70E Arc Flash Category Method?

[TwoBlocked]

Been reviewing 70E 2024. Was looking at table 130.7(C)(15)(a) Arc Flash PPE Categories for Alternating Current (ac) Systems.

There are division in the PPE Category depending on the available fault current of 25kA, 35kA, 42kA, and 65kA. I was taught long ago that you can determine the kA of a transformer by dividing the kVA by Z (impedance), divided y the V (voltage), wait is that right? I now understand that is not accurate, but seems the only reasonable rule of thumb when out in the field. That is, when there is a transformer that you can get to the nameplate.

But what about pole transformers? I am looking for a rule of thumb for determining the kA of pole transformers when out in the field. Maybe by the physical size or other characteristics like cooling fins?

And folks, I am not looking for "All I ever do is ... and it hasn't been a problem." type answers. OK?

 

[TwoBlocked]

Found a link with the kva to kA formulas: https://www.se.com/us/en/faqs/FA105006/

 

[jim dungar]

Taking the FLA/%Z gives a fault current based on having an infinite source. This is a wonderful method to use for determining the maximum SCCR and AIC, even on pole style transformers.
However using the PPE selection tables in 70E requires you to know the protective device clearing time based on the actual fault current not on a fictional infinite value.
It is amazing how relatively short a run of conductor needs to be to keep a fuse out of its current limiting region, which results in an increase in Arc Flash Incident Energy.