When I did LV landscape lights, I would always run 10-2 LV wire to help with the voltage drop. Even so, you would still have to calculate distance, # of lights, wattage of lights and voltage output of transformer. The lights I used where Hadco lighting and their transformers had several output voltages. You could crank it up (If I remember correctly) maybe 2-4 volts higher so that your output was 14-15 VAC.
