Electrical conduit plays a vital role in safeguarding electrical wiring and cables against hazards such as moisture, fire, corrosion, impact, electrocution and short circuits. With a multitude of conduit options available, engineers and contractors must carefully consider the specific location and environmental conditions for a project when selecting the appropriate conduit for an underground application. Choosing the wrong conduit can pose safety risks and lead to substantial financial consequences.
Underground conduit options available
Traditional metal conduits
Traditionally, steel or aluminum heavy metal conduits were prevalent in electrical conduit projects. However, the market has shifted towards lighter, cost-effective and durable alternatives like fiberglass conduit.
Rigid Metal Conduit (RMC): Comprising heavyweight galvanized steel, stainless steel or aluminum and installed with threaded fittings, RMC is robust but heavy, leading to higher material and labor costs.
Electrical Metallic Tubing (EMT): Thin-walled and unthreaded, EMT is commonly made of coated steel or aluminum and is suitable for indoor applications. It is not recommended for outdoor or underground use due to its susceptibility to damage and the need for special watertight fittings.
Fiberglass conduit is an affordable option with numerous advantages, including its lightweight nature and resistance to temperature, moisture and corrosion. It is a preferred choice for utilities, commercial and industrial applications that require direct burial and encased burial below ground. Its lightweight and unthreaded joints make it easy (and cost-effective) to install and its low coefficient of friction eliminates the risk of burn-through for long cable pulls. Additionally, it is nontoxic and chemically inert, contributing to its environmental friendliness.
PVC electrical conduit is known for its cost-effectiveness, durability and protection against moisture and corrosion. It has limited temperature tolerance and emits toxic fumes when melted, making it unsuitable for projects exposed to extreme heat or cold.
PVC-coated steel conduit
PVC-coated steel conduit is used in corrosive environments that also require mechanical strength. Unfortunately, its weight impacts portability in the field, making installation costly.
Considerations for electrical conduit in underground applications
Key factors to consider when selecting conduit include its protective qualities, ease of installation, installation cost, long-term durability, upfront costs, availability and lead times. It’s crucial to evaluate attributes such as corrosion resistance, temperature range and impact resistance to ensure the longevity of your installation.
Underground electrical conduit requirements
Authorities such as UL, CEC and NEC publish code requirements to guide electrical conduit in belowground applications.
UL 2420 BG
UL 2420 outlines the requirements for low-halogen belowground (Type BG) reinforced thermosetting resin (RTRC or fiberglass) conduit and fittings, designed for installation and use in compliance with CSA C22.1, the Canadian Electrical Code (CE Code), Part I and NFPA 70, the National Electrical Code (NEC).
UL 2420 encompasses ID and IPS conduit and fittings in trade sizes ranging from 1/2 (16) to 6 (155), covering both encased burial and direct burial conduit.
UL 94 HB
Underground electrical conduit must meet UL 94 HB (horizontal burn) requirements for belowground use. Horizontal burn standards are less stringent than vertical burn requirements, indicating that any conduit approved for aboveground applications is also suitable for underground installations.
NEC for underground conduit
Article 355 of the National Electrical Code (NEC) specifies the accepted use, installation and construction standards for fiberglass conduit. This rigid nonmetallic raceway with a circular cross-section, integral or associated couplings, connectors and fittings is approved for underground installations in trade sizes ranging from 1/2 to 6.
Determining the depth for burying electrical conduit underground
The depth requirements for burying electrical cables and wiring (direct burial installations) are outlined in Table 300.5 of the NEC, varying from 4 inches to 24 inches deep, depending on the wiring method employed.
Comparative costs of electrical conduit for underground applications
For direct burial installation, there can be a wide variance in installation time based on the type of conduit. For example, according to the NECA Manual of Labor Units, man-hours to install 100 feet of 4-inch SW fiberglass conduit are 8.25. For the same size and length, GRC conduit takes 30 man-hours, PVC SCH 80 takes 16.8 and PVC-coated steel takes 38 man-hours. Installation man-hours do not change. Installation of fiberglass conduit for many widths will always be more cost-efficient due to the conduit’s light weight.
To compare material and installation costs across electrical conduit types—fiberglass, PVC and various traditional metal conduits, including GRC, EMT, aluminum, PVC-coated steel and stainless steel—for underground applications, visit the Champion Fiberglass Conduit Calculator.