Heating cable installation: spacing, embedding and connection
Installing a Heating Cable: Spacing, Embedding and Connection – a Complete Technical Guide
Of all types of electric underfloor heating, the heating cable is the most versatile, yet at the same time the most technically demanding to install correctly. Whereas a heating mat is essentially a pre-defined system with a fixed wire spacing, with a cable the spacing, laying pattern and overall geometry must be designed by the installer (or an experienced technician) themselves. This is both an advantage and a burden – you have full freedom, but also full responsibility for the result. From my own experience, most mistakes occur precisely with cables: incorrect spacing, cable routed under furniture, insufficient embedding, or underestimating the length of the cold tail during connection. This article will show you how to avoid these mistakes.
If you are still deciding whether to choose a cable or a mat for your project, we recommend first reading the articles Heating Cable vs. Heating Mat: Which Solution Is Right for You? and How to Choose Electric Underfloor Heating: Cable, Mat or Foil? Here we focus exclusively on the installation of the cable itself – from designing the spacing, through correct embedding in anhydrite or cement screed, to electrical connection and first start-up.
What You Need to Know Before You Start the Installation
Output, Cable Length and Room Area
A heating cable is sold as a finished product with a fixed length and total output – it must not be shortened or extended. This means that before purchasing, you need to know how many square metres you want to cover and what specific output (W/m²) is suitable for the given space. For bathrooms as a supplementary heat source, 100–150 W/m² is sufficient; for living rooms with primary heating, 160–200 W/m² is recommended. You can find a more detailed calculation in the article What Output of Electric Underfloor Heating Do I Need for My Space?
As an example, take the HAKL TCX 10/1230W heating cable: it is a twin-conductor self-regulating cable with an output of 1230 W and a length of 123 metres. For heating an area with a specific output of 150 W/m², it covers approximately 8.2 m²; at 200 W/m², about 6.15 m². These values are crucial for calculating the spacing.
Effective Area vs. Total Room Area
A major mistake is calculating spacing based on the total room area. The cable should never be laid under fixed furniture (cabinets, kitchen units, bathtubs, shower trays, toilet bases), under load-bearing walls, or too close to the edges (at least 5–8 cm from the wall). The effective area is usually 15–30% smaller than the total room area. It is this area that you use in the formula for calculating the spacing.
Calculating the Heating Cable Spacing
The spacing between the individual loops of the cable (the so-called pitch or laying step) is the most important parameter of the entire installation. You calculate it using a simple formula:
Spacing (cm) = (Total cable length × 100) ÷ Effective area (m²) × 0.01
Or more simply, in a practical form:
Spacing (cm) = (Cable length in m × 100) ÷ Effective area in m²
Example: the HAKL TCX 10/1230W cable has a length of 123 m, the effective area of the bathroom is 5 m². Spacing = (123 × 100) ÷ 500 = 24.6 cm – round to 24 or 25 cm. To cover an effective area of 8 m²: (123 × 100) ÷ 800 = 15.4 cm – round to 15 cm.
Minimum and Maximum Spacing
The spacing should not be less than 7–8 cm – the cable must not touch itself or cross over. Too small a spacing risks overheating, shortened insulation lifespan, and in extreme cases, failure of the entire cable. The maximum spacing depends on the thickness of the floor layer and the requirements for even heating – for tiled floors with anhydrite or concrete, we generally do not recommend a spacing greater than 25–30 cm, otherwise you will feel temperature differences on the floor surface (the so-called "warm and cold stripes" effect).
In practice, the most common range is 10–20 cm. Bathrooms with tiles: 10–15 cm for intensive heating. Living rooms with anhydrite and tiles: 15–20 cm. Terraces or hallways: 20–25 cm (supplementary heating only).
Preparing the Floor Before Installation
Before installing the cable itself, the floor must be thoroughly prepared. The concrete slab or anhydrite screed must be dry, solid, and free of dust, oil stains and loose particles. If the floor is damp or contains residual moisture, the cable embedded under the screed will still work functionally, but the residual moisture can cause tile adhesion problems at first start-up.
