[Al Skierkiewicz]

For those of you that are interested, the tables that Erik has linked to are also derived from the National Electrical Code and in some locations, the NEC is the rule rather than local codes.
The transmission numbers derive from voltage drop in the wire for different currents. The desire is to have less than a specified voltage drop for a given wire length. This insures that your wall outlet maintains the 110 or 120 volt output when you plug in a toaster, space heater or hair dryer.
If Art Dutra happens to lurk by, he could be more specific than I on this subject.