AC Power Cords

The NEC states that electrical systems be grounded to a point that the maximum voltage to ground underground conductors does not exceed 150 volts. It also states that any 3 phase 4 wire, Y configurations and Delta configurations have the neutral wire be grounded as well.
 

These requirements cover any electrical systems, ranging from single phase 120 volts, all the way to three phase 480 volts AC. The conductor that needs to be grounded in these types of systems will always be the neutral wire. A bonding conductor is needed for use between the grounded conductor or neutral wire, and the equipment grounding conductor to the metallic enclosure of the main electrical service. It is important to make sure that a properly sized conductor is used to accommodate the potential surges that may occur in the electrical system. 

When grounding in a separately derived AC system, which is a system where components such as a generator, can have the grounded conductor’s continuity interrupted, possibly by a transfer switch, it is important to make sure that the grounding conductor is bonded to equipment grounding conductor of the metallic chassis on the load side of the electrical service through the use of a main bonding jumper. When continuity is not being interrupted on the load side, then it is critical to make sure that there is no bond between the neutral wire or grounding conductor and the metallic chassis of the electrical service. This will allow the main bonding to occur at the main service panel entrance point.