Somebody correct my if I am wrong...
Votage drop is pretty easy to calculate (the resistance meathod is more accurate.. but this is close) 2 x the length x amperage (load, not nominal amps) x constant (=12 for copper & 18 for aluminum) / (diveded by) cm of the wire (larger wire cm is the wire size). Size the wire for voltage drop; the bus bar 'length' is so short, you shoul be able to just use the standard rating for that amperage.