Naj,
Why do you need to know this information? For HVAC loading? (I am curious.)
Usually with large switchboards, or switchgear, the OEM will always provide you with guidance. In your case I would expect the losses to be very low (ie << 100W).
The heat generated in any switchboard is produced from a number of sources, including;
1) heat from thermal-magnetic circuit-breakers, or fuses
2) heat from the main (and any feeder) busses
I would suggest that of you really need to know the heat dissipation of your 300A switchboard, that you ask the OEM.
Regards,
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)