circuit breaker size

You need to be more specific is it for primary or secondary side and what are the voltages involved.

Assuming this is a single phase transformer, if you have the breaker on the primary side, the largest current rating for the breaker would be 1 MVA divided by the input voltage.

If you are installing it on the secondary side, the current rating would be the lesser of the current carrying capacity of the wires between the transformer and the load, or 1 MVA divided by the secondary voltage.

Without knowing the voltages involved or the transformer's overload tolerance characteristics, I can't tell you anything about the breaker type. The transformer manufacturer probably has some specific recommendations.

Hi Guest,

Do you really expect a sensible answer for your thread ? If you are a real technical person connected with Electrical Engineering you would not have asked weather any one knows? If any one does not know how all these substations work? If any one knows why he/she should bother to waste his/her valuable time to tell you that? The decency is to put a question in civilised manner. Best of luck if some one can answer your thread.

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He needs a Size 7-3/4.

If you want a simpler approach, you can use the infinite bus method wherein you will only use the impedance of the transformer to compute for the maximum fault current.