I am going to run power to a cattle barn for lights. The distance is 600 ft. Planning on using direct burial cable put in conduit. Need to know what size of wire and conduit to use. Have a 200 amp service at my house to feed off. Was planning on using a 100 amp breaker to feed the wire and using 50 or 60 amp main at the cattle barn. The load should be about 40 amps with all lights and acc on.

A 40 Amp load at 120 Volts running 600 feet will need a #4/0 copper wire to keep voltage drop less than 3%.

You may need to consider using 240 volt lighting, and/or using transformers to step up the voltage for the 600 foot run, then step down at the barn.

Assuming the 40 amp load would be half the amount if 240 volts is used, then only a #4 copper wire would be needed.

How about the alum, direct burial

Will you be surprised that with the same circuit specs the wire would be larger?

400 MCM aluminum for 600 foot fedder for a 40 amp 120 volt load.

120 volt does not agree with long distances.