Heat Management in Underground Power Cable Installations

The demand for electrical power around the world is increasing and utilities, service providers, and network owners are investing in expanding and securing the grid. This increase in demand is driven by several factors including the need for more reliable electricity, new power generation sources such as renewable energy, the growth of electric vehicles (EV), the increase in virtual currency mining, the proliferation of data centers, the migration of rural populations in many developing countries to urban areas, and the need to protect infrastructure from environmental damage. More and more, these systems are moving to underground installations that use conduit (duct, pipe) systems rather than direct burial. While historically undergrounding cable in duct has been perceived as more expensive and less practical, there is a growing trend to move towards ducted systems because of the long-term economic, operational, and safety benefits. Specifically, building the infrastructure for ducted systems provides a future-ready framework to more efficiently maintain, monitor, and upgrade cable systems.

This move to ducted systems also raises new challenges for some, particularly in managing heat generated by electrical resistance in the conductors. Some utility engineers, design consultants, manufacturers, and cable installation contractors are looking to better understand how the adverse effects of heat generated in underground cable can be better quantified and managed to curb its impact on the performance of the power grid and the environment.

Designing underground systems that inadequately manage heat can lead to reduced cable capacity, shortened service life, and increased costs. Yet, there are countless ducted cable systems around the world that operate reliably despite the heat factor. To see how many are addressing the thermal properties of ducted cable in design and practice, Polywater assembled a small group of experts to discuss the topic of heat generation and dissipation in power cables and the management of heat for new underground cable installation practices.

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Understanding Thermal Behavior in Underground Cable System Design

Power cables can generate substantial heat as electrical current flows through them. This heat, caused by electrical resistance in the conductor, is known as Joule heating and represents lost energy. The greater the current or resistance, the more heat is generated, reducing efficiency and increasing operational costs for both utilities and end users.

Underground cable installations can be susceptible to heat-related challenges due to the insulating nature of soil and the confined environment. In direct-buried cables, the jacket contacts the surrounding soil directly. In contrast, ducted systems introduce an additional heat transfer interface—the conduit wall—which generally has a larger outer surface area. Ducted cable systems can potentially dissipate heat more efficiently due to the larger outer surface area of the conduit having increased contact area with the soil.

However, both direct-buried and ducted installations can have limitations in how well they dissipate heat, especially in environments with poor thermal conductivity. This low diffusion of heat causes cable temperatures to rise, which can degrade insulation, shorten cable life, and reduce current-carrying capacity.

As Marius Engebrethsen, Project Manager at REN—a Norwegian engineering think tank—explains, “That’s why we care so much about the heat. It’s often the limiting factor of how much current you can run through the system. Too much heat can speed up the degradation process. It is critical to manage this during system design.”

Effectively managing heat begins with understanding how it moves through the system. Cable conductors, insulation, screens, jackets, ducts, air gaps, backfill, and soil all have different thermal resistivities. These materials influence how well heat dissipates from the cable core outward. To accurately assess this behavior, thermal analysis is required—typically using industry standards, specialized software, or both.

“The main use of the software,” says Engebrethsen, “is to design underground cable installations. You can draw the trench layout, run advanced calculations, and analyze how the heat builds up and spreads—even how magnetic fields behave. This allows engineers to size the cable and optimize load capacity.”

Marcel den Haan of Grid Design Solutions and Business Development of Grøft Design® further explains, “You need to understand the maximum load on the cable so you can adjust variables like cable diameter, load, or the surrounding air. You’re monitoring what’s happening around and on top of the cable to see how far you can go.”

One of the most influential factors in heat dissipation is soil composition. Soil’s ability to retain moisture—critical for thermal conductivity—depends on its granularity and pore structure. Fine-grain soils like clay retain water well due to capillary action, while coarse soils like sand drain faster but can aid vertical moisture migration.

Importantly, as the cable heats, it dries out surrounding soil, forming a “dry zone” with lower thermal conductivity. Soil or backfill with high moisture retention helps mitigate the formation of these hot zones.

“The soil might not seem interesting to electrical engineers,” says Engebrethsen, “but it’s critical to developing a good cable installation. You want soils with low thermal resistivity that can hold moisture even when the cable is heating up.”

Cable burial depth also plays a role. Deeper installations increase the distance heat must travel to dissipate, lowering ampacity. To account for these variables, comprehensive soil testing is often performed.

Jacco Smit of TenneT describes the process, “We do a soil investigation along the whole route. If we design horizontal directional drillings to 40 meters, we analyze all oil layers present with their thermal capacities and recalculate to a new thermal resistivity value as a whole.”

