My house has a 200 amp service with an exterior cutoff panel adjacent to the meter base. Breaker 1 is 70 amps for exterior AC/Heating circuits. Breaker 2 is 90 amps and controls the interior load center/subpanel.

According to the 2002 NEC load calc, I currently require 80 amps available from the existing 90 being fed to the interior load center.

I got an electrical permit (based on 2002 NEC), and am starting to wire an addition which includes master bedroom, bathroom, laundry, kitchen expansion. Can I add a third breaker to the exterior cutoff panel/load center and run a new SER, 4 wire cable cable to a new subpanel? The addition needs approx 40 amps. Should I use #4 copper for a 50' run through a crawl space?