How to find bus bar size for current carrying capacity and how we can find bus bar area

It is basically the current carrying capacity of the conductor with which the bus bar is made. There would be derating factors applicable based on the arrangement etc. Bus bar size verses current carrying capacity table should be available to you on the internet. 

Cross section of bus bar gives you the area of the bus bar. ( h x w)

Bus bar current carrying capacity depends upon the material with which is made of and ckt design. The accurate values can be find with manufacturer catalog s..

Dear..

First we should calculate the maximum current of bus-raiser, then we select the cross-section of the bus-bar depending on the rating of current , for example if the rating of bus-bar riser is2500 amp , the cross section should be1250 mm2..