However, even if you have an electrician make the final hookups at the service panel and connect the outlets and light fixtures in your garage, you can save a lot of money on the most labor-intensive part of the project—laying the underground cable from the house to the garage or outbuilding. By providing the sweat equity yourself, you’ll limit the expense of this part of the project to the cost of materials.
Wire and Cable for Underground Lines
There are a variety of ways electrical wiring can be run underground. Each method will dictate how deep to bury the cable. The method you use may depend on what is allowed or recommended by your local Code authorities, so always check with the local building inspections office to find out what is recommended in your community. Generally speaking, though, the National Electrical Code allows for three means of running underground circuits:
Laying UF (underground feeder) cable directly into the ground. This is known as direct burial, and if you do this, the UF cable must be at least 24 inches below the surface of the ground, to minimize the chance of the cable being pierced by routine shoveling. Further, the vertical runs where the cable descends into the ground at the house side and emerges up from the ground at the outbuilding side must be housed in rigid conduit, usually PVC. At the bottom of the trench, a sweep fitting is attached to the ends of the vertical conduit, and the cable is snaked through the elbows and up into the conduit at both sides. Running wire through rigid galvanized metal conduit. In this method, the conduit can be as shallow as 6 inches deep, and the individual conducting wires inside should have a “W” labeling to indicate they are waterproof. THWN-2 wire, for example, is a standard type for running through underground conduit. Use thick-walled conduit at least 3/4 inch in diameter for this application. Never use thin-wall EMT conduit for underground applications. Running wire through Schedule 40 PVC conduit. Here, the conduit must be at least 18 inches deep, and again the individual conducting wires inside the conduit should carry a “W” waterproof rating, such as THWN-2.
Running UF cable through metal or PVC conduit. Although not commonly done, it is allowable to run UF cable (but NOT NM) cable through metal or PVC conduit in an underground application. But it can be difficult to fish cable through conduit, unless the conduit is at least 3/4 inch in diameter. Most electricians prefer to fish individual waterproof THWN conductors through conduit, simply because it is easier.
Extend Existing Circuits or Add New Circuits
Although it is theoretically possible to simply extend an existing house circuit by running an additional cable out to a garage or other outbuilding, most local Code requirements will require you to run one or more new circuits. Here are standard recommendations:
For powering overhead light fixtures and one or two wall outlets in a garage: install one 15- or 20-amp 120-volt circuit. If you will be operating a small workshop with 120-volt tools, install two 20-amp circuits. If you will be running many tools or one or more 240-volt tools, have an electrical subpanel installed in your garage.
Extending an existing circuit to an outbuilding should be done only where the existing circuit is already serving a deck or outdoor outlets, and you must make sure that the new lights and outlets in the garage will not exceed the capacity of the circuit. And make sure a simple circuit extension is allowed by your local Code.
How to Run Underground Cable
In our example, we are choosing to run Schedule 40 PVC conduit and will be threading UF cable or individual THWN wires through it. The process is similar for direct-burial of UF cable or when using rigid metal conduit. Now you are ready to extend cable into the house and into the garage and complete the hookups. Make sure that all wires are contained inside conduit as they pass through the walls of the house and garage and into the interior. After wiring the outlets and lights in the garage, the last step will be to connect the circuit breakers at the main service panel.
