A loose electric underfloor heating cable gives you the flexibility to heat awkward spaces that a fixed mat cannot cover neatly – around sanitaryware, within a bay window, or across an irregularly shaped room. That flexibility only works when the cable is spaced correctly. This electric cable spacing guide explains how to achieve an even floor temperature without damaging the cable, creating cold patches or exceeding the system’s designed output.

Cable spacing is not a visual preference. It is part of the heating design. The distance between each run determines how much cable is installed per square metre, which in turn affects the heat output, warm-up performance and suitability of the system for the room.

What electric cable spacing means

Electric underfloor heating loose cable is supplied at a fixed length and wattage. Unlike a pipe system, it must never be cut, shortened or overlapped to make it fit. Spacing is the adjustment available to the installer: spreading the cable over a larger area lowers the wattage per square metre, while bringing runs closer together increases it.

The spacing is measured from the centre of one cable run to the centre of the next. It should be consistent across the free floor area unless the manufacturer’s design specifically calls for a different arrangement. A layout with uneven gaps can feel uneven underfoot, particularly beneath tile or stone finishes that conduct heat well.

The cable’s product instructions always take priority. Every kit has a designed coverage area, a minimum permitted spacing and a stated wattage. Use those figures rather than selecting a gap by eye.

Calculate cable spacing before installation

Start with the true heated area, not the overall room size. Measure the floor, then deduct fixed furniture and permanent fittings that sit directly on the floor. This commonly includes baths, shower trays, kitchen units, fitted wardrobes, toilets and vanity units without sufficient airflow underneath.

Do not run heating cable beneath furniture that prevents heat escaping. Trapped heat can cause localised overheating and may invalidate the product warranty. Movable, raised furniture can be assessed separately, but a clear heated area should always be established before the cable is laid.

The basic calculation is:

Cable spacing in centimetres = heated floor area in square metres × 100 ÷ cable length in metres

For example, if the available heated area is 4m² and the cable is 40m long, the calculation is 4 × 100 ÷ 40. The required spacing is 10cm.

That result must then be checked against the kit specification. If it falls below the minimum approved spacing, the cable is too long or too powerful for the available area. Do not solve the issue by reducing the spacing beyond the manufacturer’s limit, doubling the cable back on itself or placing sections closer together. Select a more suitable cable instead.

Why the heated area matters more than the room area

