Heat pump electrical provision: what to specify early

A domestic heat pump needs four things from the electrical design: confirmed incoming supply capacity, a dedicated circuit with outdoor isolation, controls wiring agreed with the mechanical design, and its own metering. All four are Stage 3 decisions, not delivery-day ones.

Part L has made heat pumps the default heating conversation on new work and a frequent one on retrofit. The mechanical side of that conversation is usually well developed by tender. The electrical side often is not — it arrives as a line item, "power to ASHP", and the gaps surface on site. The list below is what we ask architects to pin down while the design can still absorb the answers.

Start with the incoming supply, not the unit

Before anything is specified, someone has to confirm the property's maximum demand still works. A typical air-source heat pump draws in the low single-digit kilowatts, but it draws them for long periods, and it rarely arrives alone — the same projects tend to add EV charging, induction cooking and electric showers. On older housing stock the incoming fuse may be 60 or 80 amps, not the 100 assumed, and that changes the arithmetic.

Two practical consequences for the programme:

• The DNO needs to know. Heat pumps are notifiable to the network operator, and where the assessed demand exceeds what the supply can carry, a fuse or service upgrade has lead times measured in weeks to months. That request should go in during design, not after the unit is craned onto its pad.
• Larger houses may justify three-phase. Where a project combines a heat pump, EV charging and significant plant, a three-phase supply can be the cleaner answer than managing a single phase at its limit — but only if it is decided early enough to route and terminate properly.

A dedicated circuit, planned like one

The heat pump gets its own circuit from the board — correctly sized for the manufacturer's actual data, not a spare way that happens to be free. The details worth agreeing on a drawing rather than on site: the cable route from board to pad, any penetrations through the fabric, an IP-rated local isolator at the outdoor unit, and how the external run is contained on the elevation. On sensitive buildings that last item is a design decision, not an electrical one — surface containment on a visible elevation is exactly the kind of thing that should never be improvised.

The same drawing should settle power to everything around the unit: the cylinder and its immersion backup, circulation pumps, and any buffer or plant enclosure that needs light and a socket for maintenance.

Controls: decide who owns the heating schedule

A heat pump runs best at low flow temperatures under weather compensation — which means the worst thing that can happen to it is a third-party thermostat switching it hard on and off like a boiler. Before tender, agree which system owns the schedule: the manufacturer's own controls, or the house's control platform with the heat pump integrated properly through its supported interfaces. Both can work. What does not work is both at once, discovered at commissioning.

The wiring consequences are mundane but easy to miss: sensor and control runs between the outdoor unit, cylinder and indoor controls, and a data route to the network if the system reports remotely. They cost little on a drawing and a great deal in a finished wall.

Meter it, or the performance claims are guesses

A dedicated sub-meter on the heat-pump circuit is a small line in the spec and the only honest basis for the efficiency conversation later. It lets the client see what the system actually consumes, supports time-of-use tariffs, and turns "it seems expensive to run" into a diagnosable question rather than a dispute. On projects with monitoring ambitions it is also the datapoint every dashboard wants first.

When to decide

Treat the electrical provision as part of the Stage 3 coordination package: supply assessment and DNO application, circuit and isolation on the layout, controls ownership agreed, metering in the schedule. By Stage 4 it should be tender language, alongside the rest of the installation spec. We see the pattern most on retrofit work — in Bath the constraint is usually the building and its supply; on larger residential projects around Guildford it is total demand once the EV chargers are counted.

The short version

Confirm the supply and notify the DNO at design stage, give the unit a dedicated, drawn circuit with outdoor isolation, settle who owns the controls before tender, and sub-meter it. None of this is difficult — all of it is disruptive when it is discovered late.

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