Three questions you should figure out are:
- How long is the longest wire run?
- How much current will the wire carry?
- How much voltage drop can you tolerate?
Lets assume the longest run is 20 feet, you intend to run up to 10 Amps and you can tolerate a 1 Volt drop. From the wire table in the wikipedia link below, 14 gauge wire has a resistance of 2.525 ohms per 1000 feet, or 0.002525 ohms per foot. Since we have a feed and return wire (two wires) our total wire distance is 2 x 20 feet = 40 feet. Total resistance is 40 x 0.002525 ohms = 0.101 ohms. From ohms law we can calculate the voltage drop. V = I x R. Voltage drop = 10 Amps x 0.101 ohms = 1.01 Volt drop. Moving to a larger gauge size like 16 will increase the voltage drop due to less copper area (more resistance) of the wire. Moving to a smaller gauge size like 12 will decrease the voltage drop due to the larger copper area (less resistance) of the wire. It is also importance to buy wire that is rated to handle the voltage you are running through the wire. Its a good idea to stay away from low voltage wire (12V) intended for cars. Your trains run on around 20VAC with a peak voltage of around 28V. Buy wire with a rating higher than the peak voltage you plan to run (Peak voltage = AC Voltage x 1.414). One last thing to consider is to make sure you get stranded wire with a moderate strand count (solid wire is not very flexible).
Source for wire:
http://www.mcmaster.com/#stand...ctrical-wire/=u9o9fv
http://en.wikipedia.org/wiki/American_wire_gauge
http://en.wikipedia.org/wiki/Ohm%27s_law
I'd like to add another thing. All wire has a limit to how much current it can carry. Basically the current flowing through the wire times the voltage drop gives you the power dissipated on the wire in watts. This power dissipated heats the wire. The insulation on the wire also has an operating temperature rating of typically 105*C. The insulation is what prevents two wires of opposite polarity from shorting out and potentially causing a fire. Running excessive current through a wire can also cause a fire even without shorting out. So how much current should you run in a wire? Well you can Google "wire ampacity chart" and find various charts giving guidelines for a wires current carrying capability. I found a few different charts with different ratings on 14 gauge wire. This may be due to different standards used to calculate the data in the charts. Maybe someone else will chime in here?
While I'm rambling, poor connections at the ends of the wires create voltage drops and can melt things and even cause a fire. Be sure to crimp or solder your connections well. I'm sure you can find some good examples on Google.
