If you are doing O or S scale trains I would not use anything smaller then 18 gauge wire. Even 18 gauge wire is small depending on your transformer output. The old post war Lionel ZW's could quick fry an 18 gauge wire with a short.
With some of the new transformers(Lionel ZW-C & ZW-L, MTH Z-4000) on the market even 16 gauge wire is too small for them. These transformers are putting out close to 300 watts or more and at 20 volts, do the math; 300 divided by 20 equals 15 amps, so a 14 gauge wire would be needed to be safe. 12 gauge wire, maybe even 10 gauge, would be needed to be safe with the new Lionel ZW-L rated near 700 watts.
If you are doing H.O. then 20 gauge wire should really be the smallest.
Lee Fritz
