Bus Bar Rating

When in doubt, Google.

http://www.stormcopper.com/design/ampacity.htm

The rating of the Bus-bar has to be typically 50% more than the expected steady state current that might flow as per facility design. This is done to ensure minimal distribution losses and capability of the network to provide inrush currents that the load might draw during start-ups as well as during clearing of the faults downstream

can we get any authentic document reference (any STANDARDS-BS OR ANY OTHER)to substantiate this statement.

Uh... the bus bar rating must be higher than the incoming breaker. Otherwise the breaker does not protect the bus bar. The breaker has to trip at a current level lower than the bus bar can carry to prevent the bus bar being damaged by elevated current levels. This is based on plain old common sense. Here we use 125% but other places may use a different factor. we size a conductor be it bus bar or cable to carry 125% of the load current then select a breaker size or set the protective relays to trip below the conductor rating but above the load current if we can not find a suitable setting or rating then we will increase the conductor size a notch or two and look again.