Are you familiar with the water pipe analogy regarding current flow? The one that states that for a given pipe size, let`s say 1/2", and a given pressure which we can abritrarily state for the purpose of this discussion to be 10psig, we will get a flow of maybe .75gpm. If we increase pressure to 100psig we can get maybe 20gpm through the same 1/2" pipe. Can everyone see the relationship here?
So if we equate current flow to fluid flow and voltage to fluid pressure, the larger our flow requirements are, the higher the pressure we need through any particular conductor size, whether it be a pipe or wire. If pressure is not variable, the only thing we can increase to get more flow is increase the pipe size, same thing with wire, if the voltage is fixed at 12 we can`t get the same ampacity in a conductor that we would get if the voltage were 120.
All that I am trying to illustrate is that voltage is relevant to current flow through a particular wire size and I don`t mean to direct my statements to anyone in particular although I did quote gemini5362 merely as a point to jump into this discussion.
The notion that the primary difference between 12v conductors and 120v conductors for a given amperage is in the thickness of the insulation is misguided. With a dielectric strength of a few thousand volts per mil of insulation, insulation thickness is not relevant to this discussion. Composition and temperature rating of the insulation may be, but for other reasons related to the service environment of the installation.
Another thing, the chart from West Marine is misleading and easily misread if you aren`t careful to keep your colors in the proper order and the chart below it that suggests that 16 ga wire is sufficient for 25 amps does not delineate between momentary and continuous loads. A better chart can be found here:
American Wire Gauge table and AWG Electrical Current Load Limits which shows that 16ga wire is only good for a continuous load of 3.7 amps.
Hope this helps.