Alternator wire thickness

[J-440]

Evening all. I have a 140amp, GM style, 1 wire alternator from Powermaster. It's putting out 12.2 volts at the alternator terminal. The battery is losing volts when the engine is running and when the positive terminal on the battery is taken off, the engine dies. This alternator is new but has been sitting for 9 years. Bad internal regulator? I am also using 8 gauge wire from the alt to the battery. Powermaster calls for 6 gauge wire. This still does not explain the volts coming from the alternator terminal though. Any thoughts?