Electrical supply for heat pumps

How the circuits, protective devices, supply capacity, DNO process and certification are designed for a heat-pump system.

A heat pump is a substantial electrical load with compressor electronics, pumps and controls. The complete system can also include an electric backup heater, hot-water immersion, trace heating and indoor modules. Its electrical design must account for all of them.

Thermal output is not the number used to size the cable or breaker. The electrician works from the exact appliance data, manufacturer instructions, operating modes and the building’s existing supply.

Identify every electrical load

The selected model’s data and wiring instructions should state the supply phase, maximum input or current, starting characteristics and required protection. Separate values may apply to the compressor, fan and integrated backup heater.

The design schedule should also identify:

circulation pumps and motorised valves
an indoor hydrobox or control unit
cylinder immersion or booster heaters
frost-protection or condensate trace heating
supplementary or backup heating
controls, communications and monitoring
any other new load installed with the project

Some products use one supply; others require separate final circuits or control supplies. A generic “heat-pump circuit” description does not identify the load that determines the actual supply upgrade.

Circuit and cable design

The manufacturer may specify a dedicated circuit and permitted protective devices. The electrician then designs to the current edition of BS 7671, considering:

design current and any starting or inrush behaviour
overload and short-circuit protection
cable installation method, grouping, insulation and ambient temperature
voltage drop at the equipment during operation and starting
earth-fault loop and disconnection requirements
outdoor conditions, impact, ultraviolet light and water
isolation for maintenance
the consumer unit or distribution assembly’s ratings and instructions

The breaker curve or rating is not chosen simply to stop nuisance trips. It must protect the cable, coordinate with the appliance and operate within the fault conditions available at the property.

Heat pumps can run for long periods. IET guidance highlights the thermal interaction between adjacent, simultaneously loaded circuit-breakers and the consumer-unit manufacturer’s rated diversity factor. Finding a spare way in the board does not prove that the complete assembly can carry another sustained load.

RCD selection is product-specific

Inverter-driven compressors can produce leakage containing different frequencies and a smooth DC component. An unsuitable residual current device can trip unnecessarily or fail to respond as intended.

There is no universal rule that every heat pump needs the same RCD type. The electrician should use current BS 7671 requirements and the manufacturer’s declared residual-current data and instructions. Energy Saving Trust’s electrician guidance notes that manufacturers may specify Type F or Type B based on testing, and treats Type B as best practice where the manufacturer gives no DC earth-leakage data.

That does not justify adding a Type B device without checking selectivity, upstream devices, earthing and the rest of the installation. It also does not justify leaving a common Type AC or Type A device where the product evidence shows it can be blinded.

The commissioning record should identify the installed protective device by type and rating, as requested by the Heat Pump Association checklist.

Main supply and maximum demand

Before agreeing the design, inspect the service head, meter tails, meter, earthing, consumer unit and any existing high loads. The maximum-demand assessment should consider credible simultaneous use of the heat pump, backup or immersion heat, EV charging, cooking, showers, batteries and other loads.

An assessment should record rather than guess:

the cut-out and known fuse rating
single-phase or three-phase supply
whether the service is looped
conductor and consumer-unit capacities
available ways and assembly restrictions
existing and proposed maximum demand
any load-management arrangement and its fail-safe behaviour

A heat pump does not automatically require three-phase power, and a familiar fuse rating does not automatically make a single-phase design acceptable. The selected model and calculated demand decide.

Only the network operator or its authorised party should change a service fuse or alter the service equipment. A consumer-unit upgrade does not increase the capacity of the incoming network connection.

DNO notification or application

MCS design and installation standards require heat-pump connections to follow the ENA process, with permission obtained before installation where necessary.

The current process has two broad routes:

connect and notify, where the qualifying installation can proceed and the DNO is notified afterwards
apply to connect, where approval is required before the equipment is installed

The route depends on the appliance data, supply and other low-carbon technologies at the property. The installer should check the current ENA database and flow chart, not assume that every domestic heat pump can be notified after the event.

The handover should include the application or notification reference and any conditions. A DNO acknowledgement is not an electrical installation certificate, and an installation certificate is not DNO permission.

Isolation and the outdoor unit

Isolation should be accessible, identifiable, capable of making the required circuits safe and consistent with the manufacturer’s switching sequence. More than one supply may enter the outdoor or indoor equipment, so one switch beside the unit may not isolate controls, backup heaters or immersion circuits.

For a heat pump using R290 propane, the manufacturer defines a safety zone around potential leak points. Switchgear, sockets, drains and openings may be restricted there. The local isolator should not be positioned inside that zone merely to satisfy a generic “within sight” rule. Safe maintenance isolation can be achieved only by coordinating the electrical design with the appliance’s refrigerant-safety instructions.

The circuit protective conductor normally provides earthing for Class I outdoor equipment. PME supplies, bonding and any simultaneously accessible metalwork still need a site assessment, but a heat pump is not automatically subject to the EV charge-point open-PEN rule. Additional bonding should not be added without deciding whether a part is actually extraneous-conductive.

Surge protection and controls

Electronic compressor drives and controllers can be vulnerable to transient overvoltage. The electrician applies the current BS 7671 surge-protection requirements and risk exceptions to the whole installation rather than promising that one plug-in device protects the heat pump.

Power and communication cables should follow the manufacturer’s separation, screening and routing instructions. A control cable placed beside a high-current supply can cause unreliable sensors or communications even when its insulation is electrically safe.

Hot-water diverter valves and backup heaters must be wired to the heat pump’s control logic. Borrowing a conventional boiler wiring pattern can leave both heat sources active, operate a valve backwards or energise resistance heating more often than intended.

Inspection, testing and notification

New and altered electrical work must be inspected and tested. A new final circuit in a dwelling is normally notifiable building work in England and Wales; the routes and terminology differ across the UK, so the project should identify the applicable building-control responsibility.

The handover includes, as applicable:

the electrical installation certificate and schedules of test results
the building-regulations compliance certificate or approval record
circuit and isolator labels
protective-device types and ratings
maximum-demand and supply assessment
DNO application or notification evidence
the final electrical and control schematic
shutdown, restart and safe-isolation instructions

The heat pump should then be commissioned under load, with voltage, current, protective-device operation, phase rotation where relevant, controls and every supplementary heater checked against the design.