When using a metal water pipe as a grounding electrode, a(n) _______ is required.

When a metal water pipe is being utilized as a grounding electrode, it is necessary to use a grounding clamp. This clamp serves as a means to securely connect the grounding conductor to the metal pipe, ensuring an effective electrical bond. Grounding clamps are specifically designed for this purpose, allowing for a reliable connection that can effectively conduct grounding faults safely to the earth.

Using a grounding rod or plate would not be applicable in this scenario, as the question focuses on the connection to an existing metal water pipe. The purpose of the grounding clamp is to create a solid electrical connection at the interface of the grounding conductor and the pipe, which is critical for the system's overall safety and functionality. This ensures that any electrical fault that occurs is quickly redirected to the ground, preventing potential electrical hazards.

In summary, a grounding clamp is mandatory in this context to make a reliable connection between the grounding electrode and the water piping system.