February winds blew down my overhead service to my barn. I would like to replace the 1950's two wire service and update to a safe, sub feed using a complete new underground from a 150 amp house box. The current feed appears to be a branch from the upstairs outlet.(Not good.) The total run will be 150 feet, 50 feet being the underground sub feed.

I have 4 spaces left in the house circuit breaker box that has a 150 amp main breaker. The house has electric water heat, electric oven and range and electric dryer.

Looking for max amperage and wire size to sub feed the barn that would power lights, small power tools, possibly an air compressor and a 110 mig welder. My wife and I would not be using all the electric appliances at the same time. I am semi-retired now, but need to seek advise for this to see if I can do before spring gets here if it will be affordable to our budget. Any help is appreciated.

  • Welcome. You might revise to explain what the other 100 feet of wiring do. Is it through the house and/or the barn? Will it be in conduit?
    – isherwood
    Commented Mar 4, 2019 at 0:06
  • How deep do you want to trench it? 6", 18" or 24" of cover over top of wire/conduit? Commented Mar 4, 2019 at 0:58
  • The run would be inside from box alone ceiling 35', left turn,30' foot to outer wall, outside for the (aprox) 50' into the barn. I have looked at 2-2-2-4 Gray Stranded AL SER Cable if this would be sufficient at a 75 amp breaker feeding. Not sure if 1/0-1/0-1/0-2 Gray Stranded AL SER Cable would be better from a 100 amp breaker. Conduit is prefered and soil is such that depth is not an issue. Commented Mar 4, 2019 at 1:57
  • How many square feet is the barn? Also, how beefy of an air compressor are we looking at here? Commented Mar 4, 2019 at 2:05
  • Right now a 110 volt, 3.5 HP, 115 psi. If possible a 240 volt would be great. Barn is approx 5,000 sq foot. Commented Mar 4, 2019 at 2:31

1 Answer 1


Based on the activities you are talking about, I would say #4 aluminum.

The reason is 40A will probably suffice for immediate concerns, and you can do that with #8 copper except for the distance. That necessitates a bump to #6 copper, and a side effect of this bump is you get some more headroom, as you can technically drive that circuit as high as 60A.

However, #4 aluminum has the same ampacity as copper at half the price, so why not use it? At these large sizes, the fearmongering about aluminum does not apply, and these wires will be the newer alloy anyway.

  • I would like to max my option so that I won't regret later. Looking at distance and number of turns and best combination. For what was there, anything would be 100% better. Commented Mar 4, 2019 at 2:14
  • 1
    @KevinZirbel Well if you feel adventurous, bump it to #1 aluminum. At that point you can push it to 100A instead of 60A. Commented Mar 4, 2019 at 3:24
  • 100A would have you covered if you wanted to add electric mini split for heat/AC, or a charger for an electric car. Future owners would appreciate it.
    – Dotes
    Commented Mar 4, 2019 at 14:04
  • Exactly what I was thinking!!!! Commented Mar 4, 2019 at 23:56
  • Does 1/0,1/0,1/0 & #2 URD sound like a good candidate and if so which size conduit please.... Commented Mar 6, 2019 at 1:10

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