How to Dig a Trench for Underground Electrical Wires

Assess your outdoor electrical wire needs.

Consider how much current you’ll need now, and perhaps in the future. Will you be using conduit or cable? In some situations, you may be able to use direct burial cable and skip the conduit steps. 

Get a permit.

Installing new electrical outlets, adding wiring, and altering low-voltage systems are all types of projects that require a permit. Before you begin digging your trench, secure a permit first. This could cost anywhere from $10-500. You will need this before your first inspection occurs. Plan on getting your permit at least two weeks in advance, to keep your project on track. 

After receiving your permit, you may need up to three inspections.  You can obtain an inspection by calling the request line at the building department responsible for your area. Do this at least 24 hours in advance and have your permit information ready.

Call the utility company.

Locate your service panel or the area where you will be connecting your new wiring. From here, use chalk or spray paint to mark the path you plan to dig. Then call the utility company to check for buried pipes or wires along your designated path. 

Planning a project that uses underground electrical wire will require a few tools. These are just a few you may want to have on hand:

Digging

• shovel: to dig up soil
• mattock: helps chop into the soil
• digging bar: breaks up roots and rocks
• trenching spade (lawn edger): convenient way to create a path for wires at a measured depth
• sand for coverage

Wiring

• electrical wire
• wire strippers: removes wire insulation to make connections
• electrical terminals: to connect one or more wires to a device (may also use solder, although soldering is not better than crimping)
• voltage detector
• wire crimper
• plastic conduit: to protect wires from water underground
• propane-powered heat gun: to bend conduit if necessary
• conduit outlet body
• exterior-grade acrylic-latex caulk: to apply around the conduit outlet body and seal out water

Other handy tools might include a tape measure, fish tape for running the wires, a drill and a hole saw.

Once you’ve obtained your permit and gotten your tools together, it’s time to figure out the type of conduit you’re going to use and the size of your electrical wire.

Electrical wire size is determined by the distance between your service panel and the appliances you’re trying to power. You’ll also want to consider this distance for conduit material since some conduits are more pricey than others. Galvanized rigid metal conduit, for example, can cost over a dollar a foot but allows you to dig a trench that’s only six inches deep. This might make sense if you are dealing with tough soil and only have a short distance to travel. A more popular conduit is PVC. It’s easier to work with and cheaper, but requires a trench of at least 18 inches deep. If you’re dealing with tough soil, you might want to consider renting a trenching machine. 

You’re now ready to dig your trench. You might start with a shovel or mattock to loosen the ground. Or, if you’re lucky and the ground isn’t too solid, you can go directly in with a trenching spade. Your trench only needs to be as wide as the conduit or cable you’re using.

It’s time to create a safe environment for your outdoor electrical wire. You can use a heat gun to mold the conduit if necessary and achieve whatever shape you need to access your service panel. Remember to attach a plastic conduit outlet body to the end of the conduit. Use exterior-grade acrylic-latex caulk around conduit outlet bodies to seal out water. 

Push your cable through the conduit, or run your direct burial cable. Keep in mind that code prohibits low-voltage wires from being placed in conduit with 120/240-volt circuits. This includes low-voltage wires for garage door controls, intercoms, alarm systems, TV cables, low-volt landscape lighting, and telephone wires. Instead, you must create separate conduits, which can then be laid in the same trench next to higher-voltage conduit. 

Before you return the soil over the wires, consider covering them with sand first for extra protection. You can also apply caution tape across the length of the trench to ensure that anyone digging in the future will identify the area and not hit the electrical wire. When you’re finished, add the soil back to the trench.

You may already know how to connect two or more wires, but when dealing with electrical work that involves connecting to the electric supply, you must hire a licensed electrician. Any completed work will also require a final inspection. And that’s it. Your underground electrical wire work is complete.

Outdoor wiring projects can seem intimidating, but with the right tools you can probably do a large majority of the grunt work yourself. And knowing more about the process may better help you understand some of the costs involved.

The product statements contained in this guide are intended for general informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness, accuracy, completeness, correctness or currentness of the information provided. Information provided in this guide does not replace the use by you of any manufacturer instructions, technical product manual, or other professional resource or adviser available to you. Always read, understand and follow all manufacturer instructions.