What size wire do I need for 200 amp service 300 feet away?

A 200 amp run of 300 feet needs roughly 400 to 500 kcmil aluminum or 250 to 350 kcmil copper to keep voltage drop near 3 percent, with the exact size set by the load you really carry. The normal 200 amp sizes, 4/0 aluminum or 2/0 copper, would lose 5 to 6 percent over that distance at full load.

Distance is the whole story here. Ampacity says 4/0 aluminum is fine; the extra 300 feet of resistance says otherwise, and the NEC's drop guidance is the tiebreaker.

Money-saving reality: run the calculation at your realistic demand rather than the full 200 amps, and consider higher-voltage options for very long farm runs.

Direct-burial URD at these sizes is priced per foot, so one size up is real money; calculate rather than guess.

The long service run calculator balances ampacity and drop for any distance.