Greetings,

If you use a 100 amp breaker you should use wire as if you had a 100
amp load at the other end even if you expect to only use 75 amps "for
now". 2 AWG Al will NOT be enough for 100A with an 85 ft run at 160f
without having to worry about voltage drop. 2 AWG Cu would be.

Hope this helps,
William