Hello Steve.
In regards to your question about PoCo's Pole Mounted Transformers / Pots, and even PMTs (Pad Mounted Transformers);
They are normally the lower KVA ratings in Residential areas - mainly due to the fact that Residential Loads are rarely both high and continuous (even with heat pumps running).
They (Heat Pumps) will eventually cycle off before the 180 minute "Limit Line" is reached.
As to the SCA (Short Circuit Amperes) of a 25 KVA 1Ø 3 Wire Transformer with a Secondary Voltage rating of 120/240VAC, here's a "Worst-Case Scenario Figure":
25 KVA 120/240V 1Ø 3 Wire, Z = 1.58%:
SCA at the Transformer's Terminals would be 11,574 Amps.
PoCo Service Feeders & Secondary Distribution Feeders on Overhead Service Connections, are typically Aluminum (typical of areas using Triplex Cable to the Service Riser) - or at least Copper Clad Aluminum (in older areas, where individual Conductors are strung between poles and to the Service Riser).
Also, the Conductor sizes are small - as compared to what we who use the NEC would consider to be "Proper Size".
The PoCos use Ampacity Tables from the NESC to size Service Feeders - and the Load Calcs generated for a Project is what the Service Design Engineers base the maximum Load of a Customer's Service on.
Computed Load figures are what they use to find minimum feeder conductor sizes - so even if the Service Equipment has a 400 Amp Maximum rating (320 Amp continuous), if the Load Calcs show a Load of, say - 230 Amps, the PoCo will install something like 2/0 XHHW Aluminum for the Ungrounded Conductors, and maybe a #2 XHHW Aluminum for the Grounded Conductor.
Conductors strung between poles are something like #2 Aluminum I believe (need to verify this).
Using these small sized Aluminum Conductors greatly affects the total Fault Current between the Transformer and Customers' side of the kWh Meter.
Adding to that, the distances of which the Service Conductors + Secondary Distribution Conductors are run, has an additional affect on the overall SCA.
Now apply this to the 11.5 KAIC rating at the 25 KVA Transformer, and the SCA becomes a 2.0 KAIC fault rating at the Service, for a Customer located 4 Houses away from the Transformer.
The Residential Distribution Feeders will help limit the SCA at the Customers' end, due to the Line Impedance.
If a Bolted Fault condition occurs, this will load up the Distribution Conductors, and will result in a considerable drop in System Voltage.
As the Voltage decreases, so will the sustained Fault Current, so this will also affect the total let-through value for a fault condition.
In the Neighborhood where we live, there are 11 Houses (one of which is my wife and me) on a 37.5 KVA Pole Mounted Transformer.
Secondary Distribution Conductors are individual #4 RHW Copper-Clad Aluminum strung between Poles.
Service Drops are Three individual #6 RHW Copper-Clad Aluminum Conductors.
Average Square Footage of each House is like 1,500 Sq. Ft.
We are the last Customer on this Secondary Circuit - a good 400 Feet from the Transformer.
If that Transformer's Secondary is 1.56% Z (Impedance), the SCA at the Transformer Terminals would be no more than 17,400 Amps.
I would imagine an L-L Bolted Fault at our Service's Load side would result in maybe 4,000 Amps at the very most.
Scott35