Thermal Insulation Under the Cable
One of the most common and financially costly mistakes when installing electric underfloor heating is omitting the thermal insulation. Without insulation, a large part of the heat is lost downwards – into the floor structure or even into an unheated space below the floor. This has a direct impact on the energy efficiency of the entire system.
For electric underfloor heating, either reflective insulation with an aluminium foil (for anhydrite screeds 3–5 cm thick) or stiffer thermal insulation boards (extruded polystyrene, PIR boards) are used for greater structural thicknesses. The minimum insulation thickness depends on the floor's position: for a floor above an unheated basement space, at least 5–10 cm; for a floor between two heated storeys, 2–3 cm of reflective foil is sufficient.
Fixing Rail (Fixing Strip)
A fixing rail – a perforated steel strip that holds the cable at the required spacing – is fixed to the insulation (or directly to the concrete slab if there is no insulation). The rails are laid perpendicular to the direction of the cable loops, typically spaced 50–80 cm apart. The cable is simply clipped into the holes of the rail. An alternative is fixing the cable with staples directly into the insulation, which is, however, less precise and in practice more suitable for smaller areas.
Some installers also use concrete anchors (fixing clips) that are driven into the base. This solution is more robust but more time-consuming. For larger areas (over 10 m²), we always recommend investing in a quality fixing rail – you will save time and the cable will remain evenly tensioned and parallel throughout all loops.
Laying the Cable Itself: Patterns and Procedure
Serpentine Pattern (Series Loops)
The most common way of laying the cable. The cable runs from the thermostat outlet (cold tail) in parallel lines across the whole area and returns in the same way in the opposite direction. The distance between the loops is exactly your calculated spacing. Advantage: simple, predictable, easy to check. Disadvantage: the corners of the room require bends – the minimum bending radius of the cable is usually 5–7 cm (depends on the cable diameter, always check the manufacturer's technical documentation).
Spiral Pattern
The cable is laid in a narrowing spiral towards the centre of the room and then returns outward. This method is suitable for square rooms and, at the same spacing, allows for more even coverage in the corners. In practice, however, it is more demanding to plan and more prone to errors during installation. For typical bathrooms and kitchens, the serpentine pattern tends to work better.
Combined Pattern for Irregular Floor Plans
In practice, you will encounter rooms with alcoves, pillars, entrance zones, and so on. Here you need to plan the cable so that you achieve the required spacing in every part of the room. We recommend drawing the entire floor plan to scale on paper and planning the cable layout before starting the installation. Customers who neglect this and start laying the cable without a plan almost always end up either running out of cable (having to lay it too densely) or with a surplus length that cannot be shortened.
Dealing with Cable Bends
At the end of each loop, the cable must make a bend and turn back. This bend must not be sharp – the minimum bending radius is usually 4–6 times the cable diameter. For common heating cables (diameter 6–8 mm), this means a minimum bend radius of 3–4 cm. A sharp bend in the cable damages the internal insulation and causes a future failure – not immediately, but after several months of operation, once the cable is thermally stressed. It is also a good idea to fix the bends with a mounting rail or clip so that they do not shift during embedding.
Embedding the Cable: Anhydrite vs. Cement Screed
After the cable has been laid and fixed, embedding follows. This is a critical phase – if the cable is embedded incorrectly, heat is not transferred evenly, and air pockets can form, leading to overheating of the cable at specific spots. In practice, we encounter two basic types of embedding material: anhydrite screed and cement screed (or classic concrete with additives).
Anhydrite Screed
Anhydrite (calcium sulphate) is today the preferred choice for electric underfloor heating systems for several reasons. Anhydrite screed has excellent flowability – it literally "flows around" the cable from all sides without creating air gaps. It has a higher thermal conductivity coefficient than cement screed (approx. 2.0 W/mK vs. 1.2–1.4 W/mK), meaning faster heat transfer from the cable to the floor surface. And once cured, it has smaller volume changes, which reduces the risk of cracks around the cable.
The minimum thickness of anhydrite screed above the cable is 3 cm, with the total screed thickness usually 4.5–6 cm. Before applying tiles or another finishing layer, the anhydrite must dry – at least 28 days under normal conditions, with the first heating start-up serving precisely as the drying cycle (see below).
