Always start your calculations with 12-gauge wire. The voltage at a fixture is the actual voltage supplied (12 Volts) minus the voltage drop. The voltage supplied may be increased by use of a multi-tap transformer. These multi-taps have alternate voltage sources to counteract a long cable run with too high of a voltage drop. Another way to counteract a high voltage drop is change to a thicker wire, such as 10-guage. If you need more voltage drop than you are getting, you may change to a thinner wire, such as 14-gauge, but a thinner wire supports less wattage.
Here is a chart to determine the maximum wattage allowed for each buried wire.
|Gauge||#18 GA||#16 GA||#14 GA||#12 GA||#10 GA||#8 GA|
|Max Wattage||120 W||156 W||180 W||240 W||300 W||480 W|