What type of wire is approved for direct underground burial without conduit?

UF wire, which stands for Underground Feeder wire, is specifically designed for direct burial underground without the need for additional protective conduit. Its construction includes a moisture-resistant insulation that allows it to withstand the conditions found underground, such as dampness and potential soil contamination. This makes UF wire highly suitable for outdoor and underground applications, including running power to outdoor structures, gardens, or other electrical installations that may be exposed to the elements.

Other types of wire, such as THHN wire, are not intended for direct burial without the protection of conduit, as they do not have the necessary moisture and abrasion resistance for underground installations. Type NM wire and Romex wire are also unsuitable for this application due to their design, which is meant for indoor wiring and typically requires conduit for protection when placed underground. Thus, UF wire stands out as the approved choice for direct underground burial, ensuring safety and compliance with electrical codes.