Cement Screed
Classic cement screed is cheaper and more readily available, but requires greater care during application. Because the cement mixture is denser, air pockets can form under or around the cable if applied incorrectly. This is addressed either with special plasticisers that increase flowability, or by manually "poking" the fresh screed with a vibrating rod. Minimum thickness above the cable: 3–4 cm (cement screed has lower thermal conductivity, so a greater thickness is recommended compared to anhydrite for the same results).
Embedding Procedure, Step by Step
Before embedding, check the following: the cable is evenly laid at the prescribed spacing, does not touch itself anywhere, the bends are properly fixed, all clips/mounting rails are secure, and the cable does not jump out of position when walking on the working plank. The thermostat's temperature sensor (probe) must be housed in a protective sleeve (plastic tube) between two cable loops so that it can be replaced in the event of a fault without demolishing the floor – more on this in the section below.
Apply the screed starting from the side away from the thermostat – fill in the areas furthest from the cable outlet first and work towards it. Never step directly onto fresh screed – for larger areas, use a working plank (a board laid across several loops). After pouring, let the screed set with the window covered (no draught) – rapid drying causes cracks.
Positioning the Thermostat's Temperature Sensor
The thermostat controls the heating based on the signal from the floor temperature sensor. Correct sensor placement is crucial – if it lies too close to the cable, the measured temperature will be higher than the actual average floor temperature, and the thermostat will switch the system off prematurely. If the sensor is too far away, the floor may overheat.
Correct position: place the sensor at the middle distance between two neighbouring cable loops (i.e. halfway across the spacing), at least 50–100 cm from the wall where the cold tail of the cable enters. The sensor must be housed in a protective sleeve (a protective tube made of PVC or corrugated plastic conduit) leading from the thermostat into the floor – seal the end of the sleeve to prevent screed from entering it, and insert the sensor all the way to its position without stretching the sensor cable.
The sleeve must be accessible – with correct installation, you can pull out the old sensor from the sleeve years later and insert a new one without any demolition. In practice, it very often happens that the installer does not create the sleeve at all, or embeds it in the screed – then, when the sensor fails (which happens even with quality products after 10–15 years), a problem arises. This is a detail you should ask about even when purchasing a complete system.
Electrical Connection of the Heating Cable
Cold Tail – What It Is and Why It Matters
The heating cable consists of two parts: the active heating section (in the screed) and the so-called cold tail – a non-heating supply cable that runs from the ground into the electrical box with the thermostat. The cold tail must be long enough to comfortably reach the thermostat mounted on the wall. The standard thermostat height is 80–150 cm above the floor, and the cold tail must be routed either in a groove in the wall or in a conduit.
The joint between the heating section of the cable and the cold tail (the so-called connector/splice) must never be embedded directly in the screed without protection. In practice, this joint is always sealed in the supplied jointing kit or in a special embedding compound – check that the splice is located where there will be at least 3 cm of screed above it and that it will not be subject to mechanical load (e.g. directly under a door).
Electrical Installation Requirements
The electrical connection of the heating cable to the thermostat and mains supply must be carried out by a person with electrical qualifications (§22 or §21 under Decree No. 508/2009 Coll., or EN 50110). This is not a formality – underfloor heating is a fixed appliance embedded in a permanent structure, where any fault caused by unqualified wiring can have serious consequences (fire, electric shock).
The circuit for electric underfloor heating must be:
- a separate circuit from the distribution board (not shared with a socket circuit)
- protected by a circuit breaker with a current rating matching the cable's output (at 1230 W and 230 V, this is a current of approx. 5.35 A – a B6 or B10 breaker is sufficient)
- protected by a residual current device (RCD) with a tripping current of 30 mA
- earthed (PE conductor)
Connecting to the Thermostat
The thermostat is the heart of the whole system. For a heating cable with an output of around 1000–1500 W, a single-phase thermostat with a maximum switching output of 3500–3600 W is sufficient. As an example of quality control, we can mention the HAKL TH 900 digital thermostat – it offers weekly programming, a display showing the current floor and air temperature, and direct on-site control. For those who want remote control via a smartphone, the wifi version HAKL TH 951wifi or HAKL TH 952wifi is suitable, with advanced scheduling and consumption monitoring functions.
