Underground electrical wire may be essential if you plan to put in outdoor lighting or some other outdoor circuitry. An outdoor electrical project can be accomplished easily enough with a do-it-yourself attitude, though it is essential you comply with local codes and inspections for the safest possible end product.
It is imperative you check your local codes before proceeding with any project installing underground electrical wires, as your state or region may have specific requirements you must abide by. Some local codes may require you to cover your entire cable with conduit, while others require outdoor circuits to be protected with ground fault interrupters (GFIs), one in the circuit breaker box and the other in outlet boxes. Because all areas have different requirements, this should be done before beginning your project.
There are very specific requirements for burying electrical cable safely and the process must involve inspections by a local standards organisation. Since these guidelines may vary regionally, you should begin by contacting your local electrical authority and explaining your project to them first. Some projects may require separate inspections for each part of the process, usually beginning with a trench inspection. You might be required to have another inspection and examination of the cable when you begin laying the cable in your trenches to ensure it is laid properly underground.
For underground wiring, it is essential you use underground feeder and branch-circuit cable (UF cable). UF cable is designated for outdoor wiring, as it is weatherproof and suitable for underground burial. Since UF cable looks like ordinary cable, make sure that it is UF-designated--it should be clearly written on the sheathing. All aboveground UF cable must be protected with conduit, and special characteristics of newer UF wire should be identified on the sheathing.
Cable must be buried deeply enough to be protected from routine digging, yet not too deep to interfere with existing water or power lines when you dig your trench. Typically, UF wiring must be buried 24 inches for direct burial cable and 18 inches for rigid non-metallic conduit. For rigid and intermediate metal conduit, you might only have to dig as low as 6 inches. If your cable is powered from a ground-fault breaker, you may not have to trench as deeply; however, this is not recommended, as trees and shrubs planted over the cable can put it at risk of being cut during the digging process.