Hello again,
Calculate the diameter of the wire (or look it up) in inches from the AWG wire number N:
D=92^((36-N)/39)/200
Calculate the area of the wire (or look it up) in square inches from the diameter D:
Aw=0.25*pi*D^2
Calculate the current density knowing the wire area Aw and the current A:
J=Aw/A
J is in units of square inches per ampere.
You can also calculate the circular mils of the wire from the diameter D:
Acm=D^2*1e6
A typical straight run wire circular mils would be something like 350 circular mils per amp, but for a transformer you would want much less.
For example, a AWG wire number 10 wire has diameter 0.1019 inches and area in square inches of 0.0081548, and area in circular mils is 10383, and at 30 amps that would be about 346 circular mils/amp.