Underground, submarine and superconducting cables: How it works

Why a buried cable needs a far fatter core than overhead wire

A power cable carries the same current as an overhead line, but the bare wire in air is replaced by a thick metal core wrapped in solid insulation, so it can run under streets or along the seabed. Because insulation sits tight against the conductor, cables shed heat poorly and act like long capacitors, which sets their length limits and the choice between AC and DC.

Step by step

1. Power enters at a substation or converterShort AC cables connect to an ordinary substation. Long DC cables need a converter station at each end to turn AC into DC and back again; Viking Link joins Bicker Fen in Lincolnshire to Jutland over 765 km of land and subsea cable.
2. Current flows in a fat copper or aluminium coreBuried cables cannot cool in moving air, so the core is much thicker than an overhead wire. GB study designs use 2,500 mm² copper cores, up to three per phase for a high-rated 400 kV AC circuit, and a bundled pair of 2,500 mm² cables for a 2,000 MW, 525 kV DC link.
3. Insulation and screens hold back the voltageCross-linked polyethylene (XLPE), a tough extruded plastic, replaces air as the insulator; older subsea cables use oil-soaked paper. A metal screen keeps the electric field inside, and steel wire armour protects subsea cables from anchors.
4. Cable is laid in trench, tunnel or seabedOn land, 400 kV cables sit about 1.2 m deep in trenches about 1.5 m wide per circuit, with joint bays every 500 to 1,000 m; in cities they can run in tunnels 25 to 30 m down. At sea, ploughs or water jets bury the cable about 1 m into the seabed.
5. Superconducting optionSome ceramic tapes lose all electrical resistance when cooled by liquid nitrogen to about minus 200 C. The conductor sits inside a vacuum-insulated pipe (cryostat) with nitrogen circulating in a closed loop, so a slim cable can carry large power at a lower voltage.

The science

Resistance turns part of the current into heat, and the heat rises with the square of the current (P = I² R). Raising the voltage lets the same power flow with less current, and a thicker core lowers R, so both cut losses. A cable also forms a capacitor: conductor and screen are two metal surfaces separated by thin insulation. On a 50 Hz AC grid the voltage reverses 100 times a second, so the cable is charged and discharged continually, and this charging current takes up part of the conductor's capacity. For high-voltage cables, beyond roughly 50 km most of the current goes on charging unless large compensation equipment is added, so long links use DC: the cable charges once when switched on and then stays charged.

The engineering

The key engineering limits are heat and joints. Each drum holds only a few hundred metres to a few kilometres of cable, so sections are spliced in joint bays, which are the most common failure points. Onshore AC cables longer than about 5 km may need reactive compensation (large coils that soak up the charging current). Subsea cables are made in long continuous lengths at coastal factories and loaded onto specialised laying ships. Superconducting cables add cryogenic plants, pumps and vacuum jackets that must run continuously.

Key numbers

Sources

1. Electricity transmission costs and characteristics: final full report IET / Mott MacDonald, 2025
2. Undergrounding high voltage electricity transmission lines: the technical issues (Issue 4) National Grid, 2015
3. National Grid announces commercial operations of Viking Link National Grid, 2023
4. Laying of world record power cable between the UK and Denmark now complete National Grid Ventures, 2023
5. HVDC transmission (presentation by L. Tang, HVDC 2013) US Department of Energy, 2013
6. SuperLink superconducting cable system NKT, 2025
7. Development and demonstration of concentric-type HTS power cable for distribution grid in Shenzhen urban area Frontiers in Energy Efficiency, 2023
8. A review of potential impacts of submarine power cables on the marine environment (Taormina et al., Renewable and Sustainable Energy Reviews 96) Elsevier (via VLIZ repository), 2018