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Direct Burial Conduit: What You Need to Know

By Mar 1, 2023
Champion Fiberglass direct burial conduit
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Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, encased buried, direct buried and installation complexity—all depend on the project type and environment.

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, encased buried, direct buried and installation complexity—all depend on the project type and environment.

Aboveground and belowground conduit differences

The main difference in aboveground and belowground conduit is fire resistance. Aboveground conduit meets the fire-resistance standards of UL2515 and CSA C22.2 No. 2515. 

Belowground conduit meets UL94 HB (horizontal burn) requirements, which aren’t as stringent as vertical burn requirements.

Environmental factors in direct burial projects

Direct buried conduit is a type of conduit used in belowground installations of electrical systems in industrial and commercial construction projects. Typically, direct burial conduit is used in applications where there will be minimal digging after installation.

Direct burial serves to protect electrical systems from environmental factors such as trees, branches, wind and fire. It can also be an aesthetic solution or used for crossing roadways with minimal disruption to traffic patterns. Often, direct burial is used in applications such as data centers, wastewater treatment facilities and utilities.

Direct buried (DB) conduit must have the mechanical strength to withstand the soil load that is packed over it. In these situations, project managers have had success using Champion Fiberglass Standard Wall for ¾–4 inches in diameter, and Medium Wall for 5 inches and 6 inches (UL designates Champion Fiberglass MW for 5 inches and 6 inches as SW). In some situations, an even heavier wall should be selected.

Direct burial cable and conduit in industrial construction projects: What to consider

When determining whether your belowground conduit installation requires direct burial quality, there are several factors to consider:

  • Volatility of the environment: Frost and settlement can impact the stability of direct burial conduit, and soil corrosion can compromise the conduit.
  • Compacting of soil: For very deep trenches, special soil conditions or where a high rate of compacting can be expected, a heavier wall conduit should be selected.
  • Ease of access for later service: Consider a conduit’s coefficient of friction to evaluate for ease of pulling as well as cable fault resistance.
  • Project complexity: Challenging installations can benefit from the efficiency that carefully planned conduit use can provide for cable runs.
  • Local regulations: While the NEC (National Electrical Code) may approve a conduit use for direct burial, local regulations may call for more stringent installation techniques.
  • Future landscaping: It is important to consider whether landscaping projects may disrupt direct buried conduit.

Direct burial in action: Solar farm case study

A clean energy company was embarking on a solar farm installation consisting of the installation of five 35 kV circuits in a single 3.2-mile corridor on former working timberland.

Each circuit has the capacity of transmitting 44 MW of solar generation (a total of 22 MW) to the grid. Construction included underground burial of conduit-encased wire (0.75 mile in one continuous pull), as well as pole risers. The project involved significant trenching and the navigation of material shortages due to COVID-19.

Challenge

There were multiple circuits coming through the project. An interconnection facility was designed and installed bringing the solar facility to the grid. This involved five 35 kV circuits along 4 to 5 miles of cable to a new substation where voltage stepped up to 230 kV to connect underground to the grid.

Additionally, there were special requirements for long-segment installation due to landowner agreements forcing overhead lines underground. Buried installation posed a challenge, as concrete-encased duct banks can be a bit more complex than other types of installation. The engineers and project managers desired to eliminate as much cable, conduit and equipment as possible.

Solution

The engineers and project managers developed a scenario running cable underground in a straight-line pull. Fiberglass conduit was ideal for the job due to its broader temperature range, up to 250°F (121°C), so it tolerates more heat.

Use of fiberglass conduit allowed project managers to consider a more straightforward burial type. Durable, corrosion-resistant medium wall (MW) and heavy wall (HW) conduit provided mechanical protection so the conduit and cable could be buried directly without the need to pour concrete.

Trenches, 60 to 80 feet, were buried, and conduit was laid and connected with a slip-fit connection that was hammered into place. There was no need for epoxy, so fewer materials were needed and installation took less time.

Results

  • Optimized design that handles a significant cable load
  • Product secured in half the time of competing electrical conduit
  • Low material costs that were easy on project budgets
  • Seamless installation that shaved weeks off the timeline

Learn more

For projects requiring direct burial conduit, selecting the best underground conduit for the job is key. For more information about Champion Fiberglass conduit, reach out at http://www.championfiberglass.com or contact a rep.

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