Our local Utility providers often respond to requests for info with design letters that indicate the upstream transformer(s) and conductor sizes, lengths, and number per phase, along with their calculated AFC at the point of demarcation (usually listed as the meterbase).
Here's the thing: No matter what I do, I can never get anywhere close to that AFC value using the information they've given. Typical Utility-provided AFC values range from about 20-32kA for a standard commercial facility. However, even when I assume infinite bus, when I add the Service transformer and incoming cables to the mix, it often drops the AFC at the same point of demarcation down to less than 10kA.
Any idea why this might be? And I know this is similar to other questions I've asked before, but am I better off using their AFC, or modeling the equipment they have? Any idea how I could replicate their numbers while still modeling the equipment they've disclosed?
I've provided an example below. When I model the equipment indicated in the picture, even when assuming an infinite bus, it kicks the AFC at point of demarcation down to 5.91kA. No matter what value I set for the Utility MVA and X/R ratios, their shown transformer chokes the AFC to the secondary and beyond down to about 6.4kA.
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