Need To Wire A Subpanel To An Existing Panel 275 Feet Away - Electrical - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Need to wire a subpanel to an existing panel 275 feet away
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09-10-2012, 12:10 AM   #1
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## Need to wire a subpanel to an existing panel 275 feet away

The panel is 200 amps. I have been doing lots of research and I would like to accomplish this project on my own. I do not have any training with electricity but I have been around it my whole life. So I have never taken any classes, not a class.

Has anyone ever completed a task similar to mine?

I have a fairly good idea of how I am goin to do it. The sun panel will be 30 amps. Could I run a quadroplex 8 gage for a 30 amp or should I use a 6 gage.

09-10-2012, 06:23 AM   #2
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For a subpanel fed with 30 amps at 240 volts (using a 120/240 volt 3 wire feed plus ground), you would need #6 gauge wires. Using the chart in the National Electric Code the ground wire can be #10 (a 30 amp circuit) but I don't know how much you have to upsize that wire for the distance involved.

For the wire sizes we are dealing with (smaller than about 1/0) you may not double up wires (use say two #8 wires in the cable as a substitute for one #6 wire.)

You are allowed to run one feed between any two buildings. The most efficient way (in terms of minimum voltage drop) is to run it as a 240 volt circuit with a neutral to get 120 volts with.

In addition to the ground wire accompanying the feed, the subpanel in the separate building needs two 8' ground rods at least 6' apart.

The wires, if run underground even in a conduit, must be rated for wet areas. If not in a conduit they must be rated for direct burial.

Regardless of the supply voltage, the number of volts dropped along the way equals the number of amperes drawn at that moment times the resistance of the wires (round trip). Using the 240 volt circuit with neutral, if the 120 volt usage on one side is not equal to that on the other side, the side with the greater usage will experience a larger voltage drop. We want the reasonable worst case (the 120 volt usage will usually not be perfectly balanced) to be 4 percent or less at full load (here, 30 amps). The math is a little complicated becaus the neutral will carry the difference between the 120 volt usage on each side.

Not sure how deep you need to dig the trench (for undeground wiring). Same depth regardless of distance.

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Last edited by AllanJ; 09-10-2012 at 07:02 AM.

 09-10-2012, 06:42 AM #3 Resi EC   Join Date: Aug 2012 Location: Kansas Posts: 480 Rewards Points: 252 If you will have lighter loads than 30a, #6cu/#4al, should be fine, if your load will be 30a, you might consider #4cu/#2al. Lighter loads being just a few lights and hand tools.

 09-10-2012, 04:49 PM #4 Newbie   Join Date: Sep 2012 Posts: 27 Rewards Points: 25 I talked to my buddy and he said the burial cable needs to be at least 1 foot
09-10-2012, 05:19 PM   #5
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Quote:
 Originally Posted by Ncs0816 I talked to my buddy and he said the burial cable needs to be at least 1 foot
The direct burial quadplex needs to be buried 24".
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09-10-2012, 06:46 PM   #6
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Quote:
 Originally Posted by rjniles The direct burial quadplex needs to be buried 24".
Thanks for catching that

 09-10-2012, 06:47 PM #7 Newbie   Join Date: Sep 2012 Posts: 27 Rewards Points: 25 The one thing I'm concerned about is my voltage drop. To run a wire over 250 I will be dropping a lot. Anyone ever do something like this?
09-10-2012, 08:33 PM   #8
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Quote:
 Originally Posted by Ncs0816 The one thing I'm concerned about is my voltage drop. To run a wire over 250 I will be dropping a lot. Anyone ever do something like this?
Yes, I just ran to a 60a sub-panel about 310ft away, didn't know what they were going to do, so I ran a wire good for 60a at 310ft, 2/0AL.

#4cu/#2al will give you 30a with approximately 3% voltage drop

09-11-2012, 10:30 AM   #9
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The best advice I received was to run as big of a wire as you can afford.

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