How long must an underground metallic water pipe be in direct contact with the earth to be used as a grounding electrode according to NEC?

To be utilized as a grounding electrode, an underground metallic water pipe must be in direct contact with the earth for a minimum length of 10 feet according to the National Electrical Code (NEC). This requirement ensures that the metallic pipe has a sufficient surface area in contact with the earth, which is critical for effective grounding.

The 10-foot measurement facilitates a low-resistance grounding path, helping to dissipate electrical faults safely into the ground. Having a longer length improves the conductivity and reliability of the grounding system, reducing the risk of electrical shock or equipment damage.

Other lengths provided in the options do not meet the NEC criteria for grounding effectiveness, as a length shorter than 10 feet would not ensure adequate contact with the earth, compromising safety protocols that are paramount in electrical installations.