The biggest part of arc flash study is the data collection. However, we don't know what fault level it is before we do the analysis. So, what the category of clothing we need for this kind of work? Can we just refer to NFPA 70E table or other guide?

Thanks!

Great question. Someone, at some point in time did an SC and PDC study, see if you can get that info. Or you can look at the AIC ratings of the equipment and safely assume the available fault currents are less than that to see if you can use the tables.

Say I look at the AIC rating on 480V panel and find 65k. For many tasks found in the tables, Note 1 is called out stating that the PPE level provided is suitable for available short circuit current less than 25k.

If I want to take the cover panel off, do some thermography, and replace the cover, what PPE do I wear? It appears to be I can't use the tables!

Probally not, but AIC is not the same as available fault current as I am sure you know. You need the available fault current and clearing time of the protective device to determine if you can use the tables.

Now if only getting that available fault current information was made easy

That would spoil the fun. You can do some simple calcs based on the suppling transformer.