Connecting the thermostat: the thermostat terminals are usually marked – L (mains phase), N (neutral), PE (earth), and output terminals for the cable (L_out, N_out) and the sensor (sensor). The cold tail of the heating cable is connected to the output terminals, not directly to the supply terminals. The sensor is connected to the terminals marked "sensor" or similar – polarity does not matter for most sensors, but always check the instructions for the specific thermostat.
Choosing a thermostat is a topic in itself – we recommend the article How to Choose a Thermostat for Electric Underfloor Heating: Manual, Digital or Wifi?
Drying Cycle After Embedding and First Start-Up
After the cable has been embedded in the screed, a mandatory technological pause follows. Anhydrite screed needs at least 7 days of curing at room temperature before the first heating start-up. This is followed by a drying cycle: on the first day, set the thermostat to 25 °C, increase by 5 °C each day until you reach the system's maximum operating temperature (usually 45–50 °C for anhydrite). Heat at this temperature for 3–5 days. Then gradually reduce the temperature at the same rate.
Cement screed needs a longer curing time – at least 21–28 days at room temperature. The drying cycle is the same, but starts later. Starting the heating prematurely while the screed is not yet dry causes cracking, tile delamination and uneven drying – a mistake that repeats itself several times a season and gradually destroys the whole system.
Ventilate the room during the drying cycle – the moisture released from the screed needs somewhere to go. In a closed room, it condenses on the walls and windows, which can damage plaster or newly installed cladding materials.
Measuring Cable Resistance Before and After Embedding
We recommend every installer do the following: measure the resistance of the heating cable with an ohmmeter before embedding, after embedding, and ideally also after installing the finishing layer. The manufacturer states the nominal cable resistance in the technical documentation (at 1230 W and 230 V, the resistance is approx. 43 ohms). A deviation of ±10% is acceptable. If the measured resistance is significantly outside this range, the cable is damaged – either during installation (a sharp bend, mechanical damage) or during embedding. Damage after embedding means demolishing the screed and replacing the cable – a scenario you want to avoid at all costs.
Also measure the insulation resistance between the conductors and the earth (a terminal test with a megohmmeter at 500 V DC) – the value must be at least 1 MΩ; quality cables have tens to hundreds of MΩ. If the insulation resistance is low, the cable has damaged insulation and must be replaced.
Have these measurements recorded in the installation documentation – if the system fails later, these records are crucial for assessing the warranty and the cause of the fault. The topic of faults is covered in more detail in the article Common Faults in Electric Underfloor Heating and How to Solve Them.
Typical Mistakes from Practice and How to Avoid Them
Over years of working with electric underfloor heating installations, I have seen recurring mistakes. Here are the most common and most costly ones:
- Cable routed under furniture or the base of kitchen units: Heat accumulates, the cable overheats and the insulation is damaged. Result: replacing the cable = demolishing the floor.
- Too small a spacing combined with a thick finishing layer: For example, a cable with an 8 cm step under 2 cm thick tiles and 1 cm of adhesive over anhydrite – overheating, uneven surface, alternating warm and cold spots.
- Omitting the protective sleeve for the temperature sensor: After 10 years, the sensor is damaged and the floor has to be demolished to replace it.
- Not measuring cable resistance after installation: A fault that would have been discovered immediately is only revealed after embedding.
- Starting the heating prematurely before the screed has dried: Cracks, tile delamination, uneven drying.
- The cable crossing over itself: A hot spot, insulation failure.
- The cold tail splice embedded directly in the screed without protection: Creates a weak point – moisture, mechanical stress.
Special Situations: Cable on a Terrace, Outdoors and Under Electric Underfloor Heating
If you are considering installing a heating cable on an outdoor terrace (frost protection, pavement de-icing), different rules apply. You must use a cable certified for outdoor use and higher humidity (IP67/IP68 class for connectors), the spacing is usually 10–15 cm for continuous de-icing, and the thermostat must have an outdoor thermometer or humidity sensor. Special products exist for outdoor applications – always check the technical data sheet when choosing, and make sure the cable is certified for outdoor environments.
Different requirements apply for installations under vinyl flooring or laminate – these materials have lower thermal conductivity, and manufacturers usually limit the maximum floor temperature to 27 °C. This topic is covered in more detail in the article Electric Underfloor Heating Under Different Types of Flooring: Tiles, Vinyl, Laminate.
