Most UK electricity connections are low voltage (LV). Most large commercial and industrial sites need high voltage (HV). Knowing where the boundary sits is one of the most important decisions in the design of a new connection, because the cost difference between an LV and HV connection on the same site can be tens of thousands of pounds.
This guide explains what each one is, when each is appropriate, and how to decide.
Definitions
Low voltage (LV) in the UK distribution context means 400/230 volts three-phase or single-phase. This is the standard supply delivered to homes, shops, and small commercial premises.
High voltage (HV) means 6.6kV, 11kV, 22kV, or 33kV. This is the voltage of the regional distribution network. It is stepped down to LV through a transformer close to the point of use.
The transition between LV and HV on a site happens at a transformer, usually housed in a substation.
When an LV Connection Is Enough
LV connections are suitable for sites with total electrical demand below around 500 kVA. The exact threshold depends on the local DNO’s policies and the existing network capacity, but this is a reliable rule of thumb.
Typical LV-suitable sites:
Individual houses, including those with heat pumps and EV chargers (up to about 30 kVA).
Small commercial units such as retail shops, cafes, and small offices (typically 30 to 200 kVA).
Small light industrial units and workshops (up to around 300 kVA).
Small housing developments where each plot has its own LV service (but a site-wide transformer is needed for the estate).
When an HV Connection Is Needed
HV connections are typically needed for sites above 500 kVA total demand, and they are often more economical than LV above around 1 MVA even if LV is technically possible. Our deeper look at HV and substations for commercial developments covers the design choices in detail.
Typical HV-suitable sites:
Medium-to-large commercial developments such as retail parks, office parks, and warehouse developments.
Industrial sites with heavy loads such as manufacturing, cold storage, and food processing.
Data centres, hospitals, and large institutional sites.
Housing developments above 80 to 120 plots, where an HV feed to a site transformer is more economical than multiple LV feeds.
EV charging hubs with multiple rapid chargers, where total demand quickly exceeds LV capacity.
Cost Comparison
The comparison is not straightforward because an HV connection typically includes the cost of a transformer and substation that an LV connection does not need.
A typical 250 kVA LV connection on an existing network: £10,000 to £25,000.
A typical 500 kVA LV connection: £25,000 to £60,000 if the existing network can support it.
A typical 500 kVA HV connection with a customer substation: £60,000 to £150,000.
A typical 1 MVA HV connection with substation: £100,000 to £250,000.
A typical 2 MVA HV connection: £150,000 to £400,000.
On paper, LV looks cheaper, but the cost comparison flips above around 500 kVA because LV cables have to be run in parallel or upsized significantly, and network reinforcement costs start to dominate.
The Hidden Cost: Network Reinforcement
An LV connection that exceeds the local network’s spare capacity triggers a reinforcement charge. The DNO upgrades part of the existing network to accommodate the new load, and the cost is passed to the connecting customer.
Reinforcement costs can be anything from a few thousand pounds (for a local cable upgrade) to several hundred thousand pounds (for a new pole transformer or substation upgrade).
An HV connection bypasses much of this by feeding directly from the HV network into a customer transformer. It avoids the LV reinforcement issue entirely, which is why it is often cheaper at higher loads.
The Customer Substation
An HV connection typically requires a customer substation. This is a small building or kiosk that houses:
An HV switch (usually a ring main unit).
One or more transformers.
The LV distribution board that feeds the building.
The substation takes up space on the site and needs access for DNO maintenance vehicles. It must be sited to avoid residential windows at close range due to noise and transformer hum.
Budget £25,000 to £80,000 for a typical customer substation as part of the HV connection package.
Timeline Comparison
LV connections on an existing network are faster to deliver than HV connections.
Typical LV connection from design to energisation: six to sixteen weeks.
Typical HV connection from design to energisation: sixteen to thirty weeks.
HV timelines are longer because of the substation civils, the transformer lead time (often six to twelve weeks), and the more complex commissioning process.
How to Decide
Start by calculating the site’s diversified peak demand in kVA. A competent electrical consultant can produce this from the intended building use and equipment schedule.
If the demand is below 300 kVA, LV is almost always the right answer unless the existing network has very limited capacity.
If the demand is 300 to 500 kVA, get quotes for both LV and HV. The answer depends on local network conditions.
If the demand is above 500 kVA, start with HV as the default and only consider LV if there is a specific reason to.
Above 1 MVA, HV is effectively mandatory for cost and technical reasons.
The Bottom Line
The LV/HV decision is a load question before it is a cost question. Get the load right first, then let it drive the voltage choice. Trying to force an LV solution onto a site that really needs HV is one of the most expensive mistakes in UK electricity connection procurement, and one of the biggest drivers of the cost bands we publish for new electricity connections.