I want to install a 12KW standby generator, it comes with a load managed transfer switch. The generator will be approximately 80 feet from the transfer switch. I'm wondering what size wire should I use between them. The transfer switch will switch the entire load on a 200 amp panel in which we have selected circuits that we want to have on standby. The balance of the house on on another 200 amp panel that isn't backed up.

I would use #4 copper.