How to Calculate Busbar Ratings

I have read a previous question on the same but two units of 600MW was considered. But my question if 4 units of 660MW is connected to 400KV switchyard , how should i arrive at the main busbar current rating? Should the busbar be sized for half of the total current rating? since the total current comes to roughly 4500A and iam not sure what is the standard busbar capacities available. I heard only of 2500, 3150A. Your comments please.