In NEC Article 300.7, what is stated about conductors in ducts or plenums?

In NEC Article 300.7, it is specified that conductors installed in ducts or plenums must be listed for use in those specific spaces. This requirement is crucial because ducts and plenums often involve conditions that could affect the conductors' performance and safety, such as air circulation that can contribute to fire spread if the conductors are not designed to withstand those environments. Listing ensures that the conductors have been tested and certified for suitability in such applications, ensuring compliance with safety standards.

The other options do not align with the requirements outlined in NEC Article 300.7. For instance, grounding of conduit systems is discussed in other sections of the NEC and is not specifically a requirement for conductors in ducts or plenums. The necessity for no conductors to be installed at all is incorrect, as specific conductors can be used if they meet the required standards. Lastly, while water resistance may be critical in certain installations, it is not a universal requirement for conductors in ducts or plenums unless specified for particular applications.