The required copper wire size for a 200amp service of 120v/240v at 180 feet is 3/0. The choice to use aluminum conductors is often made because of a significant cost difference compared to copper, however the conductor size is larger, in this scenario #250 aluminum conductors would be needed.
Several factors go into determining wire conductor size. Voltage, voltage drop, distance of the run, wire type (copper/aluminum), application (overhead, underground, conduit), temperature rating and amperage are all requirements for determining proper wire size. Electricians should consult the National Electrical Code (NEC) including state and local ordinances when installing any electrical systems. Section 310-15 of the NEC is used to determine wire size.