These investigations help quantify the effects of soil conditions on cable ampacity, the formation of dry zones, and whether the heat profile of an installation needs to be adjusted.

Strategies for Managing Heat in Underground Cable Installations

Given the significant impact that heat generation and dissipation have on both the efficiency and cost of underground power systems, effective heat management is essential. Uncontrolled heat can accelerate the aging of cable components, reduce ampacity, and increase the risk of failure. On the other hand, managing thermal behavior allows utilities to maximize the load capacity and reliability of their networks.

Engineers have several tools at their disposal to control heat in underground installations. These include careful selection of materials, conductor sizing, increasing cable spacing, burial depth optimization, and thermal mitigation technologies. As Jacco Smit, Senior Advisor on Asset Management at TenneT, explains, “We always want to meet the demand that is required for the connection. There will be hotspots, of course, but then we will choose a bigger conductor cross-section or adjust the thermal properties of the surrounding soil or duct.”

Smit emphasizes that thermal design begins with understanding the system’s required load. “In my thinking,” he says, “we have an obligation to go from A to B to transport some quantity of MVAs or amperes through the connection, and we will find the best fit for cable sizing and so on.”

Beyond basic design parameters, advanced heat management can involve strategic use of subsurface water to aid heat dissipation, cross bonding to reduce sheath losses, and localized analysis of sheath current effects. In long-distance installations, cable configurations such as trefoil (triangular) layouts are used to reduce magnetic interference and maintain thermal balance.

Smit also points to Horizontal Directional Drilling (HDD) as an area where heat management becomes critical. “What we found through calculation verification with thermal monitoring,” he notes, “is that with an HDD installation, temperature peaks occur at the entry and exit points of the bore. The temperature profile spikes up, drops through the middle of the drilling, and then spikes again on the other side.”

To mitigate these localized hot spots, civil engineering measures can be implemented. “We take actions to spread the ducts at the entry and exit points, and cut the duct under the soil water level” Smit explains. “This helps distribute the temperature more evenly and create a better thermal equilibrium.”

These examples highlight that heat concerns in underground systems, while complex, are manageable. With a clear understanding of thermal dynamics and the right design and monitoring strategies, utilities can confidently move toward undergrounding without compromising performance.

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The Role of Standards in Managing Heat Dynamics

As underground cable installations become more prevalent, understanding and managing heat dynamics is critical to maintaining system performance and longevity. While material selection, design, and analysis tools help engineers address heat at the project level, international standards and guidelines provide a shared framework for modeling and managing thermal behavior in power cable systems.

These standards vary by region, reflecting environmental factors such as ambient temperature, soil conditions, rainfall, and load profiles. As Marius Engebrethsen of REN explains, the IEC 60287 standard is one of the most widely used in Europe and globally. “It’s based on analytical methods,” he says, “and is commonly used to generate calculations relevant to heat generation and dissipation.”

In the U.S., the IEEE 835 serves as a counterpart. It combines both analytical and empirical approaches, making it useful for modeling not just steady-state loads, but also cyclic and emergency conditions. Additionally, Engebrethsen highlights CIGRE’s Technical Brochure TB880, which incorporates Finite Element Analysis (FEA) and provides use cases that allow simulation software to be benchmarked against practical scenarios.

While these standards form a strong foundation, they aren’t without limitations. For example, IEC 60287 assumes constant load profiles and uses static data tables, which can be outdated or misaligned with actual field conditions. This can lead to inaccurate results in modern dynamic environments.

Marcel den Haan of Grøft Design underscores this challenge: “There are a lot of variations where you have to be more precise than what’s in the standard. Even when you work within the standard, you still need to calculate the most difficult parts. If you don’t, you risk drying out the soil or exceeding the cable’s maximum temperature.”

Some standards also assume uniform burial depths and homogeneous soil properties—assumptions that rarely hold true in real-world projects. These simplifications can lead to under- or overestimating heat-related risks.

Beyond IEC 60287 and IEEE 835, other relevant standards include IEC 60853-1 for cyclic and emergency current ratings, IEEE Std 835 for ampacity calculations in ducts and raceways, and NEC Article 310.15(B), which provides ampacity tables and adjustment factors.

As simulation software continues to advance—offering higher precision and the ability to account for localized conditions—some constraints in these standards will be eased. This evolution will enable planners and engineers to more accurately model underground cable systems and fine-tune designs for optimal heat dissipation, extending cable life and improving overall grid performance.

