Short Time rating of the bus bar

[sramesh]

I am preparing the specification for LV switch board with 2 sections connected by a Bus coupler, rating of 415 V , 4000A fed from 2.5 MVA 6% Transformer(2 Nos, one for each section). normal mode of operation is 2 Incomers ON and Buscoupler OFF. There is an option for Synchronising both the section and closing Buscoupler Breaker to have the facility of Isolating one of the Transformer for maintenance with out interruption. I would like to know how much should be the BUS BAR short time Current rating ie Calculating the short time current rating considering only one Transformer source or two Transformer Source ( Since 2 Transformers will be feeding to the BUS during SYNCRO mode operation , ofcourse only for a short duration). Kindly give your valuable opinion that will be helpful for me.