Summary
- The North West London Upgrade spans the transmission route between Sundon, Elstree, and St John’s Wood.
- A new Letchmore Heath substation will connect five data centres representing approximately 1GW of combined demand.
- More than 200km of conductor upgrades and around 60km of new cable show how far data centre infrastructure now extends beyond individual campuses.
National Grid has started a major transmission reinforcement programme in north-west London that will connect five data centres representing around 1GW of new electricity demand.
The North West London Upgrade runs from Sundon in Bedfordshire through Elstree in Hertfordshire to St John’s Wood in London, combining new substations, extensions to existing electrical sites, overhead line reinforcement, and underground cable installation.
A new 132kV gas insulated substation at Letchmore Heath, beside National Grid’s Elstree site, will provide the connection point for the five data centres. The programme also includes reinforcement at 400kV, placing the data centre load directly into a much larger transmission project rather than a series of isolated local connections.
A gigawatt reaches far beyond the campus
The North West London Upgrade includes reconductoring along approximately 35km of overhead line, amounting to more than 200km of conductor once the individual circuits and phases are counted.
National Grid will also install around 60km of new high voltage cable through the existing Elstree to St John’s Wood tunnel, which stretches for more than 20km beneath north London. The work will create another 400kV circuit and use spare capacity built into the tunnel system.
Elstree’s existing 400kV gas insulated substation will be extended, while Letchmore Heath adds the 132kV infrastructure required to distribute power towards the five data centre customers. Substation work is being led by Laing O’Rourke, HMJV is involved in the tunnel and cable programme, and Hyundai Electric and Siemens Energy are supplying major electrical equipment.
The scale puts a physical measure on London’s data centre pipeline. Five facilities with a combined connection envelope of around 1GW require far more than individual customer substations. Transmission conductors, tunnels, transformers, switchgear, reactive power equipment, protection systems, and cable routes all have to be reinforced before the load can be reliably supplied.
Those assets may sit tens of kilometres from the eventual data halls, yet delays upstream can determine when the facilities become commercially usable.
West London’s power constraint is moving upstream
London and the wider Slough and Heathrow corridor contain one of Europe’s densest concentrations of data centre capacity. Fibre, enterprise demand, and proximity to major cloud customers helped establish the market, while limited electrical headroom has become one of its strongest brakes on further development.
Connection certainty now affects land values and project financing directly. A consented site without a credible energisation date can remain stranded even after substantial development expenditure, while land attached to firm power capacity attracts a premium.
The Letchmore Heath scheme reflects a move towards network infrastructure designed around clusters of large loads. Reinforcing a transmission route to support several customers can create a more coherent system than repeated project specific works, although it requires substantial capital and long construction programmes before customer demand arrives.
Procurement sits on the same critical path. Transformers, high voltage cables, switchgear, and reactive power equipment are being ordered across grid reinforcement, renewable projects, industrial electrification, and data centre developments. Manufacturing capacity for some equipment is booked years ahead.
A delayed transformer can therefore affect both the network project and every downstream data centre depending on it. Developers increasingly need visibility beyond their own connection agreement into the transmission works, equipment packages, and commissioning programme that underpin the promised energisation date.
One gigawatt will arrive in phases
The five data centres are unlikely to draw their full combined connection capacity as soon as Letchmore Heath is energised. Large facilities are normally built and occupied in stages, with IT equipment arriving as customer demand develops.
AI infrastructure can accelerate that ramp once halls are ready. GPU clusters place unusually large loads into compact spaces, allowing a single deployment phase to add tens of megawatts more quickly than traditional enterprise occupancy.
The transmission network has to accommodate the credible maximum load while maintaining resilience across other customers and system conditions. That requires planning around contingency scenarios, maintenance, faults, and future growth rather than only the first megawatts connected.
The London programme also lands amid wider reform of the UK connections process, where strategic demand is competing for network access alongside industrial electrification, transport, storage, and generation. Data centres now represent a large enough class of load to influence where and when national transmission investment is required.
The North West London Upgrade makes that influence visible in steel, copper, cable, and switchgear. Around 1GW of proposed data centre demand is driving work from Bedfordshire into central London, extending the physical footprint of digital infrastructure far beyond the buildings where servers will eventually operate.
