[Al Skierkiewicz]

Jon,
What you use for current shunts is really a matter of how you plan to make the conversion. #10 wire is approximately one milli-ohm/foot (1x10-3 ohm)if you can read anything useable across that low a resistance. Digi-key has some current sensing resistors that you can use or you can make your own for whatever circuit you are designing. Shunts are not really rated in watts at 12 volts because there are really small voltages in these operations. Ohm's Law for a .01 resistor at 100 amps would result in only 1 volt. However, Ohm's Law solved for power is 1 volt * 100 amps= 100 watts. Now one volt is a pretty nice measurement for general purpose measurements, but you wouldn't want to drop a full volt to measure current. This leaves a maximum available to your motor of only 11 volts.
For anyone contemplating this type of sensor, please remember to adequately insulate (temp AND electric) whatever you use.
Good Luck All,