I am looking at a 125 ft run of cable to feed a branch circuit in an adjoining building. I have lots (490 ft) of left-over #2 copper cable from another project and a 125 Amp Cutler Hammer panel w/main disconnect that I could use. I have in the main house a 200 Amp Cutler Hammer panel.
--Can I put in a large double-pole breaker in the main house panel to feed this branch circuit?
--What is the largest breaker I should use in this main house panel for this branch circuit if I use the available #2 copper cable and the 125 amp panel?
--Is this 125 amp panel OK or should I purchase a different one (if so what rating)?

Use 125 amp panel. Feed with 80 amp breaker

Need to know what type of "#2 copper cable" you have.

Certain #2 wire is rated 130 amps, that will allow a 125 amp branch breaker to be used.

Not sure why an 80 amp breaker was suggested.