I want to hook up a transfer switch via generator at my shop for when the power go's out what size conductor do I need from the switch back to the load center?

I have a 600 amp service at my shop. I want to hook up a transfer switch (via generator) for when the power gos out. QWhat size conductor do I need from the transfer switch to the service. I am using around 480 amps max I don't want to oversize with the price of materials these days