What Ed and Hal give as a reference wouldn't be my 1st choice for a ground, but sounds to me like it's your best option. That's just me, old school telecom #6 to a ground buss with all grounds common and no I'm not able to get it everytime. My second choice is the electric panel ground as it is usually a common ground. I also run #10 even though most systems say #12 I'm sure what's been suggested is plenty good for what you're doing.
