If you haven't seen it, this site has good information on voltage drop/ wire sizing and a handy calculator. I'm still learning this stuff for a project on my boat, so don't take my advice as worth anymore than you paid for it.
American Wire Gauge table and AWG Electrical Current Load Limits
Based on my quick estimations a 5 or 10 foot one-way run should be okay with 1-0 cable (1 and 2% voltage drop). A 20 foot run would be closer to 4% and you may want to upgrade cable if that is the case. I don't know how much the buss bar adds to the voltage drop calculation if at all. For the AC side of the circuit with an ability to surge out 7500 watts, the drop is even less.