The primary factors are
- transformer voltage and kVA
- conductor size and distance to the panel
- possibly load contibution downstream
The basic answer can be had from a table. A more precise answer is obtained by a calculation method. Several other threads in this forum have contributions on this topic. Have you tried a search?
If you are looking for the available short circuit current at a given point, you need to know what the available current from the utility is. You also need to know the SC current available from the motors in the system.
I suggest you use a good power system analysis program. Wirenut
Suggestion: Please, provide the following:
1. Transformer kVA
2. Transformer Voltage
3. Transformer Impedance
4. Transformer Short Circuit MVA, if known
5. Cable and or bus size
6. Cable and of bus length
7. If there is a reactor, then its parameters
8. Panel number of phases
For a quick, dirty method, just divide the transformer rated full load in amps by the transformer impedance at that load, expressed as p.u., instead of percent. The actual available fault will be somewhat less than this number because of the impedance of the source and low side conductors, etc. But this gives you a number that lets you know if you need to do more study. For your case, 3000 A divided by assumed 5% impedance gives 60,000 amps. WOW!!
For more quick and dirty methods, add up your motor full-load amps and multiply by 4 to estimate short-circuit contribution from motor sources. This can be slightly refined by using 4.8 times FLA for motors 50 hp and above and 3.6 times FLA for motors smaller than 50 hp.
