Cables and insulated conductors installed in enclosures or raceways underground must be listed for use in wet locations. Cables run under a building must be installed in a raceway that extends past the outside walls of the building.

Direct buried conductors and cables must be protected from damage in accordance with (1) through (4).

1. Emerging from grade. Direct buried cables or conductors that emerge from the ground must be installed in an enclosure or raceway to protect against physical damage. Protection isn't required to extend more than 18 inches below grade, and protection above grade must extend to a height not less than 18 feet.
2. Conductors entering buildings. Conductors that enter a building must be protected to the point of entrance.
3. Service conductors. Service conductors that aren't under the exclusive control of the electric utility, and are buried 18 inches or more below grade, must have their location identified by a warning ribbon in the trench not less than 1 foot above the underground installation. Note: It's impossible to comply with the service conductor identification location requirement when service conductors are installed using directional boring equipment.
4. Enclosure or raceway damage. Where direct buried cables, enclosures, or raceways are subject to physical damage, the conductors must be installed in rigid metal conduit, intermediate metal conduit, or Schedule 80 rigid nonmetallic conduit.

Backfill material for underground wiring must not damage the underground cable raceway or contribute to the corrosion of the metal raceway. Large rocks, chunks of concrete, steel rods, mesh, or other sharp-edged objects cannot be used for backfill material because they can damage the underground conductors, cables, or raceways.

Where moisture could enter a raceway and contact energized live parts, seals must be installed at one or both ends of the raceway. This is a common problem for equipment located physically downhill from the supply or in underground equipment rooms. See 230.8 for service raceway seals and 300.7(A) for different temperature area seals. A Fine Print Note in 300.5(G) explains that hazardous explosive gases or vapors make it necessary to seal underground conduit or raceways that enter the building in accordance with 501.15. It isn't the intent of this FPN to imply that seal-offs of the types required in hazardous (classified) locations must be installed in unclassified locations, except as required in Chapter 5. This also doesn't imply that the sealing material provides a watertight seal, but only that it prevents moisture from entering.

Raceways that terminate underground must have a bushing or fitting at the end of the raceway to protect emerging cables or conductors.

All conductors of the same circuit, including the equipment-grounding (bonding) conductor, must be inside the same raceway or in close proximity to each other. See 300.3(B). There are two Exceptions associated with this rule. The first notes that conductors can be installed in parallel in accordance with 310.4. The second exception states individual sets of parallel circuit conductors can be installed in underground nonmetallic raceways, if inductive heating at raceway terminations is reduced by complying with 300.20(B) [300.3(B)(1) Ex. No. 1]. Installing phase and neutral wires in different nonmetallic raceways makes it easier to terminate larger parallel sets of conductors, but it will result in higher levels of electromagnetic fields (EMF), which can cause computer monitors to flicker in a distracting manner.

Direct buried conductors, cables, or raceways subject to movement by settlement of frost, must be arranged to prevent damage to conductors or equipment connected to the wiring.

Cables or raceways installed using directional boring equipment must be approved by the authority having jurisdiction (AHJ) for this purpose.