To install electrical wire underground, first determine the appropriate depth for the wire, and then choose the corresponding wire type for the depth chosen. Different depths require the use of different materials while offering distinct advantages and drawbacks. It is usually less expensive to dig deep, providing that the soil is easy to dig through.
Rocky or clay-filled soil generally makes deep digging difficult, forcing homeowners to resort to shallow underground electrical wire installation. When installing wire at a depth of 6 inches, use a galvanized metal rigid electrical conduit to protect the wire. Run individual waterproof conductors inside the conduit; a waterproof conductor should have the letter W printed on the label or in the product name. Although this technique allows homeowners to install any size circuit, it can cost more than $1 per foot.
At a 12-inch depth, use a simple underground feeder cable, and install a ground fault circuit interrupter. Make sure the voltage carried doesn't exceed 120 volts, and protect the circuit by installing a fuse or breaker of no more than 20 amperes.
At a 24-inch depth, you can forego the installation of a metal conduit, directly burying UF-B cable. The one restriction to this method is that a conduit must be installed where the cable is exposed on the building or at a depth of 18 inches.