Holderlin:
My run is not that long and 12 ga. was not good enough in spite of my amp not been that big, go with heavier gauge.
One point many times overlooked is the main entrance to your house/apartment electrical system. Depending on type of wheather and age of the wiring system, you might have less than ideal contact there, so since the electrician will be working at your home, ask him to clean all connections that lead to your new panel, breakers, etc....
You might get surprised on having more light out of your bulbs and therefore better current flow to your beloved target area which is your sound system!!!!!!!
Hope the investment satisfies you when done.
Luis