I don't know about the USA but calculating the voltage drop for a given cable length & current, is not a simple process in Australia.
I generally use the formula VD = (2KxIxD)/cm
where K is a typical value for the resistivity of copper, 12.9 ohms/cm-ft
I is the current
D is the distance, in feet
and cm is the circular mils of the wire.
I transpose the equation into cm = (2KxIxD)/VD
where VD is 3.6V for 120 V or 7.2 for 240 V.
So if I know the current, this tells me the wire size in circular mills. I can then select from a chart of standard gauges to the nearest size.
Our wire ampacities are selected from a table of based on what conditions the wire is used in. Most typically, it is based on no more than 3 conductors in a conduit of cable, or directly buried in earth. There are other tables for free air and other conditions.