Which single conductor of an alternating-current system is allowed to be installed alone in a metallic raceway?

In the context of electrical systems, the grounding electrode conductor is specifically designed to connect the grounding electrode system to either the grounding bus or other system elements. This conductor can be installed alone within a metallic raceway because it serves a unique purpose in providing a low-resistance path for fault currents to ground, enhancing system safety.

Metallic raceways provide a level of protection for conductors, but grounding electrode conductors are often distinguished from other types of conductors due to their role in grounding and bonding systems. They are not intended to carry current under normal operation, which justifies their installation in a raceway by themselves.

In contrast, while grounded (neutral) conductors and equipment grounding conductors typically must be run in conjunction with other conductors due to their operational functions and safety requirements, the grounding electrode conductor's primary purpose is different. This difference is what leads to the allowance for its solo installation in a metallic raceway.