It is advantageous to encase electrical cable inside a conduit of metal or PVC pipe before burying it. Outdoor wiring is different than indoor wiring and is subject to different building codes because of the dangers of water and dampness to the wire. The National Electrical Code requires encased electrical cable to be buried at least 12 inches below ground, but check your local building codes as well.
Burying UF Cable
Some building codes allow UF-type cable, which is encased in heavy plastic, to be buried without a conduit. It is recommended that this cable be buried 24 inches deep to avoid damage by digging.
Follow your city's building codes and you should have few problems. Call a professional if you are unsure of what to do. According to engineer and syndicated columnist James Dulley, "Outdoor wiring projects are not difficult for a do-it-yourselfer, but, for safety reasons, outdoor wiring codes must be followed."