Here are some probable useful web pages on cable selection:
Wire Gauge and Current Limits
http://www.powerstream.com/Wire_Size.htmCable Calculator
http://www.epanorama.net/index.php?index=calc_cableThe idea is that you first select the cable size according to the Current Limits for different cable thickness.
This is the smallest size cable you can use. You are free to use thicker if other things need that (that particular thickness not easily available, too much voltage drop on your application, maybe mechanical strength considerations on very thin wires etc..)
Then with the selected cable size you plan to use do the calculation for voltage drop you get. If that value is acceptable for you application you have right cable. If the voltage drop is too high, try with the next thicker cable available and calculate if the voltage drop is now acceptable.. If not then select the next thicker one.
Then the cable for the application has been selected then the next part is to do the necessary calculations for the proper cable protection (fuzes, breakers etc..) that protects against overload and short circuits properly according to the regulations.
My advice is just general rules of thumb for cable selection. Consult your local or national electrical code or your electrician to decide what is legal! Consulting an electrician might be a good idea in every case when selecting cable sizes, because selecting a thicker than necessary cable for the application could be much more expensive than selecting the "optimal" cable + paying some money for a professional for consulting.
