Installing an Underground Conduit and Wire to a 50 Amp Sub-Panel in a Shed
January 14, 2007
Q: I want to run a 50 amp sub feed to my shed. I have a 200 amp service, 100 amps runs my mobile home and I have 100 amps surplus. My question is what size wire do I run from my 50 amp breaker in main panel to my shed panel which has a 30 amp and a 20 amp breaker in it? I am running 50 feet from panel to shed and I am placing wire in conduit under ground.
A: #8 AWG copper wire with THHN insulation is rated for 50 amps at 75 degrees celcius and #6 AWG aluminum wire with XHHW insulation is rated for 50 amps at 75 degrees celcius. For this installation, you need 2 – hots and 1 neutral of either the #8 copper or #6 aluminum and 1 #10 AWG copper ground wire. For a circuit this small, I recommend using the copper wire, however, the aluminum wire is cheaper.
Your PVC conduit needs to be schedule 80 from 18 inches below grade to 8 feet above grade. Where it runs underground, you may use schedule 40 PVC. Here in Montana, we have very rocky soil and I prefer schedule 80 for the entire underground run. You will need 1 inch PVC buried a minimum of 18 inches deep for this installation.
Be sure that your neutrals and grounds are separated at the sub-panel in your shed and make sure the neutral bar is not bonded to the panel’s frame. Be sure all breakers are off when terminating your wires and double check with a volt-meter.
Electrical panels (load centers) and sub-panels need to be wired properly to ensure the rest of your circuits are safe and your circuit breakers will trip. I highly recommend hiring a licensed electrician to do this installation for you. In a lot of areas, homeowners are not permitted to install their own panels and sub-panels. Be sure to check with you local building codes department for your local rules and regulations before beginning any work. If you choose not to hire an electrician for this installation, you should definately get and electrical inspection to ensure all wiring is safe.
I believe that two of the most important tools you can have when working with electricity is a multi-meter and a copy of the most recently revised edition of the National Electrical Code (NEC). We offer both of these items on our website or you may find them at your local home improvement center and bookstore.
Good luck and be safe.
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