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Old 06-14-2011, 07:41 PM   #1
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Power to the outbuildings


Hi,

I am restoring an old grainery on the family farm in Door County, WI. Along with the restoration is bringing power to the grainery, barn, and machine shed with a 5 year plan of moving the grainery to a new location 250 feet further from the farm house (...for code reasons). The farm house has an updated 200A electrical panel and they left provisions at the entrance to add a 100A 2 pole distribution breaker. The farm house is 225 feet from the grainery and the final spot for the grainery is an additional 225 feet further from the grainery. The goal is to use the 100A distribution breaker as a dedicated feed to the grainery/cabin. I am planning on pulling off a 50A 2 pole off the updated 200A panel in the farm house to feed the barn and machine shed.

I beleive a 3 aught for the 100A circuit and #2 for the 50A circuit is the right size. I am using a worst case scenario of 80A load on the 100A breaker and 40A on the 50A breaker as they are rated at 80% trip. I have oversized because the total distances for both is around 550 feet. 225 feet to the grainery and another 225 feet to the machine shed and the enventual location of the grainery. I plan to run a single 2&1/2" PVC from the farm house to the grainery, which will hold (4) #2 THWN and (4) 3 aught THWN wires. The #2 will terminate in a 100A main lung panel which will have a 30A 2 pole breaker for the barn and a 20A 2 pole for the machine shed where I will just run the direct bury cable to small panels. The 3 aught will have a 100A panel in the grainery.

I want to make sure this makes sense and get a sanity check on the plan...

Thanks for any help!!

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Old 06-14-2011, 09:39 PM   #2
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Power to the outbuildings


It's about 1000' round trip to the new grainery location. The 3-aught (OOO) wires in copper will have no problem handling the load but in aluminum the 80 ampere load will suffer a voltage drop slightly over 3 percent. In aluminum the full 100 amp load will suffer a voltage drop slightly over 4%.

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Old 06-14-2011, 09:52 PM   #3
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Power to the outbuildings


Thanks for the input. Is the 3 aught too much overkill? It seems I can run #2 up to 225 feet, but then I get recommedations of the 3 aught when I start to size for the eventual 550 foot run. I had thoughts of using #2 for the first 225 feet and then when we move the service I would use 3 aught to lengthen the run.

Also, what is considered unacceptable line loss? +/- 10% from nominal seems within most tolerances of equipment. I want to be conservative, but I don't want to throw money away either....

Thanks again!
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Old 06-15-2011, 06:18 AM   #4
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Power to the outbuildings


The 3 aught is needed for the whole distance. Running thinner wire halfway will necessitate much much thicker wire for the second half in order to keep the total voltage drop within acceptable limits. Including the wires from the subpanel to the receptacles the total voltage drop should not be more than 4%. While peak load is seldom sustained, motors draw the most current at startup and significant voltage drop at that moment will cause degraded operation.
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