I have a solar installation where I have gotten push back on doing a line side tap so I am looking at the 120% busbar rating rule. I have a 600A rated switchgear with existing 400A fuses. I have 160A at 480V of max operating current for inverters (40A X 4 inverters). I have designed it with a 150KVA, 480V/240V transformer. I came up with 133.01 KVA of actual max backfeed from from the solar (160A x 1.732 x 480V / 1000 = 133.017). So on the 240V side I have 133,017/1.732 = 76,799.65/240V = 319.99A. Can I round down to a 300A breaker? Or do I need to go up to 350A?

For the 120% I have 320A to play with. If I need to go up to 350A breaker I will need to downsize the main fuses to 350A.

Trying to avoid the UL certification report for the line side tap the plan checker asked for.