Latest Tips on Underground Wire Installation

With one exception—underground wire installation—extending electrical service to a detached garage or other outbuilding is similar to installing a circuit inside the house. If you're confident enough in your wiring skills to install an electrical circuit indoors, you might be able to extend a circuit to a separate outbuilding on your own. However, installing and connecting one or more new circuit breakers at the main circuit breaker panel is required. Many individuals prefer to hire a professional to conduct this type of repair, and for good reason: working on the main service panel can result in serious or lethal shock if you don't know what you're doing.

Even if you hire an electrician to connect the outlets and light bulbs in your garage and make the final hookups at the service panel, you may save a lot of money on the most time-consuming element of the project: laying the underground wire from the house to the garage or outbuilding.

Underground Lines Wire and Cable

• Electrical underground wire installation can be done in a variety of methods. The approach you choose may be limited by what your local Code authorities allow or recommend, so always check with your local building inspections office to see what is advised in your area. The National Electrical Code, on the other hand, allows for three different ways to run underground circuits: 
• Running cable via PVC pipe (Schedule 40). Individual conducting wires inside the conduit should have a "W" waterproof rating, such as THWN-2, and the conduit must be at least 18 inches deep.

Add New Circuits or Extend Existing Circuits

Although it is theoretically possible to simply expand an existing house circuit by extending an additional wire to a garage or other outbuilding, most municipal codes will require you to install one or more new circuits. Here are some general guidelines on underground wire installation:

• Install one 15- or 20-amp 120-volt circuit in a garage to power ceiling light fixtures and one or two wall outlets.
• Install two 20-amp circuits if you'll be working in a small workshop with 120-volt tools.
• Install an electrical subpanel in your garage if you'll be using a lot of tools or one or more 240-volt tools.

Extending an existing circuit to an outbuilding should only be done if the circuit already serves a deck or outdoor outlets, and you must ensure that the new lights and outlets in the garage do not exceed the circuit's capacity. Also, check to see whether your local code permits a simple circuit extension.

How to Lay Cable in the Ground

We'll thread UF cable or individual THWN wires through Schedule 40 PVC pipe in our example. When UF cable is directly buried or rigid metal conduit is used, the technique for underground wire installation is similar.

• Using rope or a garden hose, create a pathway for the wiring to flow on the ground from the home to the garage. It is usually preferable to take the shortest and most direct path.
• Dig a small path from the house to the garage with a trenching shovel at the recommended depth for the type of installation you're undertaking. If you need to dig a large trench, you might want to think about renting trenching equipment.
• Dig the trench down to the needed depth on both sides if necessary to cross sidewalks, then bore a lateral hole beneath the sidewalk by driving a piece of rigid pipe or conduit horizontally. Then, through the hole, you dug in the sidewalk, and run the conduit.

You're now ready to do the underground wire installation and hookups by extending cable into the house and garage. As cables flow through the walls of the house and garage and into the interior, make sure they are contained inside the conduit. The circuit breakers at the main service panel will be connected after the outlets and lights in the garage have been wired.