Helpful video reference. Grant UK's "Air Source Heat Pump Installation & Set-up UK Case Study" shows a real domestic ASHP installation from start to finish, including the electrical supply circuit and local isolator. Grant UK is a major UK heat pump manufacturer so the video is specific to how these systems are installed here.
1. Read the manufacturer's data sheet
Before any cable or protection device is sized, the heat pump's data sheet must be consulted. The data sheet (usually in the installation manual) states:
- Rated current (in amps) - the running current the unit draws.
- Maximum starting current - the brief inrush when the compressor starts. Inverter-driven heat pumps have a soft-start, so this is usually close to the rated current. Older non-inverter models can spike higher.
- Recommended overcurrent protection rating - the MCB or RCBO amperage the manufacturer specifies.
- Required RCD type - whether Type A, F or B is needed to handle the DC leakage currents the inverter drive produces.
- Earthing and bonding requirements - some manufacturers specify a minimum earth conductor size or additional bonding.
Do not size anything from the rated kW figure alone. A 5 kW heat pump at 230V would in theory draw around 22A, but the actual figure in the data sheet may differ due to efficiency ratings and power factor. Use the data sheet figure.
2. Choose the right cable and overcurrent protection
Most domestic ASHPs up to around 5 kW need a 20A or 32A radial circuit using 6 mm² twin and earth. Larger models, or installations with a long cable run where voltage drop is significant, may need 10 mm².
The RCBO must not be rated above the cable's current-carrying capacity for the installed method. For 6 mm² twin and earth clipped direct to a wall, the current-carrying capacity is 47A, so there is good headroom for most domestic units.
The RCBO type matters. Standard Type AC devices only detect sinusoidal AC fault current. Inverter drives in heat pumps can produce pulsating DC leakage, which a Type AC device may not detect. Most heat pump manufacturers specify Type A as a minimum. Some require Type F (detecting high-frequency leakage up to 20 kHz) or Type B (full DC detection). Fit what the data sheet says.
3. Fit a dedicated way at the consumer unit
The heat pump must have its own circuit, not share with any other load. The electrician fits an RCBO of the correct type and amperage into the consumer unit, connects the new radial circuit cable, and labels the way clearly on the circuit schedule inside the consumer unit door.
If the consumer unit is full or does not have space for the right RCBO type (some older boards only take a limited range of device types), a small sub-board or a consumer unit upgrade may be needed. This is worth raising with the electrician before quoting stage.
4. Run the cable to the ASHP location
The cable runs from the consumer unit to the outdoor unit. Typical routes:
- Through the wall to an exterior surface, then in conduit down the outside of the house to the outdoor unit location.
- Underground in SWA (steel wire armoured) cable if crossing a garden. Buried cable must be at 600 mm depth under a cultivated area, 450 mm under a path, with cable route markers above it.
Any cable run on an external wall should be in grey round conduit to protect it and keep the installation tidy. Surface-run cable in PVC trunking is also acceptable on rendered walls where the route is straight.
5. Fit the double-pole local isolator
BS 7671 Regulation 537.3.2 requires a local means of isolation within sight of the heat pump, so any service engineer can isolate the unit without going back to the consumer unit inside the house. A weatherproof double-pole isolator (often a red rotary switch in a grey IP65 enclosure) mounted on the wall near the outdoor unit satisfies this.
The isolator must be rated for the full load current of the circuit and must break both the line and neutral conductors simultaneously.
6. Connect at the ASHP controller
Heat pumps connect to their own internal wiring centre or controller board. The supply (brown line, blue neutral, green-yellow earth) connects to the terminals the manufacturer specifies in the installation manual. Do not guess terminal positions: the wiring diagram in the manual is definitive.
The outdoor unit also needs low-voltage signal cables to the indoor cylinder unit and, in split systems, the refrigerant pipes. These are not part of the electrical supply circuit, but they run alongside it and are done by the heat pump installer (who must be F-gas registered for the refrigerant work).
7. Test, certify and commission
Before the circuit is energised, dead tests are carried out: insulation resistance between conductors and earth, continuity of the circuit protective conductor, and polarity checks. Then the circuit is energised and live tests are done: earth fault loop impedance (Zs) at the heat pump terminals, and RCD trip time (must be 40ms or less for a 30mA device at 5×I∆n).
The results go onto an Electrical Installation Certificate (EIC), which is issued to the homeowner and registered with the local authority through the electrician's Part P scheme. Keep it with your property paperwork. This certificate is what a solicitor or buyer's electrician asks for when you sell.
When to call us
Richard wires ASHP electrical supplies across Sandwich and east Kent, including the consumer unit RCBO, cable run and local isolator. He can also inspect and certify existing installations that were not properly certified at the time of installation.
Getting an ASHP wired in east Kent?
Richard provides the dedicated electrical supply, isolation and Part P certification for air source heat pump installations across Sandwich, Deal, Dover, Ramsgate and Canterbury.
Contact Richard