Heat Pump Electrical Connections: Circuit Sizing, DNO Notifications and Smart Controls

Summary

Air source heat pumps are one of the fastest-growing heating technologies in the UK, driven by the Government's Boiler Upgrade Scheme grant (currently £7,500) and the Future Homes Standard mandate for low-carbon heating in new builds. The electrical installation for an ASHP is more demanding than for a gas boiler — it requires a dedicated circuit, correct cable sizing, an accessible outdoor isolator, and in most cases either G98 or G99 DNO notification.

For tradespeople, the critical issue is that heat pump installations sit at the intersection of electrical and plumbing disciplines. The electrical circuit sizing, control wiring (OpenTherm, on/off, or smart thermostat), and integration with immersion heater backup must all be correctly executed. Errors in circuit sizing cause nuisance tripping; errors in control wiring cause the system to run inefficiently, consuming far more electricity than it should.

This article covers all aspects of heat pump electrical installation — from cable sizing and consumer unit changes through to DNO notifications and smart control integration. It is relevant to both the electrical contractor installing the circuit and the heat pump installer specifying the electrical requirements.

Key Facts

• Single-phase 230V — sufficient for most domestic ASHP up to 12kW rated input; above this, three-phase is required
• Three-phase 400V — required for ASHP input above approximately 12kW, or where single-phase capacity is insufficient at the supply
• 6mm² T&E — appropriate cable size for most domestic ASHP (16–32A depending on rated input current)
• Dedicated circuit — ASHP must have its own MCB from the consumer unit; sharing a circuit with other loads is not permitted
• Isolator requirement — a lockable outdoor isolator must be installed within 10m line-of-sight from the heat pump unit, or closer where practicable
• G99 DNO notification — required for heat pumps where the electrical input exceeds 3.68kW on a single-phase supply (>16A per phase); pre-approval required before installation
• G98 notification — applies to systems generating <16A per phase; ASHP do not generate electricity but the G98/G99 framework applies to any equipment that affects grid load
• MCS — Microgeneration Certification Scheme; certification is mandatory for installers to access BUS grant for customers
• BUS grant — Boiler Upgrade Scheme: £7,500 for ASHP (England and Wales); requires MCS-certified installer and MCS-registered product
• OpenTherm — a digital communication protocol between heat pump/boiler and thermostat; allows modulating control (variable heat output vs on/off)
• SMETS2 smart meter — required for smart tariff access; most heat pump owners benefit from Economy 7 or Agile Octopus time-of-use tariffs
• Immersion backup — most ASHP cylinders include an immersion heater element (typically 3kW) for backup or anti-Legionella pasteurisation; this is a separate Part P notifiable circuit
• Part P — new circuit from consumer unit to ASHP is notifiable under Building Regulations Part P

Quick Reference Table

Note: These are indicative. Always calculate from the manufacturer's rated input current and account for cable route length, grouping factors, and ambient temperature derating.

Detailed Guidance

Circuit Sizing Calculations

Cable sizing for ASHP follows BS 7671:2018 Table 4D5 (flat twin and earth thermoplastic cable) with derating factors applied:

1. Design current (Ib) — take from manufacturer data sheet; typically the rated input current at maximum output
2. Ambient temperature correction — if cable runs through a warm area (loft in summer), apply correction factor; standard tables assume 30°C ambient
3. Grouping factor — if cable is run grouped with other cables, apply the grouping derating factor
4. Overcurrent device selection — select MCB rated above design current but within the cable's current-carrying capacity

Example for a 6kW ASHP:

• Rated input current: 28A (from data sheet)
• Single run, not grouped, 30°C ambient: 6mm² T&E (clipped direct): rated 47A → satisfactory
• MCB: 32A Type C (motor load: use Type C or D to handle start-up current)
• Cable length: check voltage drop does not exceed 3% (BS 7671 guidance); for 6mm² at 28A, 230V, maximum length is approximately 28m before 3% is exceeded

Important: Use Type C or Type D MCBs for heat pump circuits. ASHP compressors have high start-up inrush current (typically 3–6× running current) that can trip Type B MCBs.

DNO Notification: G99 for Most Domestic ASHP

Unlike battery storage (which is about export capacity), DNO notification for heat pumps is driven by the impact on the local distribution network. As of ENA ER G99:

• Any single-phase heat pump drawing >3.68kW (16A) requires G99 approval before installation
• The DNO assesses whether the local low-voltage network can accommodate the additional load
• Most domestic ASHP between 4kW and 12kW fall under G99
• G99 applications are submitted to the regional DNO (e.g. UK Power Networks, Western Power Distribution)
• Standard G99 assessment: 45 working days; if reinforcement is required, the homeowner bears the cost

In practice: Many DNOs have developed fast-track processes for standard ASHP G99 applications. Approval is routine for most domestic installations in established residential areas. Problems arise in rural areas with long, lightly-loaded LV lines.

MCS installers are required to handle the G99 application process as part of the MCS installation process. Electrical contractors installing the circuit after MCS approval can proceed on the basis of the MCS documentation.

Outdoor Isolator Requirements

An ASHP requires an emergency isolator accessible without entering the building:

• Located within 10m line-of-sight of the ASHP external unit, where possible within 3m
• Rated for the running current plus derating for outdoor use
• IP65 minimum (outdoor, weatherproof)
• Lockable in the off position (padlockable switch or key-operated)
• Clearly labelled with the circuit it controls

The isolator must be on the load side of the main consumer unit MCB, not on the supply side. It is a service isolation point, not an overcurrent protection device.

