Underground wiring requires the use of special underground feeder (UF) direct burial cable. As its name suggests, it is designed for use underground and can be buried without a protective conduit. After you have decided what size of cable you'll need; plan where you're going to dig. Before you dig, however, check with your local council to find the locations of any buried pipes or cables. Many localities require wires be buried to a depth of at least 45 cm or 60 cm (18 inches or 24 inches) under driveways or sidewalks -- check with your municipality for the rules in your area.
Turn the power to the circuit off at the breaker box.
Hammer stakes into the ground and mark the proposed trench with mason's string. Start the trench directly below the junction box you will be using as the electrical feed.
Excavate the trench with a shovel; dig down 45 cm (18 inches), or deeper if your local municipality requires it. Check the depth with a tape measure.
Install approved outdoor rigid electrical conduit down from the junction box and attach a 90-degree sweep at the end of the conduit to rest at the bottom of the trench.
Feed the UF Direct Burial cable through the conduit upwards toward the junction box. Remove the insulation with wire strippers and connect the UF wires to the feed wires with splice connectors.
Lay the cable along the bottom of the trench to its intended destination. Leave an expansion loop where the cable exits the conduit to prevent stress on the cable.
Replace the soil and tamp it flat with the shovel.
Be sure all power has been turned off before beginning the project.