When used outside, an aluminum or copper-clad aluminum grounding electrode conductor must be installed how many inches from the earth?

The correct answer is 18 inches because, according to the National Electrical Code (NEC) requirements, when grounding electrode conductors, particularly those made of aluminum or copper-clad aluminum, are installed outside, they must be buried at a minimum depth of 18 inches below the surface of the ground. This depth helps to provide physical protection to the conductor from environmental factors, such as frost and disturbances, while ensuring it maintains a stable connection to the ground system. Proper installation depth is important for effective grounding and safety in electrical systems, as it helps to prevent accidental damage and promotes reliability in grounding methods. Adhering to this specification ensures compliance with safety standards and minimizes risks associated with electrical faults.