EMT is a common, cost-effective wiring method frequently used in commercial and residential construction for exposed or concealed interior locations. This thin-walled steel raceway provides mechanical protection for conductors within walls and ceilings. When considering an outdoor project requiring buried electrical lines, the question of whether EMT can be used underground is common. While recent code changes permit its use under specific conditions, EMT is generally not the recommended or practical choice for direct burial applications.
Why EMT is Unsuitable for Direct Burial
EMT is manufactured from thin-walled steel protected by a galvanized coating, typically zinc, applied to the exterior surface. This zinc layer provides sacrificial protection, meaning the zinc corrodes before the underlying steel, extending the tubing’s life. However, this thin protective layer is not engineered to withstand the corrosive environment found in most soils.
Soil contains moisture, varying pH levels, and dissolved minerals that accelerate the breakdown of the zinc coating through chemical and electrolytic action. Once the zinc is breached, the thin steel wall is exposed to oxygen and water, leading to rapid oxidation, commonly known as rust. This corrosion quickly compromises the tubing’s structural integrity, allowing water to enter and damage the insulated wires inside.
The failure of the conduit creates an electrical hazard, as the protective metal shell can no longer reliably serve as an equipment grounding conductor. Even though the 2023 National Electrical Code (NEC) permits EMT for direct burial with specific fittings, the practical risk of premature failure remains. For long-term reliability, especially in areas with high soil acidity or moisture, the thin-walled galvanized steel is outmatched by the underground environment.
Approved Conduit and Cable Alternatives for Underground Use
For underground electrical projects, several materials offer superior corrosion resistance and durability compared to EMT, providing a more reliable and long-lasting installation. The most common and cost-effective alternative is Polyvinyl Chloride (PVC) conduit, which is entirely non-metallic and immune to rust and electrochemical corrosion.
PVC is available in Schedule 40, suitable for most applications, and the thicker-walled Schedule 80. Schedule 80 is often required where the conduit is exposed to physical damage, such as where it emerges from the ground.
Another robust option is Rigid Metal Conduit (RMC) or Intermediate Metal Conduit (IMC), which are made from thicker galvanized steel than EMT. RMC and IMC are threaded and feature a heavier zinc coating, making them suitable for direct burial. They are often used in areas requiring maximum physical protection, such as under driveways or heavy traffic areas. While more expensive and labor-intensive than PVC, these metal conduits offer superior mechanical strength and are common in commercial and industrial settings.
For projects that do not require the ability to pull new wires in the future, Underground Feeder (UF) cable is a practical alternative that eliminates the need for conduit entirely. UF cable is a specific type of non-metallic sheathed cable jacketed in a solid, moisture-resistant thermoplastic material, allowing direct burial. This cable must still be installed at the minimum burial depths required by the NEC, but it simplifies the installation process by removing the need for a separate raceway.
Essential Code Requirements for Underground Wiring
Regardless of the material chosen, all underground wiring installations must adhere to specific safety and depth requirements outlined in the National Electrical Code. The minimum burial depth for a circuit is not a single number but varies based on the wiring method and the location of the trench. For instance, a run of PVC conduit under a residential yard requires less cover than a run of direct burial UF cable.
The required depth also increases if the wiring is installed under a driveway, a public road, or beneath a concrete slab, as these locations are subject to greater physical stress and require enhanced protection. These depth requirements prevent accidental damage from digging, excavation, or surface loads. It is necessary to check the specific NEC table for the exact depth required for the chosen conduit or cable type in the intended location.
Another important requirement is the need for physical protection where the underground wiring transitions above-ground. Even if non-metallic conduit like PVC is used underground, the section extending vertically out of the soil is vulnerable to damage from lawnmowers, vehicles, or other impacts. In these exposed locations, the code mandates the use of a more durable material, such as Schedule 80 PVC or a section of RMC. This protection must extend from the trench depth to a specified height above grade.