Duct banks explained: Expert insights on planning, design and installation
Growing energy demand, evolving security concerns and increased risks of weather-related disruptions are changing how data and electricity networks are designed. The need for more resilient infrastructure has increased interest in solutions that offer enhanced durability, reliability and simplified maintenance, such as concrete-encased duct banks.
We interviewed four underground electrical infrastructure experts — Marius Engebrethsen, Lars Solbjørg, Kåre Yttervik and Rob van Hoorn — about the opportunities and limitations of duct bank systems compared with alternatives, as well as what utilities and contractors should consider regarding duct bank design and installation to optimize project costs and outcomes.
What is a duct bank?
Duct banks are a cable protection and management solution for power and data distribution infrastructure. Typically buried underground, a duct bank consists of one or more cable ducts encased in concrete for enhanced mechanical protection, improved cable organization and easier replacement and maintenance.
Why are underground duct banks growing in popularity?
Underground duct bank runs are used to protect critical infrastructure from extreme weather events and sabotage. In an increasingly challenging geopolitical and climatic environment, securing electrical infrastructure has become essential to ensure a reliable power supply for households, healthcare services, industry, and defense.
Around 80-90% of today’s cable faults are caused by outside mechanical interferences, and concrete-encased duct banks are highly resilient against them, explains Lars Solbjørg, Founder of OPI duct bank systems:
“We have had cases where our duct banks became subject to careless excavation, landslides, flooding… And every time the encasement shielded the ducts inside, ensuring uninterrupted power distribution.”
Yet, today’s electrical networks are not at risk only during extreme events. Gradual increases in air temperatures and heat waves during summers have also been affecting the longevity of electrical infrastructure.
Cable derating is a process that occurs when bundling power cables generates excessive heat, reducing their ampacity. The effect can become more pronounced when surrounding backfill soils dry out, limiting heat dissipation from direct-buried cables.
Concrete conducts heat substantially better than dry sand, helping prevent overheating, maintain cable capacity, and extend their service life, says Marius Engebrethsen, Product Manager at Grøft Design, a specialized software for magnetic field and ampacity calculations of underground cable systems.
“Increased demand for electricity due to electric vehicle charging, data centers and reliance on air conditioning during heat waves has led to higher capacity requirements, sometimes exceeding what was planned. At the same time, high temperatures and dry soils reduce the actual cable capacity. As a result, we have recently seen more cable failures in regions affected by heat,” he adds.
Thermal resistivity of concrete vs. backfill soils |
|
|---|---|
| Concrete | ~0.6 m.K/W |
| Moist sand | ~1.0 m.K/W |
| Dry sand | ~2.5 m.K/W |
| Gravel | ~2.5-5 m.K/W |
Furthermore, underground space is becoming increasingly limited in many cities. Duct banks help improve cable management, allowing for dozens of utility lines to be organized into a single, organized structure. Thanks to the superior thermal properties of concrete, cables can be installed with smaller spacing while maintaining safe operating temperatures.
“The overall trench volume with duct banks can be reduced by 60-75% as ducts are laid as close as 3 cm together,” says Kåre Yttervik, Cable Protection Project Engineer at Pipelife Norway. "For comparison, the minimum required spacing between conventional ducts in Norway is 10 cm, and in Sweden, 5 cm.”
Moreover, increasing electrification means that underground electrical infrastructure laid today must be upgrade-ready tomorrow. Most duct banks allow cables to be pulled out and replaced or upgraded without repeated excavation, resulting in less disturbance and significant CO2 savings over their lifetime.
“When installing duct banks, the best practice is to plan for 20-50% higher capacity than currently needed. Using larger-diameter ducts or adding a few empty ones is significantly cheaper and more environmentally sustainable than performing another excavation a few years later,” says Yttervik.
