What to expect from an ASHP electrical installation

Helpful video reference. Artisan Electrics, a Cambridge-based electrical contractor with more than 340,000 YouTube subscribers and a specialist focus on domestic renewable energy installations, walk through a complete ASHP survey in this video — covering what the assessor checks, the questions they ask, and why the survey shapes the electrical specification that follows. Watch on YouTube at https://www.youtube.com/watch?v=X3uj7zNfOeM.

1. The pre-installation survey

A surveyor (or the electrician) visits the property before the installation day. They check whether the existing consumer unit has a spare way for the new circuit.

They measure the supply cable route from the consumer unit to the outdoor unit location. They also confirm the earthing arrangement. PME (TN-C-S) is standard in most UK homes, and this affects how the outdoor unit is earthed.

The outcome is a cable size, a protective device rating, and a note on any consumer unit work needed.

2. Supply cable sizing and consumer unit capacity

Most domestic ASHP units draw between 16A and 32A depending on their output. The supply cable is typically 4mm or 6mm two-core-and-earth from the consumer unit, routed to a local isolator near the unit.

If the existing consumer unit has no spare way, a piggyback breaker, a sub-board, or a consumer unit upgrade may be needed. The electrician should provide written confirmation of what they have sized and why.

3. Isolator installation

BS 7671 requires a local means of isolation for the heat pump — within sight of the unit and lockable. A 20A or 32A double-pole rotary isolator on a weatherproof enclosure is the typical solution.

The isolator lets the heat pump engineer de-energise the unit for servicing without going back to the main consumer unit indoors. It is a straightforward fitting but the enclosure rating matters: anything below IP65 is unlikely to be acceptable in an exposed outdoor position.

4. Heat pump unit connections

The supply cable terminates inside the isolator, then a short run continues into the heat pump's own connection board. Connection colours follow current BS 7671 regulations: brown for live, blue for neutral, green/yellow for earth.

The electrician checks terminal tightening torques as specified by the heat pump manufacturer. Under-torqued connections cause overheating, and overheating at the connection board is one of the more common faults on heat pumps that have been in service for a year or two.

5. Wiring centre and controls

The indoor wiring centre connects the heat pump controller to zone valves, pumps, the hot water cylinder thermostat, and room thermostats. Control wiring is typically 3-core or 4-core alarm cable or, for Modbus-capable units, a screened pair.

The electrician follows the manufacturer's wiring diagram for the specific combination of heat pump and hot water cylinder. Getting this wrong shows up immediately at commissioning, or not at all until the hot water cylinder refuses to heat in winter. Programmer, room thermostat, and cylinder stat settings are commissioned after all wiring is complete.

6. Testing, certification and notification

All new circuits are tested: insulation resistance, polarity, earth continuity, and earth fault loop impedance. The electrician issues an Electrical Installation Certificate (EIC) covering the new supply circuit and associated work.

If a registered electrician carries out the work, they self-certify to building control. You receive a completion certificate, usually within a few weeks. Keep the EIC with the property deeds: mortgage lenders and buyers ask for it.

When to call us

Richard is experienced in renewable energy electrical work across east Kent. If you have an ASHP going in and need the electrical side handled — supply cable, isolator, wiring centre — contact him early in the planning process. Getting the electrical scope agreed before the heat pump installer books in avoids the most common delays.