AllanJ 
09102012 07:23 AM 
For a subpanel fed with 30 amps at 240 volts (using a 120/240 volt 3 wire feed plus ground), you would need #6 gauge wires. Using the chart in the National Electric Code the ground wire can be #10 (a 30 amp circuit) but I don't know how much you have to upsize that wire for the distance involved.
For the wire sizes we are dealing with (smaller than about 1/0) you may not double up wires (use say two #8 wires in the cable as a substitute for one #6 wire.)
You are allowed to run one feed between any two buildings. The most efficient way (in terms of minimum voltage drop) is to run it as a 240 volt circuit with a neutral to get 120 volts with.
In addition to the ground wire accompanying the feed, the subpanel in the separate building needs two 8' ground rods at least 6' apart.
The wires, if run underground even in a conduit, must be rated for wet areas. If not in a conduit they must be rated for direct burial.
Regardless of the supply voltage, the number of volts dropped along the way equals the number of amperes drawn at that moment times the resistance of the wires (round trip). Using the 240 volt circuit with neutral, if the 120 volt usage on one side is not equal to that on the other side, the side with the greater usage will experience a larger voltage drop. We want the reasonable worst case (the 120 volt usage will usually not be perfectly balanced) to be 4 percent or less at full load (here, 30 amps). The math is a little complicated becaus the neutral will carry the difference between the 120 volt usage on each side.
Not sure how deep you need to dig the trench (for undeground wiring). Same depth regardless of distance.
