You should calculate the short circuit fault current at the 300A service location to see the kA rating required.  This fault current information at the location where you are installing the breaker may already exist in your plant.  As a rule of thumb, for service less than 600V, the kA is rated at about 42kA. However, this is only an estimate and I would oversize the breaker IA rating if the exact kA value can not be obtained.

Just a brief comment concerning the use of molded case circuit breaker for this application-and I am assumming that is what was intended. It has been my practice to choose a IC for these units which does not expose the MCB to more than 75 % of the manufacturers published interrupting capacity. There are two main reasons for this, lack of ability to maintain the unit and the fact that MCB's are tested at significantly less X/R values than air circuit breakers. If you specify a MCB with an IC of 100 KA, it is very difficult to make a mistake unless you are sitting on top of a large utility ring bus supply.

Check out the line to ground fault duty when appling molded
case circuit breakers. IEEE Standard 1015-1997 Table 3-19
states that a individual pole of a 3 pole circuit breaker
800 A Max 480/277 is subjected to 10,000 amp L-N SHORT CIRCUIT CURRENT, notice this is far below the 3 pole rating. Consult the manufacture for the breakers single pole
interrupting rating.

The electric utility should be able to supply this information. This is a place to be very conservative, since utility equipment may be changed without warning.

Suggestions:
1. Electric Utilities have their power distribution system modeled by Power Distribution Analysis software, usually. They may send you a printout and data outlined in a letter. Sometimes, one may interpret it easily, if one is familiar with SKM, Inc. program DAPPER, A_FAULT, or ETAP software computer printouts.
2. Else, you may perform the short circuit analysis by yourself; however, you will still need system impedances, and voltages from the Electric Utility to be on the accurate side.