Underground Electrical Wire

If you've ever installed utilities or run power underground, you likely know how important using the correct cable is.
Direct burial wire is no different. this wire type can be buried without conduit or metal cladding, and its rugged insulation and flexibility are perfect for underground cable applications.
No matter the type of underground work, a direct burial cable is generally required. But what counts as a direct burial wire, where do you use it, and can it be substituted for other wiring types?

What Makes a Wire Direct Burial Rated?
According to the National Electrical Code (NEC), for an electrical wire to be considered direct burial rated, it must be clearly labeled for direct burial use. Don't assume every thickly insulated wire is a direct burial cable.
Also, unlike outdoor and underground burial cables, direct burial wire has a specific UL rating noting that it has passed rigorous testing and can survive in harsh underground environments.

As a result, this wire type excels in underground utility applications. It's faster and more cost-effective to install than cables that need conduit or metal cladding and doesn't need any additional materials to support it.

How Deep Should I Bury This Stuff?
Depending on the type of direct burial wire and application, there are various rules to prevent accidents.
Underground Feeder (UF) cable must be buried at least 24 inches underground. Meanwhile, PVC conduit only has to be buried 12 inches underground. Metal conduits can be buried six inches underground - the same depth that low-voltage wiring (think pet fence wire) is buried at.
Also, if your direct burial wire terminates above ground, it must be shielded using a conduit before surfacing.

What Types of Wire Insulation are Used?
Direct burial wire is typically protected by one of several insulation materials:
XLPE - Called cross-linked polyethylene, XLPE is typically used for RHW/USE-2 (Underground Service Entrance) cable. This insulation provides excellent moisture and heat resistance, alongside abrasion and crush protection.
HMWPE - High-Molecular Weight Polyethylene is used as an insulation for Cathodic Protection Cable and tracer wire, offering protection from moisture, chemicals, and abrasion damage.
PVC - Polyvinyl Chloride jacketing is often used to protect vinyl nylon tray cable (VNTC) and UF cables from abrasion, crushing, and other environmental hazards.
Insulation thickness is also important. The cable needs enough insulation surrounding the conductors to withstand being dragged over rocks and through dirt during installation. To comply with UL ratings, direct burial cable is abrasion and crush resistant.
Once the wire or cable is installed, the insulation has to hold up across a wide temperature range, chemicals, moisture, gas, oil, and other dangers.

Where Is Direct Burial Wire Used?
Direct burial wires are used for many jobs, including:
Running electricity
Tracer wire installations for underground utilities
Pet Fencing
Phone and other telecommunication cables
Fiber Optics
Irrigation and golf courses
Cathodic Protection

Different types of cabling get buried at different depths based on the project. Smaller, low-voltage wires like pet fencing live only a few inches below the surface, but larger gauge cables like power lines and underground utilities must be buried deep enough to avoid accidental damage caused by digging.
Regardless, direct burial wire should be used in compliance with local municipality rules and guidelines to ensure safe installation.

Can you Substitute Other Wire Types for Direct Burial?
The short answer is no. The long answer is no, because the substituted wire may not be similarly rated.

There is no shortage of wire types on the market today, and they don't all do the same thing. Substitutions might seem like a great idea, especially if they save money on upfront installation costs. But if the wire isn't direct burial rated, it will quickly deteriorate and cause problems.

Suppose workers install new underground utilities using THHN instead of tracer wire. THHN may seem like a safe substitution, but it doesn't have a direct burial rating. Its nylon insulation won't provide good moisture protection.

As the years wear on, ground moisture begins breaking down the THHN insulation, cracking it, and exposing the copper conductor underneath to the elements. Eventually, the corrosion becomes too much for the conductor. After only a few years, the signal is lost, and the door has been opened for potential mishaps.

Part of working with underground wire and cable is understanding the nuances associated with external and underground usage. These projects often require unique formulations that account for the environment around them (for example, PVC insulation has both -20C and -55C variants), offer chemical resistance, and can survive in low and high temperatures.

Wire thickness will also come into play during the installation process. If the insulation is too thin, rocks, roots, and other hazards could cause abrasion damage during pulling. Thicker insulation that adheres to UL standards is needed.

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