Direct burial cable is a special type of electrical wiring, or cable, that is designed to be run in a trench underground. The electrical wires in the cable are encased in a thermoplastic sheath that seals out moisture and protects the wires within. While direct burial cable can be run inside conduit for additional protection, the cable is designed to withstand direct exposure to soil and moisture and is rated for wet, dry, and damp environments.
Types of Direct Burial Cables
The most common types of direct burial cable used in residential projects are underground service entrance (USE) and underground feeder (UF). Type USE cable is usually black and is most often used for buried lines that bring the power from the utility's transformer to individual houses. Type UF cable is usually gray and comes in rolls that look like standard non-metallic (NM) sheathed cable (often referred to by the brand name Romex). While standard NM cable is rated only for dry, interior applications, UF cable can be used outdoor as well as indoors. If you want to install cable between the house and an outdoor lamppost or to run power out to a garden shed or detached garage, UF cable is the standard choice.
The primary difference between standard NM and underground feeder (UF) cable is in the cable construction. Standard NM cable contains wires that are wrapped with paper and a relatively loose plastic sheath.
UF cable has wires that are completely encased in a solid plastic sheath. This encasement protects each wire from the others and does not allow moisture or other external elements to travel inside the cable.
The Rise of Buried Cable
Most homes in older neighborhoods in America have overhead service entrances, the power connections to the utility grid.
The disadvantages of running wires overhead include having poles in your yard and the dangers of having an exposed power line that can be run into by ladders or damaged by tree branches or other natural elements. Running power lines underground means they’re not susceptible to storm damage, and there are no exposed cables to worry about. Of course, there is one big concern with buried cable: digging (see tips below for how to dig safely).
Tips for Installing Direct Burial Cable
Direct buried cable is governed by many building and electrical code rules to ensure safe installation. Always check with your local building department to learn about specific requirements in your area. Here are some basic guidelines to keep in mind:
- Bury UF cable 18 inches deep or deeper, depending on the local code requirements.
- Call the national Call Before You Dig hotline at 8-1-1 several days before starting a project. This will alert all utilities with service lines in your area, and a representative will come out and mark the lines on your property so you know where you can and cannot dig.
- Calculate for voltage drop when running a long distance with underground cable. You may need to use larger cable to minimize a loss of voltage over long runs.
- Create a map of your property showing where you installed underground wiring, and store it in a safe place where it can be referenced for future projects. Wiring may need to be run on an angle from your home, so be sure to use something as a reference from point to point.