As others have said, the required wire size is a function of the current, length of the wiring run, voltage and the acceptable voltage drop over the length of the wires. The formula for calculating the maximum length of a given size wire is:
L = (5 x x% x V)/(I x R)
L is the maximum one way length in feet of the circuit
% is the acceptable voltage drop in the wires (normally 3% for lighting)
V is the voltage of the circuit (I use 12.5 for 12 volt systems)
I is the current to be carried
R is the resistance of 1,000 feet of wire.
Resistance of common wire sizes per 1,000 feet are
20 gauge 10.15
18 gauge 6.385
16 gauge 4.016
14 gauge 2.525
12 gauge 1.588
10 gauge 0.9989
The formula will tell you the maximum length of wire that you can use before the voltage drop goes over the desired percentage. Note that this formula is based on the one way length of the circuit in that the round trip length is built into it. The formula is based on electrical principles only and NOT ABYC suggestions.
Finally, the circuit breaker for the circuit should be based on the rated ampacity of the wire and the circuit should be fused based on the current requirements of the components the circuit supplies. So a circuit made from 16 gauge wire needs a 13 amp or less circuit breaker - typically a 10 amp breaker.