Elstree to St. John's Wood Cable Tunnel

The Elstree to St. John's Wood Cable Tunnel, known as The London Connection during construction, is a 20 km long, 3m wide tunnel beneath northwest London.

Constructed between 2001 and 2005, the tunnel carries high voltage transmission lines from Elstree substation in Hertfordshire to Lodge Road sub station in Westminster at a depth of 20m below street level.

[1] The tunnel runs beneath the A5 road for the majority of its length, and houses a single 400 kV power transmission circuit with a rated capacity of 3700 A.

There is provision for a second circuit to be installed in the future.

[2] A remotely operated, battery powered monorail system runs along the tunnel, allowing the cables to be inspected remotely using infrared cameras.