Derating factors are not required for conductors installed in which type of gutters if the number of current-carrying conductors is 30 or less?

The correct choice indicates that derating factors are not necessary for conductors installed in metallic auxiliary gutters when the number of current-carrying conductors is 30 or fewer. This adheres to specific guidelines set forth in the National Electrical Code (NEC), where certain conditions about the installation environment of conductors dictate the application of derating factors.

Metallic auxiliary gutters are designed to provide adequate heat dissipation due to their metal composition, which allows for better heat transfer than non-metallic alternatives. When the number of current-carrying conductors remains at 30 or fewer, the heat generated by these conductors is effectively managed, eliminating the need for derating to ensure safe operation and prevent overheating.

In contrast, other types of installation environments, such as non-metallic gutters or exposed raceways, may not have the same capacity for heat dissipation, which is why derating factors could be mandated in those situations to maintain safety standards and prevent potential hazards. This understanding emphasizes the importance of installation conditions and material properties in electrical conductor management.