Ed says:

There are too many variables that haven’t been addressed like total length of track, distance from power entry point(s), load of motor(s) and accessories, current output of transformer, etc. that can affect voltage drop along with amperage (current draw) to give an exact answer to your question. Bottom line, bigger wire gauge is better within reason… 1000 kcml I think would be a tad overkill however ;). Sorry I digress, but really don’t be thinking smallest you can get away with, think more on what is the largest you can fit or hide and use smaller gauge short taps to the rail. I have never thought about the mil area of say an HO rail, but it looks close to #12…maybe, give or take, and has handled all the power needs of moderate size layouts since…forever, but I’ll bet there is a book out there where someone calculated the circular mill of different code rail if someone really wanted to know. Sorry, I don’t. So, all that said, if your track lengths are fairly short, #14 feeds should be fine. If you are going, say over 20 feet, you may want to up it to #12 or #10 even #6. There comes a point that no matter how large the wire or DC current input, the voltage will drop off, due to inherent natural resistance of the wire it is traveling in. So, if your layout is that big/long, you will want several transformers placed throughout. Also use stranded wire if you can, the higher strand count the better, it will perform and handle higher current loads, run a bit cooler, and pass signal better than solid especially in DC/DCC circuits. However, even that said, if all you have is solid wire, it will work just fine too in the same sizes as above, but voltage does tends to drop off quicker in solid wire. I hope this helps a little, good luck.