RCDs and circuit protection for electric underfloor heating

Electric underfloor heating buries a live heating element in the floor, under tiles, adhesive and foot traffic. The wiring regulations treat it accordingly. BS 7671 has a dedicated section, Section 753, for floor and ceiling heating systems, and it sets out the RCD protection, the earthing arrangements, the overheating precautions and even the paperwork that must be left at the consumer unit.

This guide explains what those rules mean in plain terms: what an RCD does for a heated floor, whether the heating needs its own circuit, how big a floor one thermostat can switch, and what your electrician will test and record. It is written for homeowners planning a job and for installers who lay the mat but hand the wiring to an electrician. For the standards themselves, see our guide to BS EN 1264 and BS 7671 for underfloor heating.

Does electric underfloor heating need a 30mA RCD?

Yes, in every case. BS 7671 Section 753 requires electric floor heating to have RCD protection with a rated residual operating current not exceeding 30mA. The IET's guidance on the section spells it out for both common constructions: heating units with an earthed metallic screen or grid must be protected by an RCD rated at 30mA or less (Regulation 753.411.3.2), and circuits supplying Class II heating equipment need additional RCD protection at 30mA or less as well (Regulation 753.415.1).

The reason is the environment. The element sits in a floor that gets wet mopped, drilled by accident and walked on barefoot, and a damaged cable may not show any fault until someone is in contact with it. A 30mA RCD disconnects the supply fast enough to protect against electric shock when insulation fails.

An RCD alone is not enough, though. It does not protect against overload or short circuit, so the circuit also needs a fuse or circuit breaker. In practice most electricians fit an RCBO, a single device that does both jobs and means a fault on the floor heating does not trip other circuits in the house.

Does underfloor heating need its own circuit?

Usually it gets one, and for anything beyond a small mat it should. A dedicated radial circuit from the consumer unit, protected by its own 30mA RCBO, is the cleanest arrangement: the heating has its own protection, its own isolation and nothing else on the circuit to cause nuisance trips.

A small mat is sometimes supplied from an existing circuit through a switched fused connection unit instead. That is a judgement for your electrician, who will check the existing circuit's loading and protection. Two things to keep in mind either way:

• A new circuit is notifiable. Under Part P of the Building Regulations in England, installing a new circuit must be notified to building control unless the electrician is registered with a competent person scheme and self-certifies.
• Bathrooms are special locations. Additions or alterations to existing circuits in the zones around a bath or shower are also notifiable. Our guide to electric underfloor heating and bathroom zones covers where the mat and thermostat can go.

Nuisance tripping is also why big systems are not lumped onto one RCD. The IET guidance notes that the heating power served by one RCD should be limited to avoid unwanted tripping from normal protective-conductor currents, so an electrician will split a large installation sensibly across devices.

How much mat can one thermostat switch?

Electric underfloor heating thermostats such as the Heatmiser neoStat-e switch up to 16A. At 230V that is about 3.6kW of heating.

Mat output	Maximum area on one 16A thermostat
100W/m²	about 36m²
150W/m²	about 24m²
200W/m²	about 18m²

Those are electrical limits, not recommendations; most rooms are far smaller. For a large open area, split the floor into zones, each with its own thermostat, or have the electrician switch the load through a contactor or a 16A receiver designed for electric heating, with the thermostat doing the control. Our guide to the 16A limit on electric UFH thermostats works through the options and the sums.

What else does BS 7671 Section 753 require?

Several things your electrician and your tiler both need to know about:

• Earthed screen or grid. Heating units without exposed conductive parts must have an earthed metallic grid with a spacing of 30mm or less above them, or the heating unit must itself have an earthed metallic sheath or screen connected to the circuit's protective conductor. MaxHeatUFH electric mats have a screened cable for this reason.
• Heating-free areas. Areas where screws or nails may be driven, for example door thresholds where a door stop will be fixed, must be left unheated, and the installer must tell other trades that no penetrating fixings may be made into heated areas.
• Limited floor temperature. Where people are in contact with the floor, the surface temperature must be limited; the IET guidance gives 35°C as the example figure. The floor probe and thermostat limit setting do this job.
• Overheating measures. The system must be designed, installed or protected so the heating cannot exceed 80°C anywhere, heating units must not cross expansion joints in the floor, and cold tails must be factory joined to the heating cable, never made up on site.
• Ingress protection. Heating units installed in concrete floors must be rated at least IPX7.

