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Glossary · Energy industry & energy transition

Underground cable

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

An underground cable is an electric line that is laid below ground and transports electricity between grid points, installations or consumers. Depending on the voltage, it consists of one or more conductors, electrical insulation and protective layers. In addition, there is usually a metallic screen and an outer sheath that protects against moisture, mechanical stress and chemical influences in the soil. In the low- and medium-voltage grid, underground cables are the normal case in many regions. In the extra-high-voltage grid they are also technically possible. There, however, they place higher demands on planning, construction, operation and repair than an overhead line.

Function and voltage levels

An underground cable fulfils the same task as an overhead line: it transmits electrical power. Voltage, current, transmissible power and thermal capacity are important. The voltage level determines which insulation is necessary and which technical side effects occur. In low voltage, the voltage is usually 0.4 kilovolts. In medium voltage, it is typically about 10 to 30 kilovolts. Above this lies the high-voltage grid with usually 110 kilovolts. In the transmission grid, three-phase AC lines typically operate at 220 or 380 kilovolts. Direct current connections also use very high voltages. The higher the voltage and the transmitted power, the greater the effect of heat, electric fields, reactive power, protection technology and repair effort.

Not invisible: what remains above the route

Underground cables are often described as an "invisible" alternative to the overhead line. That is only partly true. Pylons and conductor cables disappear, and the landscape is changed less. However, construction areas and working strips during construction, joint locations and cable transition facilities remain visible. In direct current projects, converter stations are added. The use of the land above the route is also restricted. Deep-rooted woody plants, certain structures or deep interventions may be impermissible because they can damage the cable or impair heat dissipation.

Heat dissipation: the soil is part of the equipment

The most important technical difference from the overhead line lies in the surroundings of the conductor. An overhead line releases its heat directly to the air. An underground cable must dissipate heat through insulation, protective layers, bedding material and soil. The permissible continuous load, i.e. the thermal capacity, therefore depends not only on the conductor cross-section. It also depends on soil temperature, soil moisture and thermal conductivity. Dry soil or soil with poor thermal conductivity can limit the capacity considerably. In planning, cable trenches, bedding materials, distances between the cable systems and thermal reserves are therefore calculated precisely. An image for this: the cable is wrapped as if in a thermal blanket. How well this blanket lets heat pass helps determine how much current may flow.

Alternating current, reactive power and direct current

With alternating current, a further point arises. Because of their construction, underground cables have a significantly higher electrical capacitance than overhead lines: the conductor and the earthed screen lie close together and are separated only by the insulation. Long AC underground cables therefore draw a high capacitive charging current, i.e. reactive power. This current does not transport usable active power. However, it loads the line and equipment and reduces the remaining transmission capability. Long extra-high-voltage underground cables in the three-phase grid therefore need compensation installations, such as reactors. With direct current, this effect does not occur in the same form during ongoing operation. In return, a high-voltage direct current transmission requires converter stations at both ends, which convert alternating current into direct current and back again. The choice between underground cable and overhead line is therefore closely linked to whether a connection is planned with alternating or direct current. German law also distinguishes here, as described below.

Underground cables in the distribution grid

In the distribution grid, low- and medium-voltage cables are well established in cities, new development areas and many rural grids. The lines there are comparatively short, the voltage is lower and the technical risks are well manageable. Underground cables are better protected against storms, ice load and falling trees than overhead lines. In return, fault locations in the ground are harder to find and repair. A damaged cable must be exposed. This often means civil engineering works, traffic disruption and longer repair times. An overhead line is more susceptible to weather, but visible damage can often be reached and fixed more quickly. Security of supply therefore does not depend only on whether a line runs above or below ground. Grid structure, redundancy, protection technology, maintenance and the possibility of quick repair are equally important.

Acceptance and conflicts

In debates about grid expansion, the underground cable is often seen as a solution for greater acceptance. This effect is real. An underground route can reduce conflicts about the landscape, the residential environment and proximity to properties. However, it can trigger other conflicts, for example with agriculture, soil protection, groundwater, nature conservation or long construction times. The intervention shifts from permanent visibility to construction and land use. For landowners, municipalities and farms, this is not a side issue. Possible consequences are crop losses, soil compaction, problems with drainage or permanent restrictions on use, even if no pylons stand after construction.

Costs

Economically, underground cables and overhead lines differ above all in investment costs and in the effort for repairs. In the extra-high-voltage range, underground cables are generally significantly more expensive than overhead lines. The Federal Government explicitly justifies its 2026 draft bill with considerable savings through overhead lines for new direct current connections. How large the difference is exactly depends, among other things, on terrain, soil, crossings, length, voltage level, number of systems and technical design. Besides the cables themselves, civil engineering works, protective ducts, joints, thermal bedding, crossings of roads or watercourses and compensation measures cause considerable costs. These costs are not borne by the individual place that receives underground laying. Insofar as regulation recognises them, they are spread over all electricity consumers via grid charges. In this way, a local decision for greater acceptance becomes a question of costs for the economy as a whole.

Legal framework in Germany (status: November 2026)

Whether a project is built as an underground cable cannot be freely decided by the grid operator. Statutory requirements, technical rules, approval procedures and the question of which costs regulation recognises are decisive. Depending on the voltage level and the project, different rules apply in Germany:

  • High voltage up to 110 kilovolts: Under § 43h EnWG, lines on new routes are in principle to be built as underground cables. This applies if the total costs of construction and operation are at most 2.75 times those of a technically comparable overhead line and no nature conservation concerns stand in the way. Upon application, the authority may nevertheless permit an overhead line. New constructions that run predominantly within or directly next to an existing route do not count as a new route. The same applies in certain cases in which the line is routed together with another project on shared pylons.

  • Extra-high-voltage direct current: Under § 3 BBPlG, lines of the projects marked with "E" in the Federal Requirements Plan are in principle to be built as underground cables (underground cable priority). Exceptions for overhead lines on partial sections are possible only under narrow conditions; near residential development they are excluded.

  • Extra-high-voltage three-phase AC: Here, the overhead line is in principle the normal case. Underground cables are possible above all in designated pilot projects on technically and economically efficient partial sections. The reason is the physical difficulties of long three-phase cables described above.

  • Planned change in 2026: On 29 April 2026, the Federal Government adopted the draft of a Second Act amending the BBPlG (Bundestag printed paper 21/6128). Under it, the underground cable priority is to lapse for direct current lines that are newly included in the Federal Requirements Plan from the entry into force. These lines would in principle be built as overhead lines. Underground cables would remain possible if an overhead line is technically not feasible or the underground cable is more cost-efficient. The first reading in the Bundestag took place on 11 June 2026. For projects already marked, the existing underground cable priority continues to apply under the draft.

Sources

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Edited by eCMS on 11.11.2026 19:47

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