High voltage (HV) electricity connections are the backbone of any commercial development above around 500 kVA of demand. They bypass the limits of the low voltage distribution network and deliver power at 11kV (most common) or 33kV directly to a customer transformer.
This guide covers how HV connections work in the UK, the substation options, and how to plan the delivery.
Why HV
HV connections are needed whenever the demand is too large to be served economically from the LV network. See our breakdown of LV versus HV connections for the fuller comparison. The cutoff varies by site, but a useful rule is:
Below 500 kVA: LV is usually the right answer.
500 kVA to 1 MVA: assess both LV and HV options.
Above 1 MVA: HV is almost always required.
The reason is simple. LV cables have limited current-carrying capacity, and the losses become significant over longer distances. HV moves the same power at a fraction of the current, which means smaller cables, less voltage drop, and more headroom for future load growth.
The Three Main HV Configurations
There are three common HV connection configurations in UK commercial development.
Configuration One: Customer Substation, DNO Cable
The DNO lays an HV cable from its network to a customer substation on the site boundary. The substation contains an HV switch (typically a ring main unit) and a customer-owned transformer that steps down to LV for the building. The DNO owns the HV cable and the HV switch. The customer owns the transformer and all LV equipment downstream.
This is the most common configuration for individual commercial buildings.
Configuration Two: DNO Substation on Site
The DNO installs a complete substation on the customer’s site, typically in a ground-level kiosk or small building. The DNO owns the entire substation, including the transformer. LV supply is delivered from the substation to the building.
Common on larger commercial buildings and on sites where the DNO requires its own adopted asset.
Configuration Three: Ring Fed Network
For large developments (retail parks, business parks, large industrial sites), an HV ring is installed that feeds multiple substations around the site. Each substation serves one or more buildings. The ring arrangement provides resilience, so if one section of HV cable is out of service, the buildings can still be fed from the other direction.
This is standard on any site with multiple buildings or plots that need their own substations.
Substation Types
Substations vary in size, enclosure, and installation complexity.
Kiosk substations are the most common for medium commercial. They are pre-fabricated units delivered to site and sat on a concrete pad. They contain the HV switch, transformer, and LV distribution in a single weatherproof enclosure.
Indoor substations are located inside a plant room within the building. They are more expensive to install but take up less external space and can be more architecturally acceptable.
Chamber substations are below-ground installations, used in urban locations where above-ground space is at a premium.
The choice depends on site space, planning constraints, and the building’s architectural requirements.
Timeline
An HV connection has a longer lead time than LV because of the substation civils, transformer procurement, and DNO commissioning process.
Typical timeline from enquiry to energisation:
Enquiry and budget quote: two to four weeks.
Formal connection offer acceptance: two to six weeks.
Detailed design: six to ten weeks.
Civils for substation base and cable route: three to eight weeks.
Substation and transformer delivery: six to twelve weeks lead time (runs in parallel with civils).
Installation and cabling: two to four weeks.
DNO commissioning and energisation: two to four weeks.
Total: sixteen to thirty weeks from enquiry to live supply, sometimes longer on complex sites.
Cost Ranges
HV connection costs are driven by load, distance, and site conditions.
A typical 500 kVA HV connection with customer substation: £60,000 to £150,000.
A typical 1 MVA HV connection: £100,000 to £250,000.
A typical 2 MVA HV connection: £150,000 to £400,000.
A large 5 MVA HV connection: £400,000 to £800,000.
Very large connections above 10 MVA and any that require new DNO infrastructure (new primary substation, new feeder) are priced project-specifically and can run into millions.
Planning Tips
Start the HV conversation at RIBA stage 2 or 3, as soon as the load is broadly understood. DNO lead times and substation civils are the binding constraint on many commercial programmes.
Get the substation location right early. It needs access for DNO maintenance vehicles (typically 4m wide), and it needs to be clear of residential windows and other noise-sensitive areas.
Budget for the transformer lead time. Transformers are specialist kit and lead times have been extended in recent years. Twelve-week lead times are common for standard units. Larger or specialist units can run longer.
Confirm the network capacity early. The DNO’s initial feasibility will tell you whether the existing HV network can accommodate your load or whether upstream reinforcement is needed. Reinforcement can add six months to the programme.
ICP vs DNO Delivery
On HV connections, the contestable portion (substation civils, customer-side cabling, transformer installation) can be delivered by an Independent Connection Provider (ICP). The non-contestable portion (tie-in to the DNO HV network, switchgear settings, and final commissioning) stays with the DNO.
ICPs typically deliver HV connections faster and more cost-effectively than going directly to the DNO because they can run design, civils, and procurement in parallel.
What Good Looks Like
A well-run HV connection has the substation location and load locked by RIBA stage 3, DNO budget quote in hand by stage 4, formal acceptance and design by stage 5, and site civils starting on day one of construction. Energisation lines up with the building handover, rather than delaying it.
The common failure mode is leaving the HV discussion until RIBA stage 5 and discovering the DNO’s lead time exceeds the construction programme. Our realistic electricity connection timelines guide walks through the reasons in more detail. If that happens, you are looking at a three-to-six month delay that no amount of additional money can fix quickly.
The Bottom Line
HV connections are more complex and more expensive than LV, but they are the right answer above a well-defined load threshold. Plan them early, get the design done in parallel with the building design, and energisation becomes a routine project milestone rather than a cliff edge at the end of the programme.