Pretty sure that is nto electrically rated, they are discussing something else, one example has a negative AIC so that would not be very useful would it?
yeah, it is something else not Ampere interrupting Capacity.
but jahilliard has mentioned in his question title AIC rating Formula,
Indeed, AIC is the ampere interrupting capacity of a breaker. So, has to do fault calculation to find the maximium fault current and compare it to the breaker's AIC
here is a another example.
The following example illustrates a potential problem of installing underrated electrical equipment if X/R ratios and motor contribution aren't considered in the initial design.
Example design criteria:
?Utility transformer rating: 2,500kVA
?Utility transformer % impedance: 4.775%
?Service conductors: 10 sets of 600 MCM copper
?Available fault current at utility transformer secondary: 63,000A
?X/R ratio at the utility transformer secondary: 11
?Motor contribution: 400 hp
?Ampacity of service conductors: 4,000A
?Service gear tested X/R ratio: 4.9
A fault current of 62,321A is calculated at the switchgear. This value is based on the 2,500kVA utility transformer with 4.775% impedance and minimal impedance from the service conductors (11 feet of 10 sets of 600 MCM copper).
The simple form of this calculation, based on infinite bus theory, is indicated below:
2,500kVA?(√3?480V)?0.04775=62,975, or 63,000AIC at the utility transformer secondary
However, the AIC is further reduced at the service to 62,321A, based on the impedance of the service conductors. This assumes no contribution from the motors in the system or from the asymmetric component.