A closer look at undergrounding electricity lines

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27 February 2025

How is electricity transported around the grid?

The wider transmission network in Great Britain, known simply as the ‘National Grid’, transports high-voltage electricity of 275 kilovolts (kV) and 400kV around the country via tens of thousands of steel pylons, overhead lines (OHL) and underground cables. This is then ‘stepped down’ via regional substations and passed on to local distribution networks, which transport a lower voltage of up to 132 (kV) that is safe for domestic use.

In England and Wales, National Grid Electricity Transmission owns, builds and maintains the electricity transmission network. Whilst the transmission network in Scotland is divided in two, with lines in central and southern Scotland owned and maintained by SP Energy Networks, and by Scottish and Scottish & Southern Electricity Networks (SSEN) in northern Scotland.

What is undergrounding?

Undergrounding is the process by which OHL are replaced with underground cables across the transmission and distribution networks of our electricity grid. It’s a means to reduce the visual impact of our grid network, by burying power lines out of sight.

This process is used in certain circumstances, such as in nationally designated landscapes like national parks and areas of outstanding natural beauty (AONBs), and it typically applies to local distribution networks. The wider transmission network predominantly consists of transmission pylons, of which there are more than 22,000 across England and Wales, with around 16% of the network being transmitted underground.

Is it more expensive?

Although they are better protected from extreme weather, underground cables operate less efficiently and are generally larger than OHLs. This is because they require different types of electrical insulation to prevent them overheating, as well as to physically protect them from damage and ensure they do not present a danger while in service. As a result, as many as 12 underground cables, across four separate trenches, can be needed to match a 400kV double circuit OHL transmission line.

Consequently, a series of studies from National Grid, DNV Energy Systems, and the Institution of Engineering and Technology have found underground cabling to be between five to ten times more expensive than installing overhead lines, depending on factors such as local geography and ground conditions. It’s important to remember at this point that electricity transmission owners (ETOs) have legal obligations to represent value for money to consumers when developing new network infrastructure, given that increased project costs will typically be passed on to billpayers.

To take one specific example, National Grid has carried out a comparison between an OHL and underground solution for its Norwich to Tilbury proposals in East Anglia, which encompass works to reinforce and upgrade the regional transmission network. The current cost of the OHL proposal is £894.5 million, whereas an underground solution would see that figure rise to more than £5.8 billion.

Why is it more damaging to the environment?

Firstly, because the greater number of cables requires a larger work corridor, which would be around 65 metres wide during the construction period to deliver 400kV transmission. This is significantly larger than that required for an equivalent OHL line, resulting in greater constraints on land use and disruption to nearby agriculture.

Secondly, because ecological considerations also need to be made, given the area required for laying underground cables needs to be cleared of both buildings and vegetation. On average, undergrounding cables requires 14 times more earth to be excavated when compared to OHLs, and there can be lasting environmental impacts on natural habitats and hydrology, with sensitive areas like peatland particularly affected.

Thirdly, there is the question of the maintenance and replacement of cables, as well as challenges with detecting faults. Underground cables have a typical operational life of around 40 years, compared to between 50 and 70 years for steel lattice towers. When an OHL conductor reaches the end of its design life, it can be replaced with limited impact, whereas the replacement of an underground cable is significantly more intrusive, time-consuming and environmentally disruptive.

Has undergrounding been done successfully in the past?

In areas of the country where National Grid is a distribution network operator (DNO) - such as in the Midlands, southwest England, and south Wales - the network is made up of 83,900 miles of underground cables and just 60,000 miles of overhead lines. However, regional DNOs have differing ratios of overhead and underground cables, which vary by location and voltage.

For long distance transmission connections, such as the Eastern Green Links between Scotland and England, marine High Voltage Direct Current (HVDC) cabling is used to transfer electricity at subsea level. However, there is no fully offshore solution to building the grid the UK needs and power must be brought onshore somewhere, before being converted from direct current (DC) to alternating current (AC) so it can be connected to the onshore grid.

Can undergrounding become the default moving forwards?

Alongside the cost and complexity of a shift to undergrounding as the default for future grid infrastructure, one of the other key considerations is around timescales. To deliver a clean power system by 2030, in line with the Government’s ambitions, grid infrastructure needs to be upgraded quickly to connect the new generators due to come online, as well as to deal with the huge backlog of connection applications across the wider energy sector, including battery storage projects, which now stands at over 700GW.

Any delay to this would not only result in our clean energy targets being jeopardised, but also in billpayers being billions of pounds worse off, with constraint payments needing to be made to generators who are unable to distribute electricity due to insufficient grid capacity.

Can a balance be struck between OHL and underground lines?

Ofgem has allocated £500 million through the visual impact provision (VIP) project to help reduce the visual impact of existing transmission lines in English and Welsh AONBs and national parks.

Working with independent stakeholders including the National Trust, Campaign to Protect Rural England and the Ramblers Association, the VIP project has assessed more than 571km of transmission lines to date. Covering areas like the Cotswolds, Dorset, and Snowdonia, it has identified which sections of OHL have the highest visual impact on their surroundings, before recommending priority schemes to underground cables where this could be transformative for the local landscape.

It's important to note that no single technology is appropriate in every scenario, nor can one cover every circumstance, and the trade-offs of undergrounding must be acknowledged, as our recent report on the planning system makes clear. Despite its logistical and environmental challenges, undergrounding has a limited role to play, and it will continue to be the ‘strong starting presumption’ in situations where OHLs cannot avoid harm to the landscape, visual amenity or natural beauty of a nationally designated area, and it is technically and economically feasible.