Heat pump: circuit, protective devices and inspection

A heat pump is a fixed machine added to the dwelling’s electrical installation, sometimes with several supplies. Book 1 of the RGIE has no chapter of its own for it: it falls under the general rules on dedicated circuits, wiring, protective devices and documentation. Circuit, cable sizes, RCD, diagram and plan: here are those rules, and what the inspection report certifies.

A dedicated circuit

Subsection 5.2.1.2 of Book 1 provides that, in domestic installations, each fixed (mobile) appliance or machine with a rated power of 2600 W or more is supplied separately by a dedicated circuit. It adds that the appliances of a fixed electric heating system are supplied by one or more dedicated circuits. Section 2.6.1 defines a dedicated circuit as an elementary circuit intended exclusively to supply one or more current-using appliances intended for a specific use.

Depending on the model, a heat pump may have several supplies: the outdoor unit, the indoor unit or hydraulic module, and sometimes a backup heater. The manufacturer’s data sheet gives the power ratings to compare with this threshold. Book 1 does not specify whether a heat pump counts among the « appareils d’un chauffage électrique à poste fixe » (appliances of a fixed electric heating system), for which it provides dedicated circuits with no power condition: if it does, the dedicated circuit is required whatever the power. This qualification, like how the rules apply to a specific machine, is for the accredited inspection body to determine.

Think about control too. Under subsection 5.3.3.2, every current-using appliance or electrical machine is operated by a control device, even when its operation depends on a thermostat or a similar device. Among other exceptions, the text accepts that no such device is provided on the circuit if the machine itself has a main switch.

Sizing: the manual first

Book 1 refers to the rated power of the appliance (subsection 5.2.1.2): that is the electrical figure to read on the data sheet. The same subsection states that the cross-section of the wiring supplying these appliances is chosen according to their power. More generally, cross-sections are chosen, among other things, so that, unless Book 1 specifies otherwise, the current-carrying capacity is at least equal to the design current of the circuit, and so that the voltage drop under normal operating conditions remains compatible with safe operation of the appliances supplied. For voltage drop values, section 5.2.5 refers to good practice.

The manufacturer’s data sheet is the starting point for these calculations: absorbed power and current, recommended protection. The outdoor unit is sometimes far from the board, which lengthens the circuit. If the machine has a backup heater, its power adds to the dwelling’s load balance. The selection principles are covered in our article on cable sizes and ratings.

Starting current: plan for it

A heat pump’s compressor is driven by a motor. Book 1 (section 3.3.2) lists starting currents among the effects of equipment operation that must not harm other machines, appliances and wiring, or the supply of the installation: appropriate measures are taken in line with good practice.

Subsection 5.3.3.2 (point e) adds that motors are fitted, in line with good practice, with appropriate control devices if starting them without such devices abnormally disturbs the operation of other loads. For choosing the protection suited to a given machine, the manufacturer’s manual provides the starting data.

Residual-current protection

Subsection 5.3.5.3 provides that RCDs installed in domestic installations are at least type A. It also provides (point f) that, when equipment liable to carry an asymmetric current generating DC components is installed downstream of an RCD, precautions are taken so that, in the event of an earth fault, these components do not disturb the operation of the protective devices to the point of compromising people’s safety. The text mentions certain equipment with semiconductor devices. To avoid such disturbances, one of the following measures is taken:

• the equipment chosen does not produce direct current liable to disturb the operation of a protective device, as with burst-fire or symmetrical phase-angle controlled devices;
• equipment producing or using direct current is built to the rules applicable to class II;
• equipment producing direct current is supplied through a separating transformer;
• the RCD is built so that its operation remains guaranteed when an insulation fault with a disturbing DC component occurs;
• the RCD is installed together and in coordination with a residual direct current detecting device, which takes the equipment out of service when such a fault occurs.

For a heat pump with power electronics, consult the manual, which may recommend an RCD type; Book 1 sets the minimum and the obligation to take one of these measures. To put the role of RCDs in the installation back in context, see our article on 300 mA and 30 mA RCDs.

On the diagram and the plan, outdoor unit included

Under subsection 3.1.2.2, the single-line diagram of a domestic installation contains at least the characteristics of the wiring (type, cross-section, number of conductors), its installation method, the type and characteristics of the RCDs and overcurrent protective devices, and the fixed machines and appliances. The position plan shows the position of the fixed machines and appliances mentioned on the diagram (subsection 3.1.2.3).

The outdoor unit, like the indoor unit, is a fixed appliance: it appears on the plan at its actual location (subsection 3.1.2.3); using the circuit letter from the diagram makes it easier to read. If the machine has several supplies, each one appears on both documents. With Amperio, each element, drawn or dictated on site, lands in the right room on the plan, always consistent with the diagram.

What the inspection report certifies

Book 1 sets out no checks specific to heat pumps. For a domestic installation, subsection 6.4.6.4 specifies the content of the conformity inspection report. Among other things, it certifies the match:

• between the RCDs installed and the value of the earth electrode dispersion resistance;
• between the overcurrent protective devices and the cross-sections of the circuits they protect;
• of the execution of the installation with the single-line diagrams and position plans.

The report also covers the check of fixed or fixed-installed equipment (correct choice, assembly and installation on site) and of the continuity of the equipotential bonding and of the protective conductors of sockets and fixed class I appliances. The course of the visit is covered in our inspection checklist.

This article is informative. It replaces neither the RGIE (Book 1) nor the advice of your inspection body.