This is for a 15' run of wire 21A is the recommended max current. Although I doubt the amp is putting out its rated power, it will prbably still pull more than 21A, I would recommend 8awg or bigger.
Dang, i would just nab the 4 gauge u know. It isnt too much and it works great when u need to upgrade. I have the 0/1 gauge wiring just so i dont have to worry about changing it