Standards for installing underground electrical conduits are contained in the National Electrical Code, or NEC. These are the minimum standards specified; many jurisdictions have the authority to adopt stricter standards. The standards address the types of materials that may be used for underground conduits. Electrical conduits may be made of metal, such as steel or aluminum, or non-metal materials, such as plastic, fiber or fired clay.
NEC standards also allow for the use of direct-buried cables, communications and transmissions cables that are designed to be laid without any kind of extra covering, sheathing or piping for protection, but the standards usually require some form of protective enclosure known as a raceway. PVC is the most popular choice for underground conduits because it is inexpensive and durable. PVC is also waterproof and able to withstand various types of corrosive environments. However, it becomes brittle in very cold temperatures and bends or sags in extreme heat, so NEC standards specify that it be supported within 36 inches of an enclosure.
NEC standards also address the installation of underground electrical conduits by specifying their placement and the type of covering that must be used for each type of conduit. Conduits of all types that are beneath buildings generally do not require cover material or minimum depths. Several options are available for PVC underground conduits. The standards allow for either a minimum depth of 4 inches beneath a 4-inch, non-traffic bearing concrete cover, a minimum depth of 12 inches when laid in a trench with a 2-inch concrete cover, or a minimum depth of 18 inches without a concrete cover. NEC standards also specify that PVC conduits are not permitted where they may be subject to damage.