Last updated on August 22nd, 2022 by Myla Pablo
Ways to Install Underground Electric Conduit
- Directly laying UF (underground feeder) cable into the earth
- Wire is routed through a strong galvanized metal conduit
- Wire runs through Schedule 40 PVC conduit
- UF cable is routed through metal or PVC conduit
With one key exception—running underground cable—extending electrical service to a detached garage or other outbuilding is similar to installing a circuit inside the house.
If you are confident enough in your wiring skills to add an electrical circuit indoors, then running a circuit to a separate outbuilding is something you might be able to do yourself. However, you should be aware that it would necessitate work at the main circuit breaker panel to install and connect one or more new circuit breakers.
Many people prefer to have this type of work done by a professional, and for good reason: work at the main service panel can result in very 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 labor-intensive phase of the project: laying the underground wire from the house to the garage or outbuilding. By donating your own sweat equity, you will limit the cost of this aspect of the project to the cost of materials.
Wire and Cable for Underground Lines
There are several methods for burying electrical wiring. The depth of the cable will be determined by the method used. The approach you employ may be dependent on what is authorized or recommended by your local Code authorities, so always check with the local building inspections office to find out what is advised in your community. In general, the National Electrical Code allows for three methods of running underground circuits:
Directly laying UF (underground feeder) cable into the earth. This is known as direct burial, and if you do it, the UF cable must be at least 24 inches below the ground’s surface to avoid being perforated by ordinary shoveling. Additionally, the vertical runs where the cable falls into the ground on the house side and emerges from the earth on the outbuilding side must be contained in rigid conduit, often PVC. A sweep fitting is attached to the ends of the vertical conduit at the bottom of the trench, and the cable is snaked through the elbows and up into the conduit on both sides.
Wire is routed through a strong galvanized metal conduit. The conduit can be as shallow as 6 inches deep in this approach, and the individual conducting wires inside should be labeled with a “W” to indicate they are waterproof. For example, THWN-2 wire is a common type for passing through underground conduit. For this application, use thick-walled conduit with a minimum diameter of 3/4 inch. For underground applications, never utilize thin-walled EMT conduit.
Wire runs through Schedule 40 PVC conduit. The conduit must be at least 18 inches deep in this case, and the individual conducting wires within the conduit must have a “W” waterproof rating, such as THWN-2.
UF cable is routed through metal or PVC conduit. Although it is uncommon, running UF cable (but NOT NM) cable via metal or PVC conduit in an underground application is permissible. However, until the conduit is at least 3/4 inch in diameter, it can be difficult to fish cable through it. Because it is easier, most electricians prefer to fish individual watertight THWN wires through conduit.
Extend Existing Circuits or Add New Circuits
Although it is theoretically possible to simply connect an additional cable out to a garage or other outbuilding to extend an existing house circuit, most local Code requirements will need you to run one or more new circuits. Here are some general guidelines:
Install one 15- or 20-amp 120-volt circuit to power ceiling light fixtures and one or two wall outlets in a garage.
Install two 20-amp circuits if you plan to use 120-volt tools in a small workshop.
Install an electrical subpanel in your garage if you plan to use a lot of tools or one or more 240-volt tools.
Extending an existing circuit to an outbuilding should only be done if the existing circuit is already servicing 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, ensure that your local Code permits a simple circuit expansion.
How to Run Underground Cable
In this example, we will use Schedule 40 PVC pipe and thread UF cable or individual THWN wires through it. The procedure is the same whether UF wire is directly buried or stiff metal conduit is used.
Create a path for the wire to travel from the house to the garage on the ground, using rope or a garden hose. Always take the shortest, most direct route.
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 think about renting a trenching equipment.
If crossing sidewalks is required, dig a trench to the requisite depth on both sides, then bore a lateral hole under the sidewalk by driving a piece of rigid pipe or conduit horizontally. Then, through the hole you dug in the sidewalk, run the conduit.
Insert the conduit into the trench, solvent-welding the individual section joints with PVC solvent glue. Attach sweep fittings to vertical lengths of conduit reaching up out of the trench at each end.
Extend a fish tape all the way through the conduit from one end to the other. Connect the UF cable (or individual THWN wires) to the end of the fish tape and carefully drag the cable back through the conduit. This process is made much easier by having a helper feed the cable from one end while you pull on the fish tape. Applying cable lubricant will make pulling the cable simpler.
Allow enough wire at each end of the conduit line to allow an electrician to finish the hookups inside the house and garage.
If your local Code requires it, have the trench “passed” by an inspector before filling it in. The inspector will check that the conduit is buried to the right depth, then give you the go-ahead to fill in the trench and continue with the circuit installation.
You are now ready to finalize the hookups by extending cable into the house and garage. As they run through the walls of the house and garage and into the interior, ensure that all cables are contained inside conduit. The next stage will be to connect the circuit breakers at the main service panel after wiring the outlets and lights in the garage.
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