See Appendix B Section 4 and use the tables in Appendix D. It's a good question about the load for voltage drop. Perhaps use the intended service size and calculate as a non-continuous load. Or, just use 250 MCM ACWU. If more than 3 meters is above ground, you need to use the above-ground ampacity, anyway (I think). The above ground calculations are here: http://www.codemath.com/cgi-bin/Run.pl?script=CecAwgRw
Use 200A, 240V, 1P, Aluminum, 90 degree insulation, non-continuous load.