Something is not quite right here. At 460 volts, the rated secondary current is 395 amps. As “Bob on the left coast” said, that works with a 500 amp breaker. But a 300 mm2 wire is roughly equivalent to 600 MCM (from Table 8 ), and is only good for 420 amps (from Table 310.16). The next higher standard size overcurrent device is 450 amps, not 500 amps. So it appears to me that the breakers need to be replaced (or have their trip rating adjusted down, if they are of the adjustable type). The other alternative (not a pleasant one) would be to replace the cables. Or perhaps we don’t have the entire picture correct, and would need more information.
A 90 mm2 wire is roughly equivalent to a 3/0, and is good for 200 amps. At 11kV, a 315 KVA transformer has a primary current rating of 16 amps. So a 90 mm2 wire is a bit of overkill.
As to adding more load, you can’t move on that until you know what load you have. There are three ways to find that out. One is to make a list of each and every electrical component in the building, and include the VA drawn by each. Another is to attach a meter at one or more key locations, and measure the load over 30 days. Another is to get one year’s worth of peak load information from the serving utility. Unless and until you get this information, you cannot add any significant load to an existing facility.
