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Glossary · Energy industry & energy transition

Overhead line

overhead line electricity grid grid expansion transmission grid distribution grid line Infrastructure

An overhead line is an electrical line whose current-carrying conductors hang above ground from pylons. They are insulated from the pylon and the earth by insulators. It transmits or distributes electrical energy between generating plants, substations, grid nodes and load centres. In Germany, the overhead line is the classic design for power transmission at extra-high and high voltage. This applies above all to 380 kV and 220 kV in the transmission grid and to 110 kV in the high-voltage grid. In the medium- and low-voltage grids, overhead lines have been replaced by underground cables in many built-up areas. In rural areas, however, they are still widespread.

Technically, an overhead line does not consist of a single cable but of several conductors. A three-phase line carries three phases per circuit. At high voltage levels, each phase often consists of a bundled conductor with two to four sub-conductors. Bundled conductors reduce the electric field strength at the conductor surface. In this way they reduce corona discharges and the associated losses and noise. At the same time they increase the transmissible power. Earth wires, or earth wires with an integrated optical fibre, may additionally run at the top of the pylon. Earth wires protect the conductors against direct lightning strikes. Optical fibres are used for communication in grid operation. An overhead line is thus at once energy infrastructure, a protection system and part of the communication infrastructure of grid operation.

Distinction from underground cables, route and grid

The most important distinction is the one from the underground cable. An underground cable carries the current underground in solidly insulated cable systems. An overhead line, by contrast, generally uses bare conductors. Here the insulation is essentially provided by air clearances, insulators and safety distances. Different technical properties follow from this. The surrounding air cools the conductors directly. Underground cables, on the other hand, must dissipate their heat through bedding material and soil. Fault locations on overhead lines are often visible or easily accessible, so repairs are usually quicker. Underground cables are less visible. At high voltage levels, however, they generally cause higher construction costs, impose more complex thermal requirements and usually take longer to repair after faults. With alternating current, their high capacitance adds to this: long cable routes generate a lot of reactive power, which must be compensated.

An overhead line is also not the same thing as a route. The route is the spatial course, that is, the corridor in which a line is routed. Different technical designs are possible on the same route: a new overhead line, a replacement new build on an existing line, an underground cable section or several circuits on a shared pylon structure. Likewise, the overhead line is not to be equated with the power grid. The transmission grid and the distribution grid consist of lines, substations, switchgear, protection technology, control and operations management. The overhead line is one design type within this infrastructure.

Load capacity and operation

How much an overhead line can transmit does not depend on its nominal voltage alone. Current intensity, cross-section and material of the conductors, the permissible conductor temperature and the mechanical sag are also important. Weather conditions are added to this: ambient temperature, wind and solar radiation. When current flows through a conductor, it heats up due to electrical losses. As the temperature rises, the metal expands and the conductor sags more. The permissible load therefore ends not only at an electrical limit but also at a mechanical and safety-related one. The clearance to the ground, vegetation, buildings and traffic routes must be maintained at all times.

This is why weather data matter for grid operation. In cool air and wind, an overhead line can carry considerably more current than on a hot, windless summer day. In weather-dependent overhead line operation, this reserve is used: the permissible current-carrying capacity is continuously recalculated on the basis of measured or forecast weather values, instead of working with fixed, cautious assumptions. This does not replace grid expansion. In certain grid situations, however, it can provide additional transport capability and reduce congestion. This applies particularly when a lot of wind power and high line cooling by wind coincide.

Overhead lines cause grid losses. These arise mainly from the ohmic resistance of the conductors. They grow with the square of the current, with smaller corona and leakage losses added. For the same transmitted power, the current falls in inverse proportion to the voltage. Ohmic losses therefore fall roughly with the square of the voltage. An illustrative example: if the same power is transmitted at 380 kV instead of 110 kV, the current falls to less than a third. With the same conductor, the resistive losses fall to less than a tenth. This is why large amounts of energy are transported over long distances in the high- and extra-high-voltage grid.

Significance for grid expansion and security of supply

In public debate, overhead lines often stand for the visible part of grid expansion. This visibility explains part of the conflicts, but not the technical function. Generation and consumption do not automatically coincide in space and time. Wind power is often generated in regions with low local demand. Large load centres, by contrast, are located in industrial centres, cities or more southerly load regions. Overhead lines connect these areas and make generation usable across regional boundaries.

For security of supply, it is not only a question of whether enough generation capacity is available. The electrical energy must also reach the place where it is needed. If lines threaten to become overloaded, grid operators must curtail generating plants upstream of the bottleneck, ramp up others behind it, or distribute load flows differently. Such interventions are called redispatch. The available capacity of an overhead line therefore influences which plants can actually feed in and what costs arise in congestion management. It also helps determine how robustly the grid responds to the failure of individual items of equipment.

The N-1 criterion makes this connection clear. The grid must be operated in such a way that the failure of an important item of equipment does not lead to an impermissible overload of other items of equipment. Such an item of equipment is, for example, a circuit or a transformer. An overhead line is therefore not permanently loaded up to its physical limit in normal operation. Part of the apparently free capacity serves as a safety reserve for disturbances and unplanned changes in load flows.

