How can you accurately calculate fault current on a finite bus by hand?

[danielandham]

so calculating FC on an infinite bus is easy but how do you calculate it by hand for a finite bus?

here is the concept i am having an issue understanding:

let say i have a transformer that is 225KVA with 5%z and 3 phase 480 to 208.

with infinite bus i have 12491A available on the secondary of this transformer IF 12291A is flowing on the secondary then based on the voltage ratio and OHMs law 5412.8 A is flowing on the primary.

If this is true then what does it matter if i have infinite amps available on the primary or i have 5412.9 amps available 12491A should still flow in my mind.

I have run this in SKM and i see that it does matter but i just don't understand why.

 

[meBigGuy]

Let's talk in an ideal world. If you have no load, no current will flow. As you add a load to the secondary, current will flow based on the secondary voltage. Since power in == power out, then equal power will flow in the primary and you can calculate the primary current based on the primary voltage. So the key is power in == power out. Your source merely needs to be able to supply the required current.

I'm not sure where ohms law comes in since you aren't talking resistance yet. I don't quite understand your infinite vs finite either. Am I missing some basic assumption?