Carbon footprint of electrical cable replacement |
||
Direct burial (digging up a cable) |
Cable duct system (pulling out a cable) |
Duct bank system (pulling out a cable) |
| ~640 CO2 tons | ~20 CO2 tons | ~20 CO2 tons |
Overview: Duct bank systems vs. cable ducts vs. direct-buried cables
Underground electrical installations are increasingly replacing overhead lines due to improved safety, longevity and aesthetics. However, even when electrical infrastructure is placed below ground, choosing the right installation method remains critical, as it may affect the network’s reliability, maintenance and service life. National guidelines often allow for several approaches, and in many countries, direct-buried cables, cable ducts and duct banks are all used.
Direct burial of data and electrical cables is the most common underground management method due to relatively simple installation and low initial expenses. After digging a trench, the cable is laid directly into the ground and covered with soil. The trench is lined with sand, with no additional protection for the cable, or with just simple cable tiles.
This leaves direct-buried cables highly vulnerable to excavation damage and soil shifts and makes them difficult to replace. They can also become susceptible to overheating if proper installation protocols are not followed, and installation errors, such as wrong backfill material used, or the recommended spacing between cables not observed, are common realities.
Cable Ducts (or conduits) are protective pipes buried underground that enclose cables and protect them from damage. Ducts are used for both open-trench and trenchless technologies, with manholes and chambers installed between them and serving as junctions and access points. Quality underground duct systems last 50-100 years and allow the replacement and recycling of outdated electrical wiring without repeated excavation.
Ducts offer a considerably higher level of protection than direct burial; however, they can still be damaged during excavation or extreme weather events. Overheating may occur if installation guidelines are not met and conduits are placed too closely together.
Duct bank systems are typically used for cable management in critical infrastructure objects, extreme environments and dense urban areas. While duct banks have higher upfront costs, they offer the highest level of mechanical protection and boast extremely long service life compared to other underground cable management methods. Duct banks also save underground space, simplify cable organization and streamline upgrades and replacements of outdated cables without repeated digging, offering significant cost savings in the long run.
Still, planning and installing duct bank systems require specialized knowledge and should be performed by skilled professionals. Once the concrete cures, the structure cannot be easily altered.
Advantages |
Disadvantages |
|
Direct-buried cables |
• Low material and installation costs • Relatively simple installation |
• Highly susceptible to damage • Difficult to locate • Cannot be repaired or replaced without excavation |
Cable ducts |
• Good protection levels under most conditions • Simplified cable pulling • Long service life • Possibility for cable replacement/upgrades without repeated digging |
• Can be damaged during infrastructure updates or natural disasters |
Duct banks |
• Very high protection levels even under extreme conditions • Excellent service life (over 100 years) • Possibility for cable replacement/upgrades without repeated digging • Improved thermal properties |
• Higher initial costs • Higher installation complexity |
Duct bank planning, design and installation techniques
Each duct bank channel must be tailored to the specific project requirements. Before excavation, construction drawings must be made, taking into account:
• Cable type and capacity
• Required load-bearing
• Burial depth
• Crossing utilities and other obstacles
• Soil and backfill type
• Number and sizing of ducts
• Required drainage slope and bend radius
Based on these criteria, construction drawings are created to support correct trench setup and installation.
“When it comes to duct banks, most of our customers have performed calculations and created drawings themselves,” Yttervik says. “However, we also offer this service, together with our partners.”
On-site cast underground duct banks are the most common installation approach, with trenching being the first step.
• The exact width and depth of the excavation depend on project requirements and local regulations. However, the trench should be at least 100 mm wider on each side than the channel's outer width and graded toward manholes or vaults. Deep trenches must be adequately shored.
• For concrete-encased underground duct banks, original soil can be used as bedding, and coarse leveling is usually sufficient.
• Formwork holders and spacers are placed according to the drawing, and ducts are pushed in.
• After the duct system has been laid and reinforced, the conduit ends are sealed, and concrete is poured to fill the channel. Vibrating is often performed to eliminate air gaps between ducts.
• Once the concrete cures, formwork plates can be removed, and the duct bank should be examined to ensure there is no damage to the concrete or pipe misalignments or blockages.
• For concrete-encased underground duct banks, backfilling can be done using the original soil.
