ampacity chart that I use rates 12awg stranded at 9.3 amps per 1000 feet for power transmission
This statement is incomplete. You must tell us the nominal voltage and the percent of allowable drop. Wire size vs. voltage drop tables are typically for 2% drop of some nominal voltage, and they are usually for a complete out-and-back pair of conductors. For your example, for a 1000 foot loop your voltage would need to be 1500V to have a 2% drop.