Frequently Asked Questions (FAQ)
Can I shorten the heating cable myself if I have surplus length left over?
No, never shorten or extend a heating cable under any circumstances. Every cable has a fixed electrical resistance designed for a specific output and a voltage of 230 V. Shortening the cable reduces its resistance, increases the output over the remaining length, and the cable will overheat. The result is a fault or a fire. If you have surplus length left over, reduce the spacing between the loops (within the allowed limits), or choose a different cable with a shorter active length.
How far apart should the cable loops be for a 1,230 W cable in a 6 m² bathroom?
The HAKL TCX 10/1230W cable has an active length of 123 m. The effective area of a 6 m² bathroom (excluding corners and the area under the bathtub) may actually be only 4–4.5 m². Spacing: (123 × 100) ÷ 450 = 27.3 cm – that is a bit much for intensive bathroom heating. In such a case, you should consider a cable with a lower output or a larger effective area so that the spacing is no greater than 20–25 cm.
Does the cable always have to be embedded in screed, or can I lay it directly under the tiles?
The heating cable must always be completely surrounded by a thermally conductive material – anhydrite, cement screed, or a special adhesive with increased thermal conductivity. It must never have air pockets around it, because air is a thermal insulator. Laying the cable directly under the tiles without screed (only in adhesive) is possible with a thin variant, but this must be explicitly permitted by the cable's technical data sheet and confirmed by the manufacturer. Otherwise, you risk overheating and voiding the warranty.
Can I lay a heating cable on existing tiles and cover it with another layer?
Yes, under certain conditions. You must verify that the floor structure can bear the additional load (another 3–5 cm of screed + tiles = 80–120 kg/m²). The existing tiles must be solid, with no hollow spots. The surface must be roughened and treated with a primer for better adhesion of the anhydrite. Such a solution will raise the floor level, which can be a problem at doors, thresholds and the transition to a hallway. If possible, a more elegant solution is to embed a heating mat with adhesive directly under the new tiles – see the article Installing a Heating Mat Under Tiles: Step-by-Step Procedure.
What is the lifespan of a heating cable and what affects it?
A quality heating cable, correctly installed, has a lifespan of 25–40 years. The key factors are: correct spacing (no hot spots), sufficient screed thickness above the cable, absence of mechanical damage during installation, and the fact that the cable never reaches a higher output than its nominal rating (i.e. it is neither shortened nor overloaded). The thermostat and sensor tend to have a shorter lifespan (10–20 years) and are easy to replace without disturbing the floor – which is exactly why correctly installing the protective sleeve is so important.
Is it necessary to perform a heat load calculation when installing a heating cable?
For supplementary heating (bathroom, hallway), a simple estimate based on the recommended specific output (W/m²) is sufficient. For primary heating, a heat load calculation is essential – you need to determine the room's heat losses and size the system's output accordingly. Without a calculation, you risk the system being insufficient in winter, or conversely, oversized and energy-inefficient. This topic is covered in detail in the articles Electric Underfloor Heating as a Primary vs. Supplementary Heat Source and What Output of Electric Underfloor Heating Do I Need for My Space?
Conclusion: Installing a Cable Is a Craft Worth Doing Properly
At first glance, installing a heating cable looks simple – unroll it, fix it, embed it. In practice, however, it is a more complex process where every detail has a direct impact on the efficiency, lifespan and safety of the entire system. Correctly calculated spacing ensures even heat across the whole area without cold and hot spots. Precise embedding without air pockets extends the cable's lifespan by decades. Professional connection of the thermostat and protecting the sensor in a sleeve will save you from unnecessary repair costs fifteen years down the line.
If you are deciding between specific products, take a look at the HAKL TCX 10/1230W heating cable, available in two variants depending on your area. Pair it with a suitable thermostat – from a simple digital one to wifi versions with a smartphone app. You can find the entire range of electric underfloor heating, including accessories, in the electric underfloor heating category. For further questions about selection, calculation or compatibility, our experienced technicians are available – details can be found in the Frequently Asked Questions About Electric Underfloor Heating section.
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