None of this is onerous with a quality mat and a competent installation, but it is why the cable must never be cut to length, shortened or repaired casually. If a mat does not fit the room, choose a different size; see testing electric underfloor heating during installation for the checks to make before the tiles go down.

What paperwork must be left at the consumer unit?

Section 753 requires a plan of each heating system to be provided and fixed at or near the distribution board. The IET guidance lists what it must include: the manufacturer and type of heating unit, the heated area, the rated power, the layout of the heating units shown on a sketch, the rated voltage, the RCD rating, the insulation resistance of the installation and its test voltage, and the leakage current.

On top of the plan, the building owner must be given a written description of the system covering its construction and installation depth, the circuit layout, the control equipment and the maximum operating temperature, plus the instructions for use, with a copy fixed permanently at the distribution board. The owner must also be told that no fixings may be made into the heated areas of the floor.

Keep all of it. The layout sketch in particular is what saves the floor when someone later wants to fit a door stop, a toilet frame or a shower screen, and it is what a fault-finder works from if the mat is ever damaged. Our guide to electric underfloor heating fault finding shows why those recorded test readings matter.

Who can do the electrical work?

A qualified electrician must make the final connection, provide the 30mA RCD protection and test and certify the installation to BS 7671. In England, Part P of the Building Regulations applies to all fixed electrical work in dwellings, and there are three compliant routes: an electrician registered with a competent person scheme who self-certifies, a registered third-party certifier who inspects and tests the work, or notifying building control before the work starts.

You can still do the non-electrical parts yourself: preparing the floor, fitting insulation boards and laying the mat. Lay the cold tail and floor probe conduit to where the electrician wants them, and leave everything live to them. MaxHeatUFH does not install, but we can pass your job to a vetted approved installer through our installation enquiry page.

Frequently asked questions

What size RCD does electric underfloor heating need?

One rated at 30mA or less. BS 7671 Section 753 requires it for electric floor heating, whether the heating unit has an earthed screen or is Class II equipment.

Is an RCBO acceptable instead of an RCD?

Yes. An RCBO is an RCD and circuit breaker in one device, so it provides the required 30mA residual current protection plus the overcurrent protection an RCD alone does not give. It is the usual choice for a dedicated floor heating circuit.

Can electric underfloor heating be plugged into a socket?

No. It is fixed wiring: the mat's cold tail must be permanently connected through the thermostat on a properly protected circuit, installed and certified by a qualified electrician.

Why does my underfloor heating keep tripping the RCD?

Either a genuine earth fault, often damage to the cable during installation or later drilling, or too much heating on one RCD, since heating cables have a normal leakage current that adds up. An electrician can measure the insulation resistance and leakage to tell which.

Does a small bathroom mat need its own circuit?

Not always; an electrician may supply a small mat from an existing circuit through a switched fused connection unit. But work on circuits in the zones around a bath or shower is notifiable under Part P, so it still is not a DIY job.

Does the thermostat count as the RCD?

No. The thermostat only switches and controls the heating. The 30mA RCD or RCBO lives in the consumer unit, upstream of the thermostat.

What tests should exist for my floor heating?

Continuity and insulation resistance readings taken before, during and after installation, recorded on the Section 753 plan at the consumer unit, plus the electrician's BS 7671 certification. Browse electric UFH thermostats for controls that include the floor sensor these systems need.

Sources: IET Wiring Matters: floor and ceiling heating systems (BS 7671 Section 753), Approved Document P: electrical safety (2013 edition)