I am looking into buying a portable generator for emergencies. This Old House just had one on their show recently and I really like the idea. I'd like to place it outside in the back yard when I run it. I just measured it and it's around 30 -35 feet from where the transfer box will be mounted (the power inlet box will be in the back yard with the generator). HERE is the portable generator that I was interested in (also interested in any recommendations that you might have). I believe it's 30 amp unit.

What gauge wire do I need to run between the power inlet box (in the rear of the house) to the transfer box (30 - 35 feet away)?