It depends on the size of your layout and the length of the wire. The large postwar motors might need larger wire for the startup current. If you protect your accessory circuits with PolySwitches you can try different amp ratings for the load. My large accessories are fed with #16 wire. The low load stuff is fed with thermostat wire, etc. That Wire Sizes page shows uses for each size wire toward the end of the page.
Two of my operating accessory circuits have been raised from 3A (650-RUEF300) to 4A (650-UEF400) circuit protection. With several people operating different accessories at once I had to up the rating. The chain drive coaling tower and log loader together need the 4A.
For my Z-stuff switch machines circuit, I use 2 1/2 A 650-RUEF250.
