disconnect.
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Old 05-31-2007, 12:38 PM   #9
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Garage Subpanel


The 100 amp panel does not have a slot for a main so the 40 amp DP in the garage is the main. Would be a *************** if the sparks started flying in the garage and I had to run out of the garage into the house, down to the basement to throw the breaker there. The 40 amp in the garage protects the 3 #6 stranded wires in the underground conduit from the house tom the garage. It also allows me to connect the a feed to the garage withpout having to pull the meter, I can just trip the main in the house, hook up the 40 amp and then set the main and the 40amp.
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Old 06-01-2007, 04:05 PM   #10
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Garage Subpanel


In the garage, the 40 amp breaker you are using as a "main" needs to be bolted down. There are fittings for this, check the panel instructions, and I bet you will find the model #. This way, you can't pull the breaker off while it's hot.
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Old 06-02-2007, 10:49 AM   #11
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In the garage, the 40 amp breaker you are using as a "main" needs to be bolted down.
John....That is not correct. If the panel is designed for stab type breakers then you stab the main to the bus just like all the other breakers in the panel. And mark it as the main.
Now, if it is a bolt in type panel then yes, naturally you would be required to use the bolts.
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Old 06-02-2007, 10:54 AM   #12
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John....That is not correct.
John is absolutely correct.

408.36(F) Back-Fed Devices Plug-in-type overcurrent protection devices or plug-in type main lug assemblies that are backfed and used to terminate field-installed ungrounded supply conductors shall be secured in place by an additional fastener that requires other than a pull to release the device from the mounting means on the panel.

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