I'm curious how to size a service using parallel feeders. If I had a 600 amp, 3-ph service would I simply add together the ampacity ratings of a selected copper wire size or do I need to derate the wire size due to parallel use? Please tell me what size copper I would need to use. I'm assuming I would need 350-400 MCM. Thanks

A 3 phase 4 wire paralleled feeder would have 8 current carrying conductors, and be subjected to 70% derating, per NEC Table 310.(B)(2)(a). You can begin derating from the 90 Deg C column of Table 310.16, to arrive at 300 amps per conductor, and then add the sum of the allowable ampacity.

500 MCM THHN/THWN copper is rated 430 amps 90Deg C column, derated by 70% leaves 301 amps. So two 500 MCM conductors per phase will be needed for a 600 amp service.