timberframe, I seriously doubt that you can drop the wire size in the line.
BGK, take a close read on this that I dug up on the 'net in regards to wire size, motor load, starting amps and voltage drop over a 500' run.
http://answers.yahoo.com/question/index?qid=20071229090732AAmelbm If I was running the wire for myself, I wouldn't run less than 10 ga, and I'd give serious consideration to 8 ga. If you might at some time in the future want to hook up another electrical item to it, you'll be overloading the circuit if going with less than 10 ga.
What's cheaper in the long run, spending the extra now for the increased wire size and bury it once, or find out in 2 years that you really need to bury another wire?
With that said, I'm not an electrician.
