Which of the following is not permitted to serve as an equipment grounding conductor?

The correct choice indicates that a metallic water piping system is not permitted to serve as an equipment grounding conductor. While metallic water piping systems can be grounded and may serve as grounding electrodes, they have specific limitations and conditions under the National Electrical Code (NEC) that must be adhered to. One of the primary concerns is that water piping systems should not be relied on solely for grounding electrical systems, as they may not maintain a continuous conductive path that is necessary for proper equipment grounding. Additionally, changes in the plumbing system, such as replacements or alterations, can compromise this connection, leading to potential safety hazards.

In contrast, an insulated grounding wire and a bare copper wire are specifically designed to serve as grounding conductors and provide reliable paths for fault currents. A grounding rod is also an accepted method of grounding, providing a direct connection to the earth, which is crucial for safety in electrical installations. Each of these options has a well-defined role in electrical grounding systems, making them suitable choices for serving as equipment grounding conductors.