I have 200 amp service to the main box at the house. Within the main box I have a 60 amp breaker for my barn. The barn is being built 150 feet away from the house. What guage wire do I run from the main box to the barn box so that I get the most of the 60 amps at the barn? That is with as little amp drop as possible.

The barn will have it's own elect. box with a 110v service for lights, bench and power tools, etc. and a 220v service for the compressor,and arc welder.

 

There's a lot of questions there. Let me ask them in a different order.

Q: What gauge wire do I need for safety purposes?
A: This depends on the type of wire, and whether it is run in conduit, cable or free air. It's going to be either 6-gauge or 4-gauge copper wire.

Q: What gauge wire do I need to provide "as little amp drop as possible"?
A: All sizes of wire will produce absolutely no amp drop at all!! So you can use 24-gauge doorbell wire to achieve this goal, although it will melt quickly at 60 amps.

Q: What gauge of wire do I need to provide "as little voltage drop as possible"?
A: You'd need a wire larger than the size of a telephone pole! Actually, you need some kind of superconductor, or a way to cool the wire to absolute zero.

Q: What gauge of wire do I need to provide a "reasonable voltage drop"?
A: Over 150 feet and at 240 volts, 60 amps through 4-gauge copper wire will reduce the voltage 2% (i.e., 5 volts). This is usually considered reasonable.

Q: And the real question ... what gauge wire should I use?
A: 4-gauge copper.

 

You already have an answer as to the wire gauge question. Are you familiar with the rest of the steps to install the feeder to the second building? If the building is not yet built and the footers will contain any reinforcing steel there are things you need to do before the concrete is poured in order to have a good grounding electrode at the barn.
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Tom