Power over ethernet uses 48V because that is the highest voltage that can be used without

legal considerations of high voltage equipment. For long distance runs you want to send

power at a high voltage and low current otherwise wiring losses will rob you of your power.

The cable resistance goes up linearly with distance, so want to increase voltage by the

square root of distance - ie a cable good for 5m at 5V/2A would want 50V/200mA for 500m.

(ten times the voltage for 100 times the distance).

Think of the supply as a resistance - 5V at 2A is 2.5 ohms. Your cable wants to be a lot

smaller than that to keep losses down to a few percent, perhaps 0.1 ohms or less. 100 times

the cable length would then be 10 ohms or so, competely hopeless for 5V/2A. Cable of 10

times the diameter and 100 times the length would work, but thats 10,000 times as much

copper, which isn't economic.

So basically you want DC-DC converters at each end, preferably 5V->24V or more, and back

down at the other end.