Calculating Busbar Ratings

Hello Guest,

The incomer Transformer Rating should have a Standard Rating from the Makers.

It should also have a "Short Circuit Current Rating" on the secondary windings, also from the Makers.

But that Short Circuit Current Rating also depends on the Fault impedance of the Supply to the Primary of the transformer.

Then you need to calculate the Short Circuit Fault Current in the secondary connections and cabling from the transformer to the bus bar connections in your switchboard.

The cabling impedances can be calculated from cable tables, as supplied by the cable manufacturers.

So...In practice you may have a transformer rated at 31MVA (Standard), with a fault rating of some 10x that Rating = 310MVA. (That's a lot of energy, dissipated in a very short time)

Add that with the Supply Network impedance/fault current, and the secondary impedance/fault current, and you often finish up with extremely large Prospective Short Circuit Currents.

These huge but short-lived PSC currents, are capable of great destruction, due to "violent and sudden heating", plus the mechanical forces caused by large magnetic fields associated with the very large PSC currents.

It is impractical to to try and calculate for your situation, without all the data necessary to complete the calculations.

I don't know where you live, but if you actually do have a problem locally, your local Power Authority Engineer is normally able to help with a practical calculation, since he/she has access to the figures required for the calculations....

In general The Incomer Breaker and Bus bars are to be design for full load current of transformer irrespective of downstream load e.g. if rating of transformer feeding the switch board is 1500 kVA, 415 V, In this case the FLC is 2100 Amps. It is therefore suggested for 2500 A ACB on the incomer and Bus bars are to designed for full load current of transformer i.e. 2500 A.

but Sizing of Bus bars depends on ambient temperature, enclosure sizing, location of the switch board and also material of bus bar like Aluminum or copper, Short circuit with stand capacity of Bus bar, disposition of bus bar and number of bus bars and more.