For a 200 amp residential service am I correct with 2/0 cu or 4/0 al thhn. the amprage on 2/0 is 195 thats why I am not sure.

Nov, see 310.15(B)(6) and Table 310.15(B)(6), if you meet this criteria you are correct.

Roger

Don't forget to check with local codes. I don't know where your at but many places here require 3 #3ott CU.

Tom