All metal underground water piping in direct contact with the earth for how long must be used as a grounding electrode?

The correct answer is based on the National Electrical Code (NEC) requirements regarding grounding electrodes. Underground metal water piping that is in direct contact with the earth typically must be used as a grounding electrode if it is at least 10 feet long. This requirement is set to ensure that the grounding system is effective; longer sections of piping increase the likelihood of a reliable electrical connection to ground, reducing the risk of electrical shock or equipment damage due to fault conditions.

When establishing a grounding system for electrical installations, good grounding is essential for safety, and the 10-foot minimum provides a solid base for effective grounding. The NEC recognizes the importance of adequate length in grounding electrodes, which is why this specific measurement is critical in practice.