More fun with available fault current

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
If I know the available fault current (AFC) at a 480/277V panel, and I power a 480V delta to 208/120V transformer from it, what is the effect of the transformer on AFC at 208/120V?

I have an electrical drawing for the building that shows the AFC for the 480V panel and for a similar transformer fed from it. It shows the AFC for a panel connected to the secondary of the stepdown transformer as less than 1/3 of the AFC at the 480V panel; and I am trying to make sense of it. I am guessing that this is due to the %Z of the stepdown transformer; am I on the right track?
 
The available fault will also be proportional to the transformer's full load current, as well as to the inverse of its %Z.
 
The formula I have to calculate AFC for a transformer is based on the infinite bus model, but in this case I know what the AFC is on the primary and it is not infinite. Could accounting for that that affect (reduce?) the AFC on the secondary? If so, how would I bring it into the formula? My semi-educated guess is that there will be some threshold above which it won't make any difference.
 
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