Conductors used in an underground installation are said to be in a:

Conductors used in an underground installation are categorized as being in a wet location. This classification is based on the fact that these installations are typically subject to moisture from the surrounding soil, potential groundwater, and other environmental factors that can introduce water into the vicinity of the conductors.

The National Electrical Code (NEC) outlines that a wet location is one where water can accumulate or where water can drip, splash, or flow on the conductors. Since underground installations are directly affected by the presence of moisture and the potential for water ingress, they fall into this category.

While a damp location may experience some moisture, it does not consistently encounter the same level of exposure to water as a wet location. Similarly, a moist location would imply a level of humidity or dampness but does not necessarily denote standing water or frequent water exposure. Subterranean location is not an official term used in electrical wiring classifications and doesn't provide the clear understanding of moisture levels critical for safety and compliance in electrical installations. The designation as a wet location is essential for ensuring that conductors and their associated equipment can withstand the environmental conditions they are exposed to and meet the safety requirements outlined in the NEC.