If the ASHP is wall-mounted in a utility room, the isolator within the room may satisfy this requirement provided it is accessible without opening junction boxes or moving the unit.

OpenTherm vs On/Off Control

Heat pump efficiency is significantly affected by how the system is controlled:

On/Off control:

• Simple two-wire signal from thermostat to heat pump
• Heat pump runs at full output until setpoint reached, then stops
• Creates temperature cycling and reduces SCOP
• Used with standard room thermostats and basic programmers
• Lower upfront cost; simpler wiring

OpenTherm modulation:

• Digital communication protocol (two-wire, polarity-insensitive, 12V DC signal)
• Thermostat sends a heating demand signal; heat pump adjusts output to match
• Compressor runs continuously at low modulation rather than cycling on/off
• Typically improves SCOP by 0.3–0.8 compared to on/off
• Compatible thermostats: Honeywell T6R/T6H (OpenTherm), Tado, Nest (limited), Vaillant VR33

OpenTherm wiring:

• Two-wire bus from heat pump controller to thermostat
• Maximum cable distance 300m (typically no issue domestically)
• Polarity insensitive: connect either wire to either terminal
• Cannot be run alongside mains wiring (electrical interference)

BUS grant requirement: The MCS inspection process checks that the heat pump is correctly installed including controls. OpenTherm or equivalent modulating control is not mandated by MCS but is required for optimal performance targets referenced in MCS guidance.

Immersion Heater Backup Circuit

Most ASHP installations include a hot water cylinder with an integral immersion heater element. This is used for:

1. Legionella pasteurisation — heating to 60°C+ periodically (ASHP typically operates at 50–55°C max)
2. Backup heating — if ASHP fails or ambient temperature is very low
3. Emergency boost — rapid reheating in high-demand situations

The immersion heater is a separate electrical load (typically 3kW/13A) and requires:

• A dedicated 16A MCB and radial circuit from the consumer unit
• Double-pole isolator switch adjacent to the cylinder
• Timer or controller to schedule pasteurisation cycle (typically weekly, 1-hour cycle at 60°C)
• RCD protection (standard for new circuits under Part P)

This circuit must be separately notified under Part P from the ASHP circuit.

SMETS2 Smart Meter and Time-of-Use Tariffs

Most ASHP owners benefit materially from switching to a time-of-use electricity tariff:

Accessing these tariffs requires a SMETS2 smart meter, which communicates half-hourly consumption data to the supplier. SMETS1 meters (installed before 2019) cannot support all time-of-use tariffs.

Smart heat pump controls (Tado, Nest, Vaillant SensoCOMFORT with Smart home integration) can schedule heating operation to coincide with cheap overnight electricity — this is one of the most effective ways to reduce running cost.

Frequently Asked Questions

Does an ASHP installation require a new consumer unit?

Not necessarily, but it is common. If the existing consumer unit does not have a spare way with sufficient rating, or if it is a split-load unit with insufficient RCD-protected ways, a new consumer unit may be required. The additional load of the ASHP should also be checked against the incoming service fuse rating — older properties may have only 60A or 80A service fuses, while a modern ASHP may push total consumption above this.

Can a plumber do the ASHP electrical work?

No. Electrical work including new circuits and consumer unit modifications must be carried out by a Part P competent person. MCS installers who are not Part P registered must sub-contract the electrical work to a registered electrician. The Part P certificate must be issued before the MCS installation certificate can be completed.

How long does G99 approval take?

The standard G99 assessment period is 45 working days (approximately 9 calendar weeks). Some DNOs offer fast-track approval for standard domestic ASHP applications in 5–10 working days. If the application triggers a distribution reinforcement requirement, the timeline is indeterminate and costs can run to thousands of pounds — this must be flagged to the customer before proceeding.

What happens if G99 is not obtained?

Installing an ASHP without required G99 approval is a breach of the Electricity Supply Regulations and could invalidate the installation and home insurance. The DNO has the right to disconnect any equipment connected without approval. MCS certification will also be withheld, meaning the BUS grant cannot be claimed.

Is three-phase supply always needed for large heat pumps?

Not always. Some manufacturers offer single-phase versions of heat pumps up to 14–16kW rated output (with a lower input current due to higher efficiency). Always check the manufacturer's data sheet for the specific model. Installing a three-phase supply is a significant additional cost (typically £2,000–5,000+) and time burden; confirm the requirement before specifying.

Regulations & Standards

• Building Regulations Part P (2013) — electrical safety in dwellings; new circuit from consumer unit to ASHP is notifiable

• BS 7671:2018 — IET Wiring Regulations 18th Edition; cable selection, circuit design, and protection requirements

• ENA Engineering Recommendation G99 — requirements for connection of generating plant and energy storage systems to the distribution systems; applies to heat pumps >3.68kW

• MCS 001 — MCS Installer Certification Standard; required for BUS grant eligibility

• Building Regulations Part L — conservation of fuel and power; heat pump COP requirements for new build

• ENA G99 Guidance — Energy Networks Association

• MCS Installer Standards — Microgeneration Certification Scheme

• OFGEM: Boiler Upgrade Scheme — Office of Gas and Electricity Markets

• IET Guidance Note 7: Special Locations — Institution of Engineering and Technology

• Heat Pump Association Technical Guidance — Heat Pump Association

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