Amtech is one of the top electrical calculations software packages out there, but very expensive. Solutions electrical is accurate, but not easy to use (can’t even copy paste a circuit on the version I use). There is also ID-Spec from Schnieder for basic distribution design.
Re copper Bus Bar capacity it’s a peace of copper and you can use the same calculation as for cable calculations.
Use IET guidance notes for electrical designers for electrical design calculations formulas. Well they’re good for UK engineers anyway. Good luck