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The utility company provide short circuit at the Point of Entry, and usually utility company's cable connect to the service switchboard directly. Therefore, when doing power system study, the service switchboard will have ASCC higher than equipment withstand rating and/or OCPD (breaker or fuse)...

For low voltage system 480/277V 3Phase 4 wire system. When utility short circuit X/R is higher than equipment test X/R. Then Equipment will face derating. I am in NYC, I have see many buildings have 200kA 480/277 3Phase incoming short circuit with high X/R, then service equipment with 200kAic...