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Mike, Master Electrician
Category: Electrical
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Experience:  Licensed Master Electrician - OnQ Certified Data Voice Audio Video Installer
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I would like to run electrical power to my shed from the panel

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I would like to run electrical power to my shed from the panel in my house. I need two 120 v, 20 amp circuits in the shed. I plan to use a 4 conductor wire (black, red, white and ground) to get power to the shed. When I connect to the main panel box in the house, do I use a double circuit breaker (if so, what size?) or two single circuit breakers? I plan to split the power into two circuits in the shed.

Thanks for your help on this and anything else I should know concerning this project.

T. J.
About how far away from the panel is the shed? What all do you plan on running in the shed (how many recpts, lights, AC unit)?
Customer: replied 7 years ago.

70 ft to shed from panel. Will use one circuit for two fluorescent lights and ceiling fan. The other circuit for small power tools (saw, router, drill only one at a time)

And is this shed detached from the house?
Customer: replied 7 years ago.
Yes, shed is detached
Ok... since the shed is detached, you will need to install sub-panel in the shed with a main breaker. A 40 amp rated circuit should be fine for what you're doing, but I always recommend going up at least one size at the very least - it's very rare that a homeowner doensn't end up adding at least a couple of things that he/she never thought they would within 6 months of completing a project like this. But you are certainly free to choose here.

You could run #10 THHN and put it on a 40 amp double breaker breaker in the supply panel (in your house), and connect it to your main breaker in the shed panel (but I recommend at least a 50A breaker with #8 THHN, 3 #8's for two hots and the neutral and 1 #10 for the ground) Then you'll install individual breakers in the shed panel (20A single poles) to supply your receptacle and lighting loads. You will need to run THHN in conduit or UF direct burial cable if you don't want to run conduit.

You will also need to drive two ground rods outside the building on the other side of the wall behind the panel in your shed. You will then need to connect them with a #8 bare copper ground wire with one continuous run to the ground bar in the panel to properly ground things.

On a side note, it doesn't really matter what size the main breaker is in your shed panel since the conductors are already protected by the 40 or 50 breaker supplying them. This purpose of this breaker is only to serve as a means of disconnect. So if you can only find a 60 amp breaker panel for instance, that will be fine.

I know this is probably a lot more than you were expecting, but it's code, and for good reason. Let me know if you have any additional questions. And good luck!

Customer: replied 7 years ago.
Mike, this is exactly what I needed to know. I am not clear on one thing, however. The double breaker in the house You specifed 40 amp. Is this 40 amp on each leg of the double breaker or 20 amp on each leg?
A 40 amp breaker is rated for a total of 40 amps, but technically if will supply up to 40 amps on either leg, but will trip somewhere around 45-50 amps total (total of both legs). This will be adequate to install two 20 amp breakers in your shed sub-panel. You can install more than this, as the load isn't calculated by adding up the sum of all the breakers, it's calculated by what you'll be using at any given time plus a margin of error. Actually, the formula is much more complicated than that, but that's it in a nut shell and it's just to let you know that you can certainly install three 20 amp breakers in your shed sub-panel if you'd like, but just know that if you had all three circuits maxed out at 20 amps, the breaker would trip.

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Actually, if you run #10, you'll have to put it on a 30A breaker (even though #10 THHN has a rated ampacity of 40 amps, code requires no larger than a 30A breaker - there are special circumstances for small conductors like #10, #12, and #14). This will still serve your load just fine though. If you run #8 THHN you can go up to a 50A breaker.