Hey, Andrew Rasken.
Jon C. pretty much summed it up for you.
There is no "Code" within the NEC (National Electrical Code) that I am aware of that dictates where the change should occur.
There is no mathematical formula that decides where, for example, a 12 volt nominal system should stop and a 24 volt nominal system should start and so on.
But think about it this way...
If you have a 5000 watt PV array wired for 12 volts nominal you will have to provide ample size conductors and supporting electrical devices like fuses, breakers, charge controllers, and all in order to safely utilize a total of 400 amps, give or take.
That same 5000 watts at 48 volts nominal would equal only 100 amps give or take. This translates into big $$$ savings.
IMPORTANT NOTE!(These are not real world examples of an actual PV system. These are examples to help illustrate the gist of Ohm's law.)
Its not just for PV arrays ether. This applies to all electrical installations from the biggest nuclear power plant down to the battery in kids small toy. If you would like to learn more, just search - Ohm's law and then price the difference between 500 mcm and #2 - copper wire per foot.
Another important fact that Jon C. pointed out is power loss. When speaking in terms of conductors, such as but not limited to wire, Lower voltages suffer a much higher rate of power loss over a compareable distance to a higher voltage. This is the sole reason that Tesla won out over Edison in the AC vs. DC war.
Alternating currents can be transformed by way of inducing the current flow from one set of windings (pronounced: wine-dings) onto a different set of windings because it is constantly going from, pos. to off to neg. to off and so on. Think about those high tension power lines you see all over the country side and in the cities. Chances are the are carrying some where around 200,000 vac more or less but by the time it gets to a typical home its been transformed several times down to 120/240 vac. Each time the amperage increases. (Not taking into account the number of "taps" for a multitude of homes and business's or industries.)
An example of this in a Direct current can be found in the ignition system of earlier automobile engines. One set of windings inside the coil receives 12 vdc through the closed points but when the points open (turn off), the electric field inside the coil collapses and is induce on another set of windings that provide nearly 10,000 vdc potential to the grounded spark plug gap. But it is at a much lower amperage than the 12 volt line side.
If your really interested in an RE system for your home or whatever, I can tell you, one of the best investments that I made was a subscription to
http://www.homepower.com/home/ it had the greatest payback value of all in my opinion.
