When two services are installed on the same building, how must they be connected according to NEC?

When two services are installed in the same building, they are required to be connected to the same grounding electrode system to ensure safety and effective grounding. This requirement is outlined in the National Electrical Code (NEC). Connecting both services to the same grounding electrode system allows for a common reference point for the electrical ground, which helps prevent differences in potential that could lead to hazardous conditions or equipment malfunction.

A common grounding electrode system promotes better performance of protective devices and maintains a consistent electrical potential throughout the building. This uniformity is critical for the safety of both persons and equipment, reducing the risk of electric shock and ensuring that overcurrent protective devices work effectively. By utilizing the same system, any fault current is directed safely to ground through a single path, avoiding potential issues that could arise from having two separated systems.

The other options, while they may seem plausible, do not adhere to NEC standards. For example, separate grounding electrode systems could create potential differences, and using any available grounding system or the nearest grounding electrode might not provide the effective grounding needed when two services are in close proximity. Therefore, the requirement for both services to connect to the same grounding electrode system is crucial for maintaining safety and reliability within the electrical installation.