No simple answer to that, I'm afraid. Difference gauges of wire for different parts of your layout. I'm not a good electrician however, so I'll refer you to the model railway portion of these forums.
http://www.trains.com/trccs/forums/88/ShowForum.aspx
Go here for my rail shots! http://www.railpictures.net/showphotos.php?userid=9296
Building the CPR Kootenay division in N scale, blog here: http://kootenaymodelrailway.wordpress.com/
I'll also reccomend the Model Railroader fourum. This one's for......actually, I don't really know what this is for!