Legal framework in Germany (as of November 2026)

  • Plan approval: Under Section 43(1), first sentence, no. 1 EnWG (German Energy Industry Act), the construction, operation and modification of high-voltage overhead lines with a nominal voltage of 110 kV or more require plan approval by the authority responsible under state law. Excluded are long-distance railway power lines and overhead lines up to 200 metres in total length outside Natura 2000 sites. For these short lines, the project developer may apply for plan approval. Separate rules apply to lines within the scope of the NABEG (Grid Expansion Acceleration Act). The approval procedure for overhead lines below 110 kV is not governed by this provision. They can only be included in a procedure under certain conditions, for example when they are carried on a shared pylon structure.

  • Weighing of interests: Under paragraph 3a, lines under Section 43(1), first sentence, nos. 1 to 4 EnWG are in the overriding public interest. Their accelerated expansion is to be included as a priority concern in the weighing of protected interests until electricity supply is almost greenhouse-gas neutral. For modifications, replacement or parallel new builds on existing routes, the examination of alternatives is in principle limited to the area in and immediately next to the existing route.

  • Fields: Covered 50 Hz low-frequency installations must comply with the limit values of the 26th BImSchV (German Federal Immission Control Ordinance) at places where people stay other than temporarily. These are 5 kV/m for electric field strength and 100 µT for magnetic flux density, even at maximum operational load. Such places include homes, schools and workplaces. The 100 µT is half of the general value in Annex 1a. Under Section 43 EnWG, the operator must notify the immission control authority of compliance at least two weeks before construction, commissioning or modification.

  • High-voltage direct current transmission (HVDC): Since the end of 2015, underground cables have had priority for the direct current projects identified in the Federal Requirements Plan. Previously, overhead lines were the rule and underground cables the exception. The Ultranet project is exempt from the priority for underground cables. A government draft amending the Federal Requirements Plan Act (Bundesbedarfsplangesetz) of May 2026 provides that newly added HVDC lines are in principle to be built as overhead lines. Underground cable sections are to be permissible only as an exception. Projects already identified are to keep their status. Whether and in what version the amendment has since entered into force should be checked in the current legal text.

Typical misunderstandings

A common misunderstanding is that overhead lines are outdated technology that could in principle be replaced by underground cables. This equation overlooks voltage level, length, transmission capacity, repair concept, reactive power, ground interventions and costs. At extra-high voltage level, underground cables can be technically sensible or required by law, for example for certain direct current links or in sensitive sections. However, they are not a simple underground variant of the same line. They change construction method, operation, fault repair and cost structure.

Conversely, an overhead line is not automatically the cheapest or most suitable solution. In densely built-up areas, for short grid connections, in protected areas or in the low-voltage grid, underground cables can have considerable advantages. Which design is appropriate depends on several factors: voltage level, length, local conditions, approval requirements and the relationship between investment costs, operating costs and interventions in space and the environment.

Another misunderstanding concerns the role of the overhead line in the energy transition. Lines do not generate electricity and replace neither storage nor flexible consumers or power plants. They do, however, change which areas of the power system are connected with one another. Without sufficient transport capacity, low-cost renewable generation remains regionally limited, while more expensive or more emission-intensive generation has to step in elsewhere. With more line capacity, regional differences can be balanced out better, but only within the physical and operational limits of the grid.

The term acceptance, too, is often used too loosely. Conflicts over overhead lines do not arise only from a lack of information. Property rights, landscape appearance, municipal planning, nature conservation, agricultural use, construction periods and the distribution of benefits and burdens are affected. The benefit of a line is often supraregional, while the visible burden is local. This spatial asymmetry shapes many planning and approval procedures.

Institutional and economic context

Overhead lines are planned, financed and operated in regulated grids. Grid operators do not decide freely, like ordinary companies, where they build lines. They are subject to statutory tasks, grid development planning, approval procedures and the regulation of their revenues. The investments are included in the recognised grid costs and are refinanced through grid charges. The choice of design therefore also affects the distribution of costs in the power system. This is a key reason for the debate on overhead line or underground cable for new direct current links.

The economic assessment of an overhead line must therefore not stop at construction costs. A line can reduce congestion costs, reduce the curtailment of renewable plants, reduce redispatch and facilitate the integration of new consumers, such as heat pumps, electric vehicles or electrolysers. At the same time, costs arise for planning, compensation measures, land rights, operation, maintenance and, where applicable, the dismantling of old installations. A robust assessment must consider these effects together.

The distinction from flexibility is also important. Overhead lines shift electricity spatially. Flexibility shifts feed-in or consumption in time or adapts it to the grid situation. The two can complement each other, but cannot replace each other arbitrarily. A flexible consumer can ease a local bottleneck if it responds at the right time at the right grid node. It does not, however, replace permanently missing transport capacity between large generation and consumption areas. Conversely, a new overhead line does not solve every short-term fluctuation in residual load or every local distribution grid problem.

An overhead line is thus a concrete item of equipment with a visible spatial effect and far-reaching technical consequences. It makes electrical power transportable by means of air insulation, pylons, conductors and safety clearances. Its significance lies in the combination of physics, space, costs, approval and grid operation. Anyone who talks about overhead lines is therefore also talking about how far electricity is to be transported, which bottlenecks are accepted, which costs arise and which burdens are borne locally.

Sources

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Edited by eCMS on 12.11.2026 05:05

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.