Replies

1. 
   joeh | Nov 04, 2002 01:29am | #1

   If you do a search for "sub panel" you'll find way more opinions than you can read this afternoon. This is asked almost weekly. Joe H

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=3549.1&search=y

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=10986.1&search=y

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=5728.1&search=y

2. 
   FrankB89 | Nov 04, 2002 04:10am | #2

   Your electrical supplier most assuredly has a little chart that shows wire size/voltage drop/distance for overhead or buried service.

    

3. 
   BarryO | Nov 04, 2002 08:40am | #3

   Well, I did the same thing when I built my barn 13 years ago, except it the run was from a separate 200A service on the pump house, rather than the house panel.  But the distance was the same, and the barn panel (100A) was the same size.

   At that distance, you'll need alot bigger wire than you would ordinarily need for a 100A feeder, in order to avoid voltage drop.  I used 4/0 aluminum for the two hot wires, and 2/0 aluminum for the neutral.  I used type USE wire and buried them directly in the trench.

   This will not be cheap; I think I paid ~$700 for the wire.  Since the barn is a separate building, and assuming there is no other metallic path between the barn and your house (i.e., don't run a phone wire to the barn). you do not need to run a ground wire from house to barn.  In these cases, the code allows you to tie the neutral to the ground bus at the barn panel, which also must have a ground rod system installed.

   Also, note that the 4/0 wire will probably not fit in the connecting lugs on the 100A breaker in the main panel.  You'll need to stub on some 2/0 pigtails to do this.  Use the proper type of connector to do this (AL-rated connectors with hex key screws), which you'll need to insulate with electrical and friction tape.  Do not try to put these connectors inside the main panel, and there almost certainly won't be enough room.  4/0 is pretty stiff stuff, so you'll need a big junction box to do this in (at least 9"x9").

   An alternative would be to call the power company and ask about installing a second meter and service at the barn.

1. 
   BossHog | Nov 04, 2002 05:11pm | #4

   You can download a voltage drop calculator here:

   http://www.mikeholt.com/technical.php?id=technicalvoltagedropDemocracy must be something more than two wolves and a sheep voting on what to have for dinner. -- James Bovard(1994)

4. 
   booch | Nov 04, 2002 06:13pm | #5

   You don't want more than a 3% drop in voltage.

   100 yards = 300 foot

   Per the I.A.E.A table 4/0 copper will go 323 foot before the voltage drops 3% from your nominal. The next logical size is 250MCM which will go 383 foot.

   Data from the Gould Shawmut Book of electrical information.

   Someone stated the 100 amp panel won't take the wire size. True. There are load centers that will take convertable main lugs but you need to talk with your supplier to see what is the maximum main lug the load center will take.

   PS. torque to fasten removable lugs is important. Otherwise you let out the magical smoke that powers all electrical products.

   If using Aluminum make sure the lugs & wire are compatable. It will say so and will be stamped on the lug.

   Last thought, you could use a smaller wire and then get a buck boost transformer to trim the voltage back up to the correct level. This should generate some discussion before you do it.

5. 
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