I am installing a 40 amp sub panel in my detached garage. I understand I need two ground rods for the sub panel in the detached garage. Since I am feeding the sub panel with a ground wire from the properly grounded main panel, can I consider the main panel grounding as one of the two ground rods for the sub panel? Or do I need to install a second ground rod for the detached garage?
Furthermore, if I have two ground rods can they be connected in series, or is parallel required.