Ampacity calculations for duct banks
Heat management is crucial for all underground electrical installations and, ideally, should be accounted for in the planning stage, explains Engebrethsen. Insufficient heat dissipation leads to a reduction of cable capacity, shortens their service life and increases maintenance and repair expenses. While concrete has excellent thermal properties and, unlike sand, it is not susceptible to dryouts, laying a high number of cables closely together requires advanced thermal and magnetic field calculations:
“The commonly used analytical models rely on simplifications and may not be accurate enough when it comes to large-scale or higher-complexity duct bank designs.”
Grøft Design software utilizes COMSOL Multiphysics, which allows for advanced thermal and electromagnetic simulations of underground installations, incorporating a high variety of electrical engineering parameters, such as cable type and arrangement, conduit type and placement, concrete class, burial depth, soil and backfill properties, weather conditions and others.
“We often run simulations for highly specific conditions and designs, for example, mixed voltage cables cast within a single duct bank, added aluminum shields, inserted water-filled cooling pipes or nearby-located district heating lines,” Engebrethsen adds. “The goal is to compare various possible solutions to identify the optimal one that will ensure reliable performance, safety and regulatory compliance.”
Cable protection Pipes for duct banks
Since the concrete encasement bears most of the load, a wide range of conduit systems can be safely used in duct runs, including U-PVC, PP, HDPE. Plastic pipes are the most common choice across Europe due to their lighter weight, suitability for greater pull lengths and excellent corrosion resistance, explains Yttervik:
“The main criterion is that the pipes must be watertight. Concrete-encased duct banks actually allow for thinner-wall ducts to be used, SN4 stiffness class being the minimum in our projects.”
“When it comes to duct selection, established practices often dictate the choice,” adds Solbjørg. “For example, in Norway, ducts meeting the European Standard EN 50626-2 are preferred, compared to other countries that use ducts according to EN 50626-1.”
Concrete specifications for duct banks
In Europe, the standard concrete class for underground duct bank runs laid 20-30 cm deep is C20/25. It offers great balance between workability and durability, allowing for formwork removal within 24-48 hours.
Higher-strength grades and/or special mixes can be used for heavy-load applications, such as airports or military complexes, as well as ground-level or above-ground.
“Reliance on simple, handmade spacers and formplates can increase material costs for duct banks and slow down the installation process. Just a few extra centimeters left on one side or on top will require much more concrete to fill the bank. On the other hand, if there are a few centimeters missing, this can negatively affect the overall strength of the cured structure,” says Solbjørg.
OPI: industry-leading duct bank solution
For nearly three decades, Pipelife has worked with OPI to provide highly reliable, easy-to-install duct banks for even the most complex installation scenarios. Since its invention in 1990, the OPI system has been supplied to more than 15 countries, with over 25,000 km of cable ducts encased.
The OPI portfolio includes reusable formwork holders, formwork plates, and more than 125 spacer types, providing a complete solution for duct bank setup and casting, whether in a controlled setting or on-site.
While most underground duct bank runs are set up manually or with relatively simple supports, OPI’s interlocking modular spacer system is highly adaptable to various project needs, speeding up installation and minimizing the risk of alignment issues.
“Most spacer systems are designed for relatively small, simple configurations, housing two to eight, maybe up to 20 ducts. For comparison, the largest completed OPI duct bank to date contains 130 conduits, and we have even larger developments in progress,” says Solbjørg.
Advantages of OPI duct banks
Straightforward setup and installation: Supporting duct sizes from 32 to 250 mm, the OPI spacers enable simple, error-free setup of even complex, large-scale duct bank designs. The system is also suitable for joint installations, combining electrical cables of various voltages or housing data and electrical cables in the same bank.
Higher installation speed: The state-of-the-art spacer system facilitates and accelerates duct bank setup, enabling casting over 100 meters per day.
Effortless cable pulling: The precise designs of OPI duct banks enable longer cable runs, reducing installation time as well as the number of vaults and manholes required. Over-2400-meter cable pulls have been completed in one run, even with 90-degree bends.
