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edmart2 03-17-2009 03:11 PM

detached garage subpanel help
I would like to add a subpanel in my detached garage and be able to run 240v also. Can I pop in a 40 A breaker using #8 AWG to maybe a 70A subpanel in the garage. I want to put another 40 A breaker to run 240V to a receptacle and a 15A breaker for my lights and a couple receptacles. Do I have to use a ground rod for that subpanel and if I do, do I bond the neutral bus and ground bus? Or can I run the ground back to the main panel?

AllanJ 03-17-2009 04:54 PM

You need a ground rod at the subpanel and should also have a ground wire running back to the main panel. The ground wire going back to the house is also mandatory if there are any other metallic connections such as cable TV wires or plumbing going to the house. (Grounding the subpanel to a metal water pipe going into the ground can be substituted for the ground rod.)

Only one feed is allowed out to any one separate building so with your 70 amp subpanel you might as well run #6 wires for 60 amps @ 240 volts.

Neutrals and grounds are kept separate all the way except for the connection in the main panel.

jbfan 03-17-2009 09:04 PM

Why would you need a 40 amp receptacle in the garage?

Scuba_Dave 03-17-2009 09:22 PM

What is your main feed? 200a ?
Have you done a load calc on your house to see how much power you are using? I'd think about just running a 100a sub if you really need a 40a 240v recaptacle out there
Then run everything out of the sub

joed 03-17-2009 10:02 PM

Under 2008 NEC you are required to run a four wire feed to a detached garage. You also need a ground rod. The neutral and ground must NOT be bonded in the garage panel.

edmart2 03-18-2009 05:44 PM

the main panel is 125 amp im not sure I can run 100amp in the garage, can someone give me an idea what size subpanel i can safely get?

Scuba_Dave 03-18-2009 05:58 PM

What you can run depends upon what you are currently using for electric.
You can "safely" run a 100a panel
But you would never be able to use all of the power unless you plan to upgrade to a 200a main service in the future
Or unless you are using very little electric in the house

So the best bet would probably be a 60a panel
Again, depending upon what your current electric use is
60a breakers are easy to find, not sure about a 70a
The breakers that are most commonly used are generally less $$

J. V. 03-19-2009 12:01 PM

If you have two spare breaker slots, a place to land a neutral and a place to land a ground you can install a sub panel of any size greater than 60 amp in the garage. It must have a main breaker or disconnect. Just keep this in mind. If you feed the sub panel with a 70 amp breaker, that is the maximum amount of power you will have at the garage, even if you install a 100 amp sub panel. You need to run 4 wires H-H-N-G. 2 ground rods, wire to match the feeder breaker. And yes, they make 70 amp breakers.
Search this site for sub panels for the most information on this subject possible. Sub panels need special attention. It's for sure a "HOT TOPIC". This is a good DIY project. But please ask before you do anything. It's much easier to address any concerns BEFORE you start.

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