# feeding a 3 ph 120/208 100a sub panel



## erics37 (May 7, 2009)

Well if you have to go 500 feet away, I'd run the feeder at the higher voltage, and put the step-down transformer at the far end, near the panel. You can get by with smaller conductors, smaller conduit size, and probably less % voltage drop over a comparable lower voltage run.


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## MarcDuke (Dec 22, 2010)

any specific reason


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## erics37 (May 7, 2009)

MarcDuke said:


> any specific reason


Uh, yeah:



erics37 said:


> Well if you have to go 500 feet away, I'd run the feeder at the higher voltage, and put the step-down transformer at the far end, near the panel. *You can get by with smaller conductors, smaller conduit size, and probably less % voltage drop over a comparable lower voltage run.*


Same reason the power company doesn't distribute power over the grid at 208 volts.


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## Shockdoc (Mar 4, 2010)

Why not just land the x former over by the panel? You can then run a much smaller conduit and conductors over that way.


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## MarcDuke (Dec 22, 2010)

Thanks, so this makes much more sense.

500ft of #8 in 3/4 emt to the xformer


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## uconduit (Jun 6, 2012)

why not #4


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## glen1971 (Oct 10, 2012)

MarcDuke said:


> Thanks, so this makes much more sense.
> 
> 500ft of #8 in 3/4 emt to the xformer


I'd use at least #4s... 125% on a 45 kva is about 68 amps on the primary..At 500' and loaded the Vdrop is about 3% with #4s.


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## 751 (Dec 1, 2011)

My calculation comes out with #4's at 480 for 500'


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## Spark Master (Jul 3, 2012)

Higher voltages are standard for long distance transmission. 500' is pretty far, even for 277/480.


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## glen1971 (Oct 10, 2012)

Spark Master said:


> Higher voltages are standard for long distance transmission. 500' is pretty far, even for 277/480.


It is long, but definately not uncommon...


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## l0sts0ul (May 7, 2011)

Running at 208 is not the best way. 


500' at 480 is still a lot of distance. My calc says 1/0 aluminum for 100A at 500' for. 2.48% vdrop. 

You said aluminum.


For the savings I would quickly price out stepping up to 600v and back Down again on the far end. I've had to do this a few times due to restrictions of wire available. 

It sometimes works out cheaper


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