Space savings: In the OPI system, conduits can be set as close as 3 cm together, offering up to 75% space savings compared to direct-buried wiring or cable ducts.
Cost savings: Compared to manually constructed plywood setups, OPI helps optimize project costs by minimizing concrete waste and accelerating installation.
Optimized energy transfer: For setups under increased electrical loads, special cooling pipes can be added in the bank, enabling air or water circulation to further enhance heat dissipation.
Suitability for prefabrication and challenging conditions: OPI is suitable for deep and shallow trenches, as well as underwater and above-ground installations. Furthermore, the highly adaptable spacer design makes OPI ideal for precast duct banks, enabling fast, error-free prefabrication of various configurations.
Extreme durability and protection: Once cured, the OPI duct banks are nearly indestructible and offer reliable protection to critical infrastructure for over 100 years.
Conclusion
As demand for reliable power and data transfer continues to grow, developers, contractors and utilities are increasingly seeking strategic and cost-effective solutions that can ensure superior reliability and adaptability of tomorrow’s electrical networks.
“Long-term performance and futureproofing of power networks are becoming fundamental design principles,” sums up Rob Van Hoorn, Cable Protection Category Manager in Pipelife. “Duct banks do not provide a quick-fix solution, but it is something that you build, and it will last, serving and protecting communities and industries for generations to come.”
Questions and answers
1. WHAT IS THE PURPOSE OF A DUCT BANK?
Duct banks are an advanced underground electrical infrastructure solution used to protect and organize critical power, data and telecommunications cables.
The main advantages of duct banks compared to direct-buried cables are:
• Advanced protection
• Improved cable organization and space savings
• Easy replacement and upgrades of outdated wiring
• Reduced environmental impacts
2. HOW DEEP DOES A DUCT BANK NEED TO BE?
The installation requirements of duct banks can be country-specific; therefore, checking local regulations is essential.
The OPI duct bank system is typically buried ~30 cm underground; however, it is also suitable for underwater, ground-level, and above-ground installations, provided the concrete specifications meet the required application.
3. WHERE ARE DUCT BANKS REQUIRED?
Underground duct bank runs are commonly used in large-scale infrastructure developments, such as airports, railways, logistics hubs, power plants, healthcare facilities, industrial complexes, military sites and data centers where reliable transmission of power and data is critical.
Nowadays, duct banks are also increasingly installed in urban areas as these systems save the coveted underground space and allow for electrical cable replacements and upgrades without repeated excavation.
4. ARE DUCT BANKS EASY TO INSTALL?
Duct banks must always be installed by professionals; however, installation complexity can be significantly reduced by using specialized formplate and spacer systems rather than conventional plywood setups.
5. WHAT PIPES SHOULD BE USED FOR DUCT BANKS?
In Europe, PP, HDPE and PVC conduits are the most common choice for duct runs due to their lighter weight and excellent corrosion resistance. Compared to other alternatives, plastic ducts have a lower coefficient of friction, allowing for longer pulls and fewer pull points per run.
The European standard EN 50626 is the most common reference for cable ducting. Still, ducts that meet Part 1 of the standard may lack watertight couplers, crucial for duct bank systems. Part 2 of the standard sets more stringent quality requirements for ducts and supports fixed-angle bends with a large radius. Therefore, products meeting EN 50626-2 are the optimal choice for duct banks.
6. WHAT ARE PRECAST DUCT BANKS?
Precast duct banks are used in cases when on-site pouring would be too complex or impossible, for example, on steep slopes, under roads or waterways, in areas with unstable soils or extreme weather conditions.
Prefabrication of duct bank components off-site can significantly reduce on-site installation time, allowing for a speedy setup once the trench is prepared.
7. ARE DUCT BANKS SUSTAINABLE?
Duct bank systems allow for the pulling of cables and the replacement of outdated electrical wiring without repeated digging, which minimizes their environmental impact and lifecycle carbon footprint. Moreover, duct banks boast exceptionally long service life and are engineered to last for over 100 years.
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