A vertical run of 4/0 AWG copper must be supported at intervals not exceeding _____ feet.

The correct interval for supporting a vertical run of 4/0 AWG copper wire is 100 feet. According to the National Electrical Code (NEC) requirements, conductors must be supported at specified intervals to ensure safety and to prevent sagging or undue stress on the wiring.

In particular, NEC Article 300 specifies that vertical runs of conductors should be supported at intervals not exceeding 100 feet. This support is critical to maintain the integrity of the installation over time, accommodating any environmental factors such as changes in temperature and structural settling that could affect the conductor's position or condition.

While the other options suggest shorter intervals, they do not align with the NEC standards for this size of conductor. Thus, the provision for a maximum support interval of 100 feet ensures that the installation remains compliant and secure.