What type of wiring can THHN not be used for?

THHN is a type of wire that is used primarily for specific applications, and its characteristics limit its use in certain situations. The correct response indicates that THHN cannot be used for direct burial installations. This is because THHN is designed for dry locations and has insulation that is not suitable for being buried underground without additional protection.

In direct burial applications, wires need to have the capability to withstand moisture and environmental conditions over time, and thus they typically require a more robust type of insulation, like that which is provided by UF (Underground Feeder) cables.

In contrast, THHN can be effectively used for applications such as residential wiring and industrial applications where environmental exposure is limited and installation occurs in conduits or trays, making it a versatile choice for many indoor wiring tasks. Ground wire is also an application for THHN when it adheres to the specifications of the installation, further illustrating the limited scope of use for direct burial installations.