Improper burial depth of liquid-tight conduits leads to costly repairs and potential electrical hazards in industrial settings. Weather exposure, mechanical stress, and ground movements can severely damage insufficiently buried conduits, resulting in system failures and safety violations.
This comprehensive guide outlines NEC requirements, burial specifications, and key considerations for different conduit types to help you implement secure, code-compliant installations that protect your electrical infrastructure.
National Electrical Code (NEC) Requirements
The National Electrical Code (NEC) provides minimum depth requirements for burying conduit to ensure the wiring is adequately protected from damage. The 2020 NEC generally requires underground conduit to be buried at least 18 inches below grade. This applies to both metal and nonmetallic conduit.
However, there are some exceptions and variations to this general requirement depending on the specific installation circumstances and type of conduit used. Local jurisdictions may also have additional or more stringent depth requirements that need to be followed.
Exceptions to the 18″ NEC Minimum Depth Requirement
Under Driveways or Concrete Slabs
When running conduit under driveways, parking lots, or concrete slabs that will support vehicular traffic, the NEC requires increasing the burial depth to at least 24 inches. This extra depth helps protect the conduit from the additional load and potential damage from vehicles driving or parking over the underground area.
Areas with Heavy Frost
The NEC requires a minimum burial depth of 24 inches in areas where frost is a concern, but local codes may mandate even deeper burial depending on the severity of local frost conditions.
Low-Voltage Wiring
For low-voltage wiring systems of 30 volts or less, such as landscape lighting or irrigation controls, the NEC reduces the minimum burial depth to just 6 inches. However, it’s still recommended to provide additional protection by running low-voltage wiring through conduit, especially in areas with aggressive soils or a greater risk of damage.
Conduit Type and Minimum Burial Depth
Liquid Tight Flexible Metal Conduit (LFMC)
LFMC is made from spiral-wound metal strips with a flexible plastic outer jacket. It provides good protection and flexibility for transitioning between buried conduit and exposed areas. The NEC allows LFMC to be buried at the standard 18″ depth as long as the plastic jacket is intact and suitable for direct burial.
Liquid Tight Flexible Nonmetallic Conduit (LFNC)
LFNC is a nonmetallic conduit made from PVC with reinforcing fibers that provide flexibility and strength. When using LFNC rated for direct burial, the NEC permits reducing the minimum burial depth to just 12 inches for areas not subject to vehicular traffic. This can make installations easier in tight spaces or shallow trenching situations. However, LFNC should still be buried 18-24 inches when under driveways or in frost-prone regions per the exceptions noted earlier.
Factors Influencing Burial Depth
Soil Conditions
The composition and stability of the soil is a primary factor in selecting burial depth. In areas with loose, sandy, or unstable soil, the conduit should be buried deeper to prevent shifting or damage. Clay-rich soils that expand and contract with moisture changes may also require greater depth. Soil with good drainage properties and moderate compaction is ideal.
Trenches should be dug to a consistent depth, with the bottom leveled and cleared of rocks, debris, or sharp objects that could damage the conduit. A layer of sand or fine gravel can be added below and above the conduit for additional protection in challenging soil conditions.
Proximity to Other Utilities
Liquid tight conduit must maintain sufficient clearance from existing underground utilities such as gas lines, water mains, sewer pipes, or telecom cables. Consult local utility companies and “call before you dig” services to identify and mark any buried lines in the area.
Electrical conduit should be separated from other utilities by at least 12 inches of soil. If this spacing cannot be maintained, rigid concrete or PVC spacers can be used. Greater burial depth may be necessary to achieve proper clearance in congested utility areas.
Environmental Factors
The local climate and environmental conditions impact burial depth decisions. In regions with heavy rainfall, snowmelt, or overland flooding, conduit should be buried below the frost line and engineered to resist water infiltration. Waterproofing compounds, sealed fittings, and additional drainage layers help protect against moisture damage.
Extreme temperature swings cause soil to expand and contract, potentially shifting the conduit. In cold climates, the burial depth must be below the maximum frost line to prevent freezing. Hot, arid regions may require special techniques to dissipate excessive heat build-up around the wires.
Traffic and Load Considerations
Under pedestrian walkways, a minimum cover of 18 inches is typically sufficient. However, conduit placed below driveways, parking lots, or roads with vehicular traffic must be buried at least 24 inches deep to withstand the increased load and vibration.
Conduit installed below concrete slabs should be covered by at least 4 inches of concrete on all sides. If heavy equipment, trucking, or industrial activities are anticipated, even greater slab thickness and burial depths are advisable.
Flood-Prone or Wet Areas
The conduit should be rated for prolonged submersion and sealed to prevent water entry when installing liquid tight conduit in flood zones, wetlands, or areas with high water tables. Locate the conduit above the high-water mark if possible, or bury it substantially deeper to minimize the risk of damage from erosion or floating debris during flooding events.
Use GFCI protection on circuits in wet environments and install additional grounding and equipotential bonding to prevent shock hazards.
FAQs
Can you bury seal tight conduit
Yes. But standard seal-tight (LFMC – Liquid-Flexible Metal Conduit) cannot be directly buried underground. For underground installations, you must use LFNC-B (Liquid-Flexible Nonmetallic Conduit Type B) which is specifically rated and UL-listed for direct burial applications according to NEC standards.