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Burgertown · Aug 23, 2009, 01:05 AM

What gauge wire do I need to run a 100 amps from my house to my shed that is 270 feet away?

donf · Aug 23, 2009, 04:38 AM

120 VAC, copper = 4/0 (#8 EGC). Al = 300 kcmil. (#6 EGC)

240 VAC, copper = 1, (#8 EGC) Al, 1/0 (#6 EGC)

tkrussell · Aug 23, 2009, 05:01 AM

Don, and Burgertown, when line conductors are increased in size due to voltage drop, the EGC must be increased in size proportionally.

You have given sizes based on single voltages, but the service voltage is 120/240, and some loads may be 240, while others may be 120 volt.

Some loads may be just lights or small hand tools, while others, don't know without asking or assuming, maybe a large motor or welder. So need to determine types and voltages of loads

Now what?

Perhaps a compromise of using #4/0 Aluminum, 4 wire URD direct burial cable, with a reduced neutral of #2/0?

This is a common standard product available at a reasonable cost.

stanfortyman · Aug 23, 2009, 05:58 AM

Depending on the actual load, I come up with some different numbers.

This is one of the few places TK and I sort of disagree. I feel a feeder should be calculated at 240v since as the load increases it actually becomes more balanced. It is extremely difficult to heavily imbalance a residential 120/240v feeder. That is something you would have to almost do intentionally, which is not going to happen. And all 240v loads are already perfectly balanced.

The loads is the other thing. Everyone says "What wire do I need for 100A?", when what they almost always mean is what wire for a 100A sub-panel.

Using the "standard" 80% figure I come up with #1cu for the feeders. Like TK said, the ground also need to be increased. Without doing all the math, a #6 would be fine.

At a full 100A load that jumps to 1/0cu. Again, the full 100A number is something that will likely never get seen if maybe for a very short duration.

Burger, if you provide a little more detail than "What wire do I need for 100A" then we can give you even more detail.

tkrussell · Aug 23, 2009, 06:12 AM

Was just making a point to Don, that there is more to it than appears, and judgments need to be made, of course, would be better to know as many details as possible.

While I do agree with as the 240 volt loads increase the panel gets more balanced, and the neutch current remains fairly low, my concern, of course, without knowing details of the types of loads, is that say a large 120 volt is the only load running, or by chance only 120 volt loads are on one leg,and the neutch has 30-40 amps running on it.

I wish here was a simple answer for these types of questions. There just is not.

Reality may be even though there is a 100 amp feeder, more likely the load will be minimal, and #2, maybe #1, aluminum will suffice. Who really needs 100 amps, in a shed? Must be one helluva shed.

stanfortyman · Aug 23, 2009, 07:11 AM

Originally Posted by tkrussell

Reality may be even though there is a 100 amp feeder, more likely the load will be minimal, and #2, maybe #1, aluminum will suffice. Who really needs 100 amps, in a shed? Must be one helluva shed.

Yeah, I was thinking the same thing. :D

donf · Aug 23, 2009, 10:37 AM

TK, how do you arrive at a 120/240 VAC for a 100 Amp load cable sizing?

Can you show me the formula you use?

Also, I assume (very dangerous) that this will always be a feed line to a sub panel. So in this instance, the question appeared to me that he was asking what is the size of the cable I need to feed a subpanel 270' away from my main panel.

tkrussell · Aug 24, 2009, 02:50 AM

That is my point, there is no one formula for 120/240 volts. You need to know what 120 volt loads would be on at one time, and understand what phase or leg they are on, so as to accurately calculate what the unbalanced neutral amps would be to be able to size the neutral and either/or of the live legs.

Then the types of loads must be considered, as I have said before, large motors, welders, and high amp draw of inductive loads, which have high inrush starting amps.

And I am of the philosophy of having a conductor larger than what appears to be sufficient for an typical load, as who knows if and when more or additional inductive losd will be applied. Once the wire is pulled you need to live with it, or spend more money later.

Sizing wire for voltage drop requires knowledge, experience and judgment.