Air source heat pumps require a dedicated circuit sized to the compressor input, typically 32A to 63A. This guide covers ASHP circuit sizing, DNO notification, immersion backup circuits, UFH zone controls wiring, MCS certification, and the Boiler Upgrade Scheme grant.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
A domestic ASHP needs a dedicated circuit sized to the compressor electrical input (not the thermal output) — typically a 16A circuit on 2.5mm² for a 5–8kW unit, rising to 32–40A on 6mm² for 16–20kW units. Most run on single-phase 230V. Add an immersion backup circuit (usually 16A) and controls wiring, then test and certify the work to BS 7671:2018+A4:2026 and issue an EIC.
Always size from the manufacturer’s electrical data sheet — rated current, maximum current and starting current all matter. Check the DNO supply fuse has spare capacity before connecting.
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Air source heat pumps (ASHPs) are the central technology in the UK's heat decarbonisation programme. Government grant funding (the Boiler Upgrade Scheme), rising gas prices, and the heat pump installation target of 600,000 per year by 2028 mean that heat pump electrical work is a significant and growing opportunity for UK electricians.
The electrician's role in a heat pump installation typically covers: the dedicated circuit from the consumer unit to the outdoor unit, the immersion heater backup circuit to the hot water cylinder, the controls wiring for UFH zone valves and room thermostats, and the testing and certification of all electrical work under BS 7671:2018+A4:2026.
This guide covers circuit sizing, DNO notification, immersion backup circuits, UFH controls wiring, MCS certification and the BUS grant, and the testing and certification requirements for heat pump electrical installations.
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The dedicated circuit for an ASHP must be sized for the compressor electrical input, not the thermal output. The COP (Coefficient of Performance) determines the relationship between thermal output and electrical input:
| Heat output (COP ~3) | Electrical input | Indicative circuit | Cable (T&E) |
|---|---|---|---|
| 5kW – 8kW | 1.7kW – 2.7kW | 16A RCBO | 2.5mm² |
| 10kW – 12kW | 3.3kW – 4kW | 20A – 25A RCBO | 4mm² |
| 16kW – 20kW | 5.3kW – 6.7kW | 32A – 40A RCBO | 6mm² |
Indicative only. Confirm the conductor size against the actual run length and installation method — a long run to an outdoor unit can force the next size up on voltage drop, and a high compressor starting current may need a time-delay device at the consumer unit.
Always request the electrical specification sheet from the heat pump manufacturer before sizing the circuit. The rated current, maximum current, and starting current are all relevant. Heat pump compressors draw a high inrush at start-up, so check the inrush figure against the protective device characteristic. Use the cable sizing calculator to confirm the cable size for the actual run length from the consumer unit to the outdoor unit position.
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Heat pumps consume electricity — they do not generate or export. This means that the DNO notification requirements that apply to solar PV and battery storage (G98, G99) do not directly apply. However, there are still DNO considerations:
Heat pump cylinders include a 3kW immersion heater element as a backup. The electrical installation for the immersion heater must include:
Where the heat pump heats the property via underfloor heating (wet UFH), the controls wiring is a significant element of the electrical installation:
The controls wiring for a heat pump UFH system is low-voltage and is excluded from the BS 7671 EIC scope in terms of circuit testing, but the mains supply to the controls hub or room thermostats (if mains-powered) is included in the EIC scope.
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Start free trialThe Boiler Upgrade Scheme (BUS) provides a grant of up to £7,500 for an ASHP installation (2024/25 rates — check GOV.UK for current rates). To access the grant:
The electrical installation for a heat pump must be inspected and tested as part of initial verification under BS 7671:2018+A4:2026 Chapter 64. Carry out the dead tests before energising, then the live tests, recording the results on the Schedule of Test Results. The test sequence for the dedicated ASHP circuit:
| Test | BS 7671 reference | What good looks like |
|---|---|---|
| Continuity of conductors / CPC | Reg 643.2 | Low, stable end-to-end reading on the protective conductor from the MET to the heat pump enclosure earth terminal. |
| Insulation resistance | Reg 643.3, Table 64 | Tested at 500V DC for a 230/400V circuit; minimum 1.0 MΩ. Disconnect the heat pump at the local isolator so its electronics do not influence or suffer from the test. |
| Polarity | Reg 643.6 | Correct polarity confirmed at the local isolator and at the heat pump supply terminals. |
| Earth fault loop impedance (Zs) | Reg 643.7.3 | Measured Zs at the heat pump supply terminals within the maximum permitted for the protective device and disconnection time. |
| RCD additional protection | Reg 643.8 | Where a 30mA RCD/RCBO provides additional protection, it disconnects within 300ms at rated residual operating current for a general non-delay type. |
| Functional test | Reg 643.10 | Energise and verify the isolator, controls and compressor operate correctly — the compressor starts and runs as intended. |
An Electrical Installation Certificate must be issued covering all new circuits. The EIC should note the heat pump model, the dedicated circuit rating, and the local isolator position. A copy of the EIC is provided to the MCS-certified heat pump installer for inclusion in the commissioning documentation.
Heat pump installations are a growing market and the electrical package — dedicated circuit, immersion backup, and controls wiring — is typically worth £500 to £1,500 per installation as a sub-contract element. Building relationships with MCS-certified heat pump installers generates recurring referrals.
Before agreeing to connect the heat pump, check the DNO supply fuse rating and the existing consumer unit maximum demand. If the supply fuse is undersized for the additional ASHP load, a supply fuse upgrade is required — this involves the DNO and takes time. Flag this on the survey to avoid programme delays.
The heat pump installer needs the EIC to complete their MCS commissioning documentation and to register the BUS grant. A prompt EIC — issued the same day using the EIC Certificate app — makes you the preferred electrical sub-contractor for heat pump installers who care about their customer experience.
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