AVmechaniCalc, the Aviation Mehanic's  and avionics technician's software, will eliminate all the AC43.13B formulas and chart interpolation so awkward to do...

Enter your desired amperage, run length, system voltage and desired voltage drop and get the gauge needed.  

You can find requirements to 0.1 amp and 0.1 feet for best efficiency, unlike the charts in the 43.13B, where you're

 lucky to interpolate to 1 amp and 1 foot.  Then, enter all the required derating factors and get a fully derated wire in seconds with no algebra.   You can build a wire bundle very conveniently as well with the wire bundle builder.  Click on the "?" in the form below to view instructions for this worksheet.