Which characteristic is required for all underground metallic piping?

For underground metallic piping, a minimum cover requirement is crucial to ensure safety and protect the piping from physical damage and environmental factors. The requirement for a minimum cover of 12 inches helps safeguard the pipes from above-ground activities such as vehicle traffic, excavation, or landscaping, which could otherwise lead to potential ruptures or leaks. Additionally, adequate cover helps minimize the effect of freeze-thaw cycles and ground movement, ensuring the integrity of the system.

The other options do not universally apply to all underground metallic piping. While color coding may be used but is not a requirement for identification, insulating to prevent corrosion is not always feasible for all types of pipes, and reinforcement at joints is specific to certain applications and not a general requirement. Thus, the minimum cover is the most universally applicable characteristic for ensuring the safe operation of underground metallic piping.