Heat Pump Electrical Install London — ASHP Supply, Circuits, G99

It depends on the heat output of the unit and the rest of the household load. A 5–7 kW ASHP comfortably runs on a 60A or 80A single-phase supply with a smart-load module. An 8–9 kW ASHP works on a 100A single-phase supply with careful load management. A 10 kW+ ASHP or any ASHP installed alongside a 7kW EV charger and an induction hob on a 60–80A supply usually benefits from a three-phase upgrade — both for headroom and for the lower per-phase inrush current at compressor start.

Strictly: any ASHP with input current below 16A per phase notifies under G98 and no DNO pre-approval is needed. Above 16A per phase, G99 applies — most 8 kW+ output ASHPs hit this threshold. Practical maximum on a 100A single-phase London supply is around 12 kW heat output (≈ 25–30A peak input) with smart load balancing — beyond that the three-phase upgrade case is strong.

BUS provides £7,500 toward an air-source heat pump and £7,500 toward a ground-source heat pump install in England and Wales. Eligibility requires: a domestic property in scope (including BTL), a valid EPC (no F or G rating), no outstanding loft or cavity insulation recommendations on the EPC (unless exempt), and the install must be MCS-certified. Larger commercial premises use the Public Sector Decarbonisation Scheme route instead.

A dedicated radial circuit from the consumer unit to a lockable isolator within 2 metres of the ASHP outdoor unit. Cable size sized to the manufacturer’s rated input plus 25% headroom and the route length (typically 6mm² for a 5–7 kW unit, 10mm² for an 8–10 kW unit). A Type A RCBO at the board provides 30 mA leakage protection and overcurrent. Single-pole isolator at the unit allows safe maintenance. BS 7671:2018 + A4:2026 compliant.

Within 2 metres of the outdoor unit, in line of sight, lockable in the off position, and accessible without tools. The lockable requirement means the engineer servicing the refrigerant circuit can secure the supply against accidental re-energisation. We install the isolator on a weatherproof IP65 enclosure on the wall closest to the ASHP unit.

Weather compensation modulates the flow temperature against the outdoor temperature — the single biggest determinant of seasonal coefficient of performance (SCOP). Most modern ASHPs ship with a weather sensor that wires from the outdoor unit to the controller — typically 2-core 0.5mm² screened cable, separate from the power circuit, with the sensor mounted on a north-facing external wall. We run the control cabling at the same visit as the power circuit.

Best practice — yes. The immersion remains on its own dedicated circuit (usually 3 kW / 16A) as legionella-defence on weekly schedule and as cold-snap backup if the ASHP defrosts or trips. We size the immersion circuit and the ASHP circuit independently so both can run simultaneously without exceeding board limits.

Yes — strictly. BUS, SEG (for any solar PV that is part of the package) and most BTL lender refinance favouring depend on MCS certification. For ASHP, MCS scheme MIS 3005 applies; we work with MCS-certified ASHP installers and handle the electrical side of the install under the same MCS umbrella project number.