Interior metal water piping must be within what distance from the point of entrance to be used as part of the grounding electrode system?

The correct distance for interior metal water piping to be used as part of the grounding electrode system is 8 feet from the point of entrance. This requirement aligns with the National Electrical Code (NEC) standards, which stipulate that metallic water piping systems can serve as grounding electrodes if they are within this specified distance. The reason for this limitation is that the grounding system needs to be effective in directing electrical fault currents to the earth to enhance safety and prevent electrical shock hazards.

The 8-foot distance ensures that the metal piping remains within a favorable range to connect to the grounding system while also considering practical installation factors and ease of access for maintenance. Using metal water piping for grounding can significantly improve the effectiveness of the grounding system as it provides a low-resistance path to ground, which is critically important for mitigating electrical hazards.

The other options do not meet the established code requirements and would therefore not be compliant for use in this context. A distance shorter than 8 feet could potentially compromise the grounding effectiveness, while longer distances would exceed the code's specified limit, rendering the use of the piping as a grounding electrode inapplicable.