There is no fixed rule for what gauge of wire to use for track power or accessory power. It all depends on the length of wire and how many amps it will carry. A voltage drop calculator (Google it) can be used to figure adequate wire gauge for a specific application.
Good point. When you look at the charts, multiply the resistance listed by the expected amps on the wire and determine if you can stand the voltage drop. The voltage drop for our low 18 vac circuits are more critical than the 120 vac some of the charts' recommend gauges.
In other words, a 2 vac drop on an 18 vac circuit is more significant than a 2 vac drop on a 120 vac circuit.