Three-phase supply upgrades

When a home may need three phases, how demand and generation are assessed, and how DNO, metering and internal electrical work fit together.

A single-phase home receives one line conductor. A three-phase supply provides three line conductors whose voltage waveforms are offset from one another. Under the GB declared-voltage framework, each phase is nominally 230V to neutral and the phase-to-phase voltage is nominally 400V.

The practical benefit is additional capacity and the ability to connect equipment designed for three-phase operation. It is not automatically necessary for every heat pump, EV charger, solar array or battery. The decision should follow a maximum-demand calculation and DNO assessment for the actual property.

Capacity is set by the connection, not the phase count alone

The usable supply capacity depends on the service cable, cut-out, fuse or protective device and the capacity the DNO agrees. A three-phase head with modest per-phase capacity is not the same as an unrestricted high-power connection.

As one DNO example, UK Power Networks offers domestic and small-business upgrades from single to three phase up to 70kVA in its area. That is not a national default and not the capacity every site will receive.

Before requesting three phase, establish:

the present number of phases
the confirmed main-fuse rating and agreed capacity
the calculated maximum demand after diversity and load control
any equipment that specifically requires three phase
the proposed generation and export arrangement
whether the local network can provide the requested capacity

An electrician can calculate the customer’s demand. The DNO confirms the network and connection capacity.

When an upgrade may be justified

Three phase may be appropriate where:

calculated demand exceeds an acceptable single-phase connection
a piece of equipment requires a three-phase supply
several large loads need to operate with less restriction
a development needs multiple metered supplies
the DNO’s accepted generation design requires a multi-phase connection

It is not the only option. A fuse or capacity upgrade within single phase, dynamic load management, a lower charging rate or controlled sequencing of loads may meet the requirement at lower cost. Those alternatives still need proper demand calculations and any required DNO acceptance.

Do not size the supply by adding every appliance nameplate at full output with no diversity, or by assuming smart control will always prevent overload. The design should state what can run together, the control failure response and the demand requested from the DNO.

Generation and export need their own assessment

G98 applies to fully type-tested micro-generators up to and including 16A per phase. A three-phase G98 installation can therefore have generating capacity on each phase within the standard’s conditions. This is a connection threshold, not an automatic right to export 11.04kW or any other calculated total.

Equipment outside G98 follows G99. A larger system may also use G100 limitation where the DNO agrees a Maximum Export Limit below the installed generating capacity.

Moving a property to three phase does not automatically cancel an existing export limit or approve a new inverter. The generation application should show:

existing and proposed generating devices
Registered Capacity per phase
phase allocation and any imbalance
storage coupling
proposed Maximum Export Limit
limitation and protection evidence

The DNO assesses the proposal against the local network. The accepted connection arrangement must be known before equipment selection relies on three phases to resolve an export constraint.

Existing single-phase solar and batteries

An existing single-phase inverter may be able to remain connected to one phase after a supply upgrade, but that needs to be part of the new design. The designer must consider per-phase current, phase imbalance, metering, export control and the requirements of any new three-phase equipment.

Alternatives include retaining some single-phase devices, adding equipment on other phases or replacing them with a three-phase system. There is no universal best arrangement. Compatibility, warranty, communications and the DNO-approved schematic matter more than the marketing description.

A three-phase grid connection also does not guarantee three-phase backup. Some battery systems support only one backed-up phase, some support selected circuits and others can form a three-phase island only in specific configurations. Backup power, neutral switching, earthing and phase balance must be designed separately.

Who does the work

The project crosses three ownership boundaries:

The DNO changes or provides the service cable, cut-out and agreed connection capacity.
The electricity supplier or its meter operator changes the meter.
The customer’s electrician changes the consumer-side installation and certifies it.

UK Power Networks explicitly directs meter upgrades to the electricity supplier rather than the DNO. SSEN describes its capacity-change work as including the service cable and cut-out, with the project prepared for the supplier to install the meter.

Internal work may include a new distribution board, meter tails, phase allocation, protective devices, equipment circuits and changes to generation or backup wiring. The required work follows the actual design; a three-phase consumer unit and wholesale rewire should not be assumed without a survey.

Application and project sequence

A sensible sequence is:

Survey the existing service, meter position and customer installation.
Calculate present and future maximum demand.
Decide whether single-phase alternatives remain viable.
Apply to the DNO for the required three-phase capacity.
Review the DNO design, exclusions, land requirements, price and programme.
Coordinate the supplier’s meter change and the electrician’s work.
Submit or amend generation applications where solar or storage changes.
Complete, test and certify the installation.

SSEN’s published process asks for the current and required capacity, a site plan, access, project responsibility and whether groundworks should be included. It warns customers not to complete site-preparation work until they have spoken to the project manager.

That is sound practice in any region. Duct routes, meter boxes and excavation must match the DNO’s accepted design.

Cost and timing

There is no dependable UK-wide average. Cost depends on how close suitable three-phase network is, the requested capacity, reinforcement, excavation, highway work, land rights, metering and internal electrical changes.

The DNO connection offer separates:

contestable and non-contestable connection work
private-land and highway excavation
reinforcement or abnormal network work
land and permit costs
meter work outside the DNO scope
customer-side electrical work
generation-connection changes

Timescales are equally site-specific. SSEN publishes target stages for its own connection-offer and works process, but legal land rights and permits can extend them. The dated programme in the accepted connection offer still has supplier and electrician dependencies. A headline DNO start date is not the same as a fully commissioned three-phase installation.

Metering and tariffs

Confirm with the intended supplier that it can provide the required three-phase meter and support the chosen import and export products. Do not assume that the existing tariff, standing charge, half-hourly data or export account will transfer unchanged.

After the work, check that the import and export records contain the correct MPANs, meter serial number, phase configuration and opening readings.

Handover records

Keep:

the DNO’s accepted capacity and design
service and meter work records
the electrical installation certificate
the as-built phase and circuit schedule
updated G98, G99 or G100 records
import and export MPAN and meter details
settings for load management and export control

These records prevent a future installer from treating “three phase” as a complete description of the connection.