I'm going to be helping someone install a sub-panel soon. I've done this before so I'm not scared of the job or anything, but there is one thing I'm not sure of for this specific job, since I've never done one this big.

The guy I'm helping wants a 80A feed to his garage for the subpanel. Inside his house, he has a Square D QO breaker box which we'll be drawing the subpanel feed from. I know Square D makes QO breakers that go all the way up to 80A, but my concern is, should he ever start pulling a lot of load (I doubt he'll ever come close to 80A, or even 80% of that, but you never know), that the load bars/strips in the main breaker panel in the house may heat up. I'm assuming that since Square D makes QO breakers as big as 80A, that it is safe to put a 80A breaker in a QO load center, but I hate to make assumptions. Will it be okay to put in a 80A breaker to feed the sub-panel with?

Also, what gauge wire would I be best served using for this? I'm thinking #4 copper underground burial wire be fine? I'm thinking it will, just wanted to make sure since I've never put a sub-panel with this much load capacity before.

Thanks!

How far are you going? You may have to derate the wire size... if less than 100 feet then I'd say AWG # 4 ( table 310.15 (B) (6) )would be fine... remember to observe your depths from table 300.5 . And don't worry, Square D QO is a dependable product. Remember, if you are running this to an outbuilding (not attached to the house) you are limited to 6 breakers, after 6 you are required to have a main breaker in the subpanel. Also remember that the neutrals and grounds must be isolated from each other in a subpanel.

Just a slight clarification.
There is no requirement for a "main breaker". The requirement is for a disconnect. This can be one of several different types.
It is true that a main breaker panel is by far the easiest.