Hello and thanks in advance for any help. I have a detached garage just under 300' from the 200 amp service at the house. I plan to run the wire underground in conduit. I am a little confused as to what (exact) type of wire I should have for this run. I am going to have a 100 amp sub-panel in the garage with the ground field at the garage. One of the quotes I have states "100 amp URD" for the wire. At the local "big box" store they told me that this is 2-2-2 wire (aluminum). My question: exactly what wire am I looking for? In the garage I will have lights, a small bench grinder, a 110 volt portable air compressor and maybe some hand held power tools (drill, etc.).

URD cable comes in 3 wire and 4 wire. You will need 4 wire for a subpanel.

URD cable is rated for direct burial. It comes in a twisted assembly.

If this is pulled into a conduit, the conduit will need to be a bit oversized, as pulling twisted cable assembly through conduit is difficult. For a 4 wire #2 cable, probably min 2 inch conduit would work, depending on how many bends in the conduit. Any 90 Deg sweeps should be steel, as the friction of pulling cable that long will burn through any plastic sweep.

You can use straight conduit for the long run as protection and easy replacement of the cable in he future, and not have any sweeps or bends at the ends, letting the cable just lay in the earth near the ends and rise into conduit at both buildings, since the cable is rated for direct burial.

Using #2 Al for 300 feet will only allow a 20 amp 120 volt load to operate and keep the voltage drop at 3%. 40 Amps at 240 volts will be 3 % Vd.

The #2 will be fine as long as your loads don't go too high. which what you describe sounds OK.