When considering the appropriate wire size for a circuit, don’t forget to take the circuit length into consideration. Electrical wire, no matter what material it is made out of, has a certain resistance to it. With an increase in cable length, the resistance causes a voltage drop in the circuit.
The solution is easy enough. You should just choose the next sized larger ampacity wire size to lessen the effects, for instance, on a 15-amp circuit, you would normally use 14-gauge wire to supply the circuit. However, if the run is over 50′ long, you could use a 12-gauge wire instead which is rated for a 20-amp circuit.
Likewise, a 20-amp circuit that normally is run with 12-gauge wire, should be run with 10-gauge wire if the the run is over 50′ long.
Voltage drop on wires due to resistance causes hotter wires and less voltage to run the devices attached to it. You may have noticed the same effects of voltage drop when using an electric drill that is attached to a long extension cord. The drill still runs, but is often slower, with less torque, and tends to heat up more quickly. Voltage drop has a devastating effect on motors.
Planning ahead for circuit size, the distance it must travel, and what is going to be connected to it will save you money and frustration later.