Re: how do determine copper wire thikness

Wire is sized by it's current carrying capacity.  Dividing the wattage by 
the voltage will give you the required amperes.  As to the actual wire 
size needed, that is more a local law than a science and, it varies from 
country to country.  If you can contact your local Electrical Engineering 
Society, they can tell you where to find that information.

But, as an example, in the US, standard copper wire can be anywhere from 
0.03 to about 75 mm.  Normal wire sizes used in household wiring are a 
#10, #12, and a #14 - these sizes set by our National Bureau or Standards, 
and the larger the number the smaller the wire.  A #10 wire is 2.5 mm in 
diameter and will safely carry 15 amps.  A #12 wire for 10 amps, has a 
diameter of 2 mm.  And, a #14 wire, with a current carrying capacity of 
about 6 amps has a diameter of about 1.6 mm.

These values are conservative and can be increased by the use of better 
electrical insulation, or when the wire is not used in a very hot 
environment.