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I am looking to wire a pole barn. To avoid trenching from the box at the meter. I am wondering if I can put a 100 amp breaker in my 200 amp box in my house and pull from there. If so, what size of copper or aluminum wire would I need to use for a run of 120-150'. Can I even use aluminum wire from a breaker box inside of my home to output to another building? I do understand that I would not be able to run everything in the barn and the house at the same time, but I don't plan on running that much in the pole barn. Mainly lights and a fan in the summer. Worst case, I would be running a welder that requires around 40 amps. Any information would be greatly appreciated. Thanks!

  • Are you planning on running a direct buried cable, or conduit? Also, how much load does your house pull? – ThreePhaseEel Apr 9 at 4:38
  • I hadn’t planned on putting it in conduit but I can. Is there a rule of thumb for conduit size for wire for burial? – jdeere51 Apr 9 at 12:05
  • Can you post photos of the existing breaker box at the home, by the way? – ThreePhaseEel Apr 9 at 23:33
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When digging a trench, conduit means you only dig it once, and makes future wire damage much less likely. Up to you, but often the price of conduit + wires is not worse, and sometimes better than, direct burial cable. I am also a big fan of putting in an extra conduit for future use, say if you decided to extend your network coverage to the barn. I prefer schedule 80 PVC in the gound unless I have a special consideration where rigid metallic conduit (RMC, it's like galvanized water pipe) makes more sense. In Barns, I prefer to keep using EMT conduit for the individual circuits, as rodents, wires, and barns can be a recipe for barn fires otherwise.

Aluminum wire is a perfectly reasonable choice for a heavy feeder - the modern alloys don't have the issues the old stuff used for 15/20 amp interior circuits did, and the cost is far less than equivalent copper, even though aluminum has to be a larger size. It is the standard material used for heavy feeders, though if you are unfamiliar with proper preparation for connecting it, it's worth having an electrician drop by to make those connections correctly (torque matters, anti-oxidant paste matters, and prepping the ends for the paste matters.)

For 150 feet at 100 amps with 75C connections and wire insulation, 1/0 aluminum is conservatively adequate. 90c would drop that to #1, but 90C wire insulation is much easier to come by than 90C rated connections, from what I recall when shopping. #4 is adequate for the equipment grounding conductor.

That information is not some magic I know, just me using one of the many web wire calculators that reference back to the NEC requirements. I would do the same for conduit size once a wire size/insulation type is chosen, though I highly recommend making the conduit bigger than the minimum size for your own ease of pulling - it's not very expensive to upsize conduit, and pulling near maximum fill is a pain.

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For a shorter run, there is no question that #1 aluminum would be correct. (#3 copper would be equally correct but 3-4 times the price and with no safety improvement, since aluminum is reliable in these large feeders).

Long runs are vulnerable to voltage drop. Normally at about 115' we think about bumping a wire size. We would do that if you expected to run the circuit at/near limits much of the time. However you do not. That makes yours is a textbook example that you base voltage drop calcs on expected load, not breaker trip rating.

So it's back to #1 aluminum final answer.

The next size up is #1/0 aluminum, you might price the difference.

How do you arrive at this? Any of the various voltage drop calculators out there, but you make 2 passes through it.

  • First, you punch in aluminum, your breaker trip capacity, voltage, 40% allowable voltage drop, and 1 foot distance. This tells you the minimum size of wire you must use given that circuit breaker.

  • Then, you punch in aluminum, your actual expected loads, voltage, 8% allowable voltage drop, and your actual distance. Now, keep lowering that voltage drop number until you get the same wire size. That will tell you the practical drop to expect on that wire.

Never take the calc's default 3% voltage drop. Many, in fact most "size bumps" I see are cases where without the bump, the voltage drop would be 3.3% at breaker trip. 3% isn't even in the electrical code, it's an apocryphal number! facepalm... The people were oblivious that the bump was unnecessary. That's why you have to cross-examine the calc a bit.

  • Was that first voltage drop really meant to be 40%? – JPhi1618 Apr 10 at 2:48
  • Yup. @JPhi1618. I would have said 99% but was afraid that would be interpreted as 1%. Goal is to remove v.drop from the calculation. – Harper Apr 10 at 8:56

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