Electricity demand is accelerating across the United States while the transmission system’s ability to serve new generation and new loads remains severely constrained. Data centers, advanced manufacturing, electrification and broader economic growth are increasing the amount of electricity that must be delivered, yet new transmission corridors can take many years to plan, permit and build. This has intensified interest in reconductoring existing lines with advanced conductors and deploying other Grid Enhancing Technologies that can unlock more value from infrastructure already in place.
Recent studies have made the need for action increasingly difficult to ignore. CERA Consulting’s August 2026 report, Powering Growth and Affordability: The Role of Transmission in Economic and National Security, describes transmission as essential infrastructure supporting economic growth, energy affordability and national competitiveness. Reports from the U.S. Department of Energy, Americans for a Clean Energy Grid, Grid Strategies, GridLab, Lawrence Berkeley National Laboratory, universities and other respected organizations also underscore the importance of increasing transmission capacity more quickly and cost-effectively.
These reports have helped move advanced conductors into the center of the grid-modernization conversation. That is encouraging, but it also makes it important to define “advanced conductor” carefully.
Some reports and promotional materials group nearly every high-temperature conductor into the same broad category. This can create the impression that ACSS, ACSS/TW and modern composite-core conductors offer substantially equivalent performance. They do not. ACSS can operate at high temperatures and remains useful in certain applications, but a high-temperature rating is not the same thing as high performance – and calculated ampacity is not the same thing as usable / delivered transmission capacity.
What the Technical Literature Tells Us
One of the most influential analyses of reconductoring is the UC Berkeley and GridLab study published in the Proceedings of the National Academy of Sciences, Accelerating Transmission Capacity Expansion by Using Advanced Conductors in Existing Right-of-Way. Its central finding is not that every high-temperature conductor can double transmission capacity. The study specifically examines the ability of advanced composite-core conductors to cost-effectively double capacity within existing rights-of-way.
The associated technical paper, The 2035 Report: Reconductoring With Advanced Conductors Can Accelerate the Rapid Transmission Expansion Required for a Clean Grid, is even more explicit. It explains that ACSS was introduced in the 1970s using fully annealed aluminum to enable operation at higher temperatures. It then identifies ACSS’s fundamental drawback: higher-temperature operation creates a risk of excessive sag due to the relatively high thermal expansion of its steel core.
The report notes that this characteristic can require taller structures or shorter spans on new lines. In reconductoring applications, it may require structural modifications to preserve existing clearances or force the line to be operated below its calculated thermal rating. Either outcome can reduce the practical benefit of selecting a conductor primarily for its high-temperature capability.
When the report introduces what it calls the “new path forward,” it turns specifically to advanced composite-core conductors that replace the conventional steel core with a smaller, lighter and lower-expansion composite core. Its national loadability analysis uses ACCC® Conductor as the modeling reference because ACCC Conductor combines approximately twice the thermal limit of the baseline ACSR conductor with lower resistance and substantially reduced thermal sag.
The paper does not suggest that ACSS can never be used. What it demonstrates is more meaningful: when researchers modeled the opportunity to double transmission capacity within existing rights-of-way, they relied on the combined performance of composite-core technology – not simply the ability of a steel-core conductor to withstand a higher temperature.
Ampacity Is Not Necessarily Usable Capacity
ACSS uses fully annealed aluminum that can operate at temperatures that would cause conventional hard-drawn aluminum to lose strength over time. This can produce an impressive thermal ampacity rating, particularly when calculations assume operation at 200°C or higher.
Transmission lines, however, do not operate inside ampacity tables. They operate between structures and above roads, rivers, buildings, vegetation and other facilities where minimum electrical clearances must be maintained.
As current increases, conductor temperature rises. As temperature rises, the conductor expands and sags. Once allowable clearance is reached, the practical capacity of the line has been reached – even if the conductor materials can withstand a much higher temperature.
At elevated temperature, the annealed aluminum in an ACSS conductor carries relatively little tensile load, and the steel core increasingly governs sag behavior. Steel has a much higher coefficient of thermal expansion than the carbon-fiber composite core used in ACCC Conductor. As the steel heats, it lengthens and the conductor sags.
Increasing the strength of the steel core does not eliminate this limitation. Strength and coefficient of thermal expansion are different material properties. A higher-strength steel core may support greater mechanical load, but unless the core material has a substantially different coefficient of thermal expansion, it will continue to expand as its temperature rises.
When sag is the constraint, strength alone does not solve thermal expansion.
ACSS/TW Helps Address Resistance – but Adds Weight
It is important to acknowledge that ACSS can be manufactured with compact trapezoidal aluminum strands. ACSS/TW packs more aluminum into a given conductor diameter than conventional round-wire ACSS. The additional aluminum reduces electrical resistance, lowers line losses at a given load and can improve calculated current-carrying capability.
Those are legitimate benefits, but the additional aluminum also increases conductor weight. While aluminum is relatively lightweight, adding substantially more of it along the full length of a transmission span creates a significant cumulative mechanical load. At elevated operating temperature, the steel core must support much of that additional weight as the fully annealed aluminum transfers most of its tensile load to the core.
The ACSS/TW designer therefore encounters an important tradeoff. Adding aluminum reduces resistance, but it also adds weight. A stronger steel core may help support that weight, but greater strength does not inherently change the thermal expansion of the steel-core system. As operating temperature rises, the combination of additional conductor weight and continuing steel-core expansion can increase sag and consume the clearance margin needed to use the conductor’s calculated thermal ampacity.
An ACSS/TW conductor may therefore look attractive in an electrical comparison while still falling short in the complete transmission-line design. It may contain more aluminum, exhibit lower resistance than round-wire ACSS and offer a high thermal rating. But if its sag-temperature curve causes it to reach required clearances before attaining that rating, part of its theoretical capacity cannot be used.
The conductor may be thermally capable but clearance constrained.
The Objective Is Delivered Power
Electrical losses add another dimension that simple ampacity comparisons often overlook. Transmission losses increase approximately with the square of current. When current doubles, resistive losses at a given resistance increase by roughly four times. Those losses consume generation, increase conductor temperature and reduce the amount of electricity delivered to the receiving end of the line.
A conductor’s ability to survive at 200°C, 225°C or 250°C does not mean routine operation at those temperatures is economically desirable. Extreme-temperature capability can be useful during emergencies, but moving more current by operating a conductor at a very high temperature is not the same thing as delivering more electricity efficiently.
The objective is not simply to put more amperes into one end of a transmission line. It is to deliver more usable electricity at the other end while maintaining clearances, respecting structure limitations, controlling losses and supporting long-term reliability.
This is why conductor evaluations should examine resistance, operating temperature, sag, strength, weight, structural impacts and delivered energy together. A thermal ampacity number alone cannot describe the performance of the complete transmission line.
ACCC Conductor Changes the Architecture
ACCC Conductor was engineered to address these interacting constraints simultaneously. Its hybrid carbon- and glass-fiber composite core is approximately 70 percent lighter than a conventional steel core and provides substantially greater strength-to-weight performance. The weight saved at the center of the conductor creates a “weight budget” that can be reinvested in highly conductive aluminum.
Compact trapezoidal strands allow ACCC Conductor to incorporate approximately 28 percent more aluminum than conventional ACSR of the same diameter and weight. More aluminum reduces electrical resistance. Lower resistance reduces line losses and operating temperature at a given load. Lower temperature further reduces resistance and sag, allowing the benefits to reinforce one another.
Just as importantly, the ACCC Conductor’s composite core has a coefficient of thermal expansion roughly an order of magnitude lower than steel. Above the conductor’s thermal knee point, the composite core governs high-temperature sag. Rather than merely increasing the strength of a core that continues to expand significantly when heated, ACCC Conductor changes the material responsible for that expansion.
This is the fundamental architectural difference. ACSS enables high-temperature operation by placing fully annealed aluminum around a steel core. ACSS/TW adds conductive aluminum but also adds weight. Higher-strength steel can improve mechanical capability, but it does not inherently change the system’s thermal expansion.
ACCC Conductor replaces steel with a lighter, stronger and extremely low-expansion composite core. This enables the use of additional aluminum without a comparable total-weight penalty while simultaneously controlling high-temperature sag. Capacity, efficiency, strength, weight and sag performance are improved as part of an integrated conductor system rather than through a series of compromises.
Advanced Should Mean Measurably Better
ACSS has served the utility industry for decades and remains useful where high-temperature capability is needed, sufficient clearance is available and losses or structure loading are not the governing concerns. Recognizing those applications, however, does not make ACSS equivalent to a modern “advanced” composite-core conductor.
The development history tells the story. ACSS was created to address the temperature limitation of conventional ACSR by replacing hard-drawn aluminum with fully annealed aluminum. Composite-core conductors were subsequently developed to address the limitations that remained – particularly steel-core weight, thermal expansion, sag and the difficulty of adding substantial aluminum without increasing total conductor weight.
As utilities, regulators and policymakers accelerate grid investment, conductor classifications should be based on measurable performance rather than broad marketing categories. The important questions are not limited to how hot a conductor can get or how many amps it can theoretically carry. Utilities must determine how much power the line can actually deliver while maintaining required clearances, how much energy will be lost, what modifications existing structures will require and what the solution will cost customers over its operating life.
ACSS can increase calculated thermal ampacity. ACSS/TW can add aluminum and reduce resistance, but that added aluminum increases weight while the conductor remains governed by the thermal expansion of its steel core. Higher-strength steel can improve mechanical capability, but increased strength does not eliminate thermal expansion.
ACCC Conductor addresses the complete challenge. Its lightweight composite core supports greater aluminum content. Greater aluminum content reduces resistance and losses. Its extremely low coefficient of thermal expansion controls sag. Its high strength supports demanding mechanical applications. Together, these attributes allow utilities to deliver more power more efficiently while making substantially better use of existing structures and rights-of-way.
The grid does not simply need conductors that can tolerate more heat. It needs conductors that transform existing transmission corridors into more capable, efficient and valuable infrastructure.
When the objective is simply to achieve a higher temperature rating, ACSS remains one available option. When the objective is to maximize usable capacity, maintain clearance, reduce electrical losses, limit structural modifications and deliver more electricity to customers over the life of the asset, ACCC Conductor offers the more complete engineering solution.