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Evolving Thermal Management in Underground Cable System Design

Underground ducted cable installation has a history spanning more than a century in many parts of the world. Over this time, power utilities, engineering consultants, civil contractors, and manufacturers have accumulated valuable knowledge and practical experience. These lessons, built on both successes and challenges, have shaped the current approach to underground cable system design. Still, misconceptions about how to manage heat in ducted cable systems persist, often rooted in outdated information and practices.

In early underground ducted cable installations, pipes containing asbestos were commonly used. This material was highly fire-resistant and non-conductive, but it was also known for its high heat-insulating properties. To compensate for the heat produced when a cable was installed in duct, its maximum current capacity was reduced using a universal de-rating factor. Yet, with the introduction of plastic cable jackets and duct materials, this factor no longer accurately reflects current thermal conditions.

“The influence of pipes on the long-term permissible current of cable lines is diverse, and it cannot be described by introducing a universal correction factor,” says Mikhail Dimitriev, electric power system consultant at Voltplace. “In many cases, laying cables in pipes does not reduce the permissible current—on the contrary, it increases it,” Dimitriev asserts.

A lack of understanding regarding the thermal characteristics of newer duct and jacket materials has, at times, discouraged broader use of ducted installations. Many utilities have defaulted to open-trench methods, assuming they offer better heat dissipation and lower construction costs. Though this perception is increasingly being challenged.

One method used to manage heat involves flooding ducted cable systems with a bentonite clay slurry. Initially bentonite was applied to lubricate the cable during installation, but it was also found to assist in heat dissipation. However, this method of flooding with bentonite has fallen out of favor for several reasons.

First, the introduction of engineered water-based lubricants in the latter half of the 20th century proved far more effective at reducing friction during cable pulls. These lubricants enable longer cable lengths to be installed safely over greater distances.

Second, it was discovered that, over time, bentonite clay caused the cable to bond to the duct, making removal or replacement extremely difficult. In many cases, conduit remediation was required before new cables could be installed.

Despite this, bentonite clay is still used in some ducted cable installation processes—most notably during horizontal directional drilling (HDD). In this context, it is applied around the outside of the pipe to ease duct installation. The viscous consistency of the slurry also helps maintain the integrity of the borehole by supporting its walls and preventing collapse during drilling and installation.

Regardless of growing evidence and field experience, misconceptions about heat dissipation in ducted cable systems persist in many regions. These misunderstandings risk overshadowing the many benefits of conduit-based installations, including enhanced protection from mechanical damage and fire, increased system reliability and efficiency, simplified maintenance and faster fault location, and infrastructure readiness for future upgrades.

Most importantly, ducted systems support the long-term flexibility required to meet rapidly growing power demands. Many engineers now design duct systems for use over the course of several cable lifespans.

“In our new cable policy, we specify a certain quality of conduit in such a way that the duct can to be used a minimum of two times,” says Jacco Smit of TenneT. “So, we install it once, and during the lifetime of 50 years of the cable system, the cable will be pulled out and after cleaning the duct a new cable will be pulled in. Then, within 50 years when power demands grow and a larger cable is needed, the duct and infrastructure will be present —which makes retrofitting less impactful on the environment and civil planning.”

Smit adds, “We also take into account that the conductors will be bigger in the future due to new cable development, so the duct is robust and big enough to have a lifetime of 100 years.”
Advancements over the last few decades in cable and duct materials—along with improved planning and installation practices—have significantly enhanced the thermal performance and current-carrying capacity of underground systems.

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Conclusion

As underground ducted cable installations increase to meet rising power demands, a solid understanding of thermodynamics is essential. All power cables generate heat, and when that heat isn’t properly dissipated, it can compromise both the efficiency and lifespan of medium- and high-voltage systems. To maintain reliable service and maximize current-carrying capacity, heat must be carefully predicted and managed.

Optimizing thermal performance depends on several factors—from system design and material selection to accurate modeling and adherence to standards. Awareness of how these elements interact, along with the ability to apply and interpret thermal calculations, is critical to successful project planning.

By advancing the understanding of thermal factors in underground ducted cable networks and implementing heat management strategies, utilities open the door to the numerous long-term economic, operational, and environmental benefits that underground power networks provide.

Meet the Contributors

Jacco Smit
Senior Technology Advisor Cable and Lines
TenneT
LinkedIn

Marcel den Haan
Grid Design Solutions and Business Development
Grøft Design®
LinkedIn

Marius Engebrethsen
Project Manager
REN
LinkedIn

Mikhail Dimitriev
Electric Power System Consultant
Voltplace
LinkedIn 

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