what gauge wire for a bus wire

Since voltage drop is also a concern, courtesy of the CRC Handbook of Chemistry and Physics here's a table of resistance verses wire gauge. The wire type is assumed to be annealed copper wire, and resistance is given in ohms per thousand feet at 20 degrees celcius(68 degrees farenheit)

Gauge.........Ohms per Thousand Feet

10................. .9989

12................  1.588

14................  2.525

16................  4.016

18................  6.385

So, with a length of 50 feet, 16 gauge wire would have a resistance of about .2 ohms. Since V=IR, with a current of 5 amps, the voltage drop would be just over 1 volt. If you go to 14 gauge, the voltage drop with 5 amps becomes .7 volts. Go to 12 gauge wire, and this drops to .4 volts. 

Thus, with drops this long, you can see the necessity of using the largest gauge wire feasible. 

By the way, if you wanted to limit voltage drop with a 5 ampere current to .01 volt, essentially zero for our purposes, you would need to use 0000 gauge wire.