Busbar Sizing for Panel

Let's calculate the BUSBAR size of the panel together with the short circuit

Data Available :

Transformer, 11/ 0.415 kV , 100KVA

Transformer Secondary Current = (Transformer Rating /1.73*Secondary Voltage)

Current = (100KVA / 1.73 * 415V)     = 139.12 Ampere

So we can select the next standard higher size available i.e = 250A BUSBAR

"The breaker Size will be 250 Ampere as well"

Here Impedance for the transformer is 4%

The short circuit current for the BUSBAR would be :

Isc = Transformer Secondary Current / Impedance

Isc = 139.12 / 4 %        = (139.12*100) / 4     = 3478 Ampere 

Isc = 3.478kA

In the case of a 2 Transformer or stand-by transformer, we need to add up the Secondary current of the individual.

Few illustration Pictures for reference.