How To Install Underground Cable

Underground Electric Cables
Jared Earle/Creative Commons/Flickr

Extending electrical service to a detached garage or other outbuilding isn't much different than adding a circuit inside the house, with one important exception—running underground cable.  If you are sufficiently confident of your wiring skills that you would feel comfortable adding an electrical circuit indoors, then running a circuit to a detached outbuilding is something you may be able to do yourself.

But be aware that it involves work at the main circuit-breaker panel to install and connect one or more new circuit breakers. Many people choose to have this kind of work done by a professional, and with good reason—work at the main service panel has the potential for very serious or fatal shock if you don't know what you are doing. 

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.

Wire and Cable for Underground Lines

There are a variety of ways electrical cables can be run underground. 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. 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. It can be difficult to fish cable through conduit, though, 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 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: 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 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 through it.

The process is similar for direct-burial of UF cable or using rigid metal conduit. 

  1. Layout a pathway for the wiring run on the ground from the house to the garage, using rope or a garden hose. The shortest, most direct route is always best.
  2. Use a trenching shovel to dig a narrow channel from the house to the garage, at the recommended depth for the type of installation you are doing. If you have to dig a long trench, you may want to consider renting a trenching machine for this job.
  3. If necessary to cross sidewalks, dig the trench down to the required depth on both sides, then bore a lateral hole under sidewalk by driving a piece of rigid pipe or conduit horizontally. Then, run the conduit beneath the sidewalk through the hole you bored. 
  4. Lay the conduit into the trench, solvent-welding the joints of individual sections with PVC solvent glue. At each end, attach sweep fittings connected to vertical lengths of conduit extending up out of the trench. 
  5. Extend a fish tape down through one end of the conduit all the way through to the opposite side. Attach the end of the UF cable (or individual THWN wires) to the end of the fish tape, then carefully pull the cable back through the conduit. Having a helper feed the cable from one end as you pull on the fish tape makes this job much easier. Applying cable lubricant may make it easier to pull the cable. 
  6. Leave plenty of excess cable at each end of the conduit run, to allow for an electrician to continue the hookups inside the house and inside the garage. 
  7. Before filling in the trench, make sure to have it "passed" by the inspector, if this is required by your local Code. The inspector will verify that your conduit is buried to the proper depth, then give you the go-ahead to fill in the trench and continue with the circuit installation. 

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.