RRAddict:
One thing to think about. With respect to current it is not the size of your layout but the size (power) of the transformer you are using and with respect to voltage drop the length of the wire runs. If your transformer is pumping out 7-10 amps, 20 awg or greater may start overheating on you. You will most certainly experience a voltage drop from the transformer to the track which means you will need to crank up the transformer voltage to get proper track voltage. You may want to try this. Figure out the longest wire run you will have and cut pairs of 20 awg versus 16 awg wire to that length. Attach to your transformer and to the track and measure the voltage at the transformer versus the voltage at the track and see if there is any difference between the two gauges.
As part of writing this post I did a goggle search on wire current carrying capacity and have to admit I got confused. I saw anywhere from 4 to 11 amps for 20 awg. The 11 amps may be for short runs on chassis boards but this is way outside my area of expertise. If you still wish to use 20 awg wire I would suggest running 12-16 awg from the transformer to a terminal board then making shorter runs of 20 awg to track just be safe and error on the side of caution.
As Susan points out the thin wires are used for turnouts and lighting where current draw is lower or intermittent (on-off for turnouts). The heavier wire is for track power where current draw is continuous.
Let me know how things turn out
Joe
