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Old 03-19-2013, 07:22 AM   #1
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separate structure sub panel

Hello All,

I have a quick question, after 6 months of building my "storage barn" is now complete and I am ready to get power to it.

275' run from main panel (in home which is main structure on property) to the barn.

My plan is as follows:

Main Panel in basement of home; install 100amp 2P breaker in the existing CH 200a panel, run 1/0-1/0-1/0-1 AL URD. I have a three foot run up from this panel to the outside where it will be ran underground using 2" PVC with a 90 deg sweep.

From the sweep leaving the house I will direct bury the cable until I get to my barn 275 away where as I poured the slab with a 3" 90 deg PVC sweep positioned directly below where my sub-panel will go. I have purchased a floating neutral bar for my Homeline 100a panel and will not bond ground to the box.

1. I get mixed opinions on the URD inside the house I have been told both to use as is or that I need to use service entrance with a jbox going to the urd on the outside of the house. I really don't want to but i want it safe and correct.
2. ground at the building, since I am bringing a ground from the main do I need any sort of earth ground.
3. When I did my voltage drop calcs I came up with 4/0-4/0-4/0-2/0, what do you think about my wire size.

lastly, my area requires no electrician to sign off the work, my local inspector is the one dictating the wire size. I have went over my plan with him and he is fine with about anything as long as I pay my permit it seems. I am more than confident on the wiring, less than confident its not exactly to code. Any and all help is much appreciated. I apologize for my lack of terminology and general ignorance.


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Old 03-19-2013, 06:09 PM   #2
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The feeder will need to be 4 wires from the main panel.

You will need a grounding electrode system at the outbuilding.

The 3" will not fit in a typical panel depth.

Voltage drop will be affected by the load on the circuit. This will in turn affect the wire size needed.


Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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Old 03-20-2013, 01:51 AM   #3
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For 3% voltage drop with 90*C wire, 75*C terminations, 240Volts, 26-30*C ambient, I get.

1/0 AL = 66Amps load max

4/0 AL = 133Amps load max

It is code to use a Torque Wrench or Torque Screwdriver on the terminations. Cu is very forgiving, AL is not.

Please go to the Quick Links drop down menu in the header of this page, click Edit Your Details, scroll down and put in your location. Thx

Last edited by Glennsparky; 03-20-2013 at 02:12 AM.
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Old 03-20-2013, 07:02 AM   #4
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URD can only be ran inside if it is dual rated RHH/RHW.
All responses based on the 2011 NEC.
If you live in New Jersey click here. All other states click here.
Please check with local, county and state officials as laws may vary.
Sizing motors here. Online motor calculator here. Online calculators here.
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aluminum wiring , garage , shop , sub-panel

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