Track Wire Gauge selection at a minimum must take into consideration the acceptable voltage drop for the given current. The minimal acceptable standard for track wiring is that a short circuit anywhere on the layout on every single piece of track will result in the booster or DCC system shutting down until the short is removed. Any time there is not DCC power shutdown when a short circuit is present, represents a high heat (melting of track ties) of worse a fire hazard (Burning of wires in the locomotive or the track bus wires themselves). I am not making this up. Its not a what if. Its happened.
For new layout wiring. wire gauge considerations would also take into account the Best Practices on how connections are made and standardize a just a few wire gauges required to wire the whole layout.
For more information go here:Wiring: Standardizing Color and Connections
WIRE DEFINITIONS
WIRE: A single wire that carries current.
CABLE: Two or more wires. It may or may not have a outer insulating jacket. A twisted pair of wires form a cable.
BUS: A cable the performs a specific electrical function.
DCC WIRING DEFINITIONS
TRACK BUS: A pair of heavy gauge wires that carries the booster current to both rails of the track. Since there are two rails, there are two wires. A 30Ft track bus run has 60 feet of wire in it.
TRACK FEEDERS: A short wire, usually in pairs, that is a smaller gauge of wire than the track bus. It is used to connect the heavy gauge track bus wire's power to the small rail of the track. It small size is to make the track feeder connection to the rail as invisible as impossible. Because of it very short length and multiple track feeders work together, the use of the small gauge is not a major electrical consequence.
DC RESISTANCE CONSIDERATIONS
All wires have resistance. The resistance is a mechanical function of:
a) Wire gauge (diameter of the wire). For a given length of wire, the HIGHER THE GAUGE number gets the SMALLER DIAMETER the wire gets and consequently the higher the resistance it has.
b) Wire Length (How long the wire is end to end.) For a given gauge of wire, the longer the wire gets and consequently the higher the resistance it has.
Wires have voltage drops when the carry current. Ohms Law. Putting this together:
For a given current and a given wire length, the smaller the wire gauge the higher the resistance and consequently the voltage drop.
For a given current and a given wire gauge, the longer the wire length the higher the resistance and consequently the voltage drop.
For a given wire gauge and given wire length, the higher the current gets the higher the voltage drop.
All three of these factors conspire together at the same time.
Voltage drops in wires are something you wish to minimize for it reduces the reliability and performance of the DCC system.
As the voltage drop goes up on wires that carry track current, at some point it will be so great it defeat DCC systems short circuit overload protection. At the extreme, the wire itself burns up when there is a short. This is why one talks about using heavy gauge wire for your track power wires.
AC RESISTANCE (IMPEDANCE) CONSIDERATIONS (Large Layouts)
This section really applies to large layout of layouts with high current booster above 5Amps.
Wire also have capacitance and inductance properties of wires that become know when AC power (current) is flowing in them. (DCC is a special form of AC current).
To learn more about wire properties go here: Electrical Properties.
Of the two properties, Inductance and Capacitance, the inductance is the one of concern when long DCC track bus wires are run.
When the wire gets longer the 60 feet (30Ft of cable) the inductance become large enough that under short circuit conditions, it can create a voltage spike large enough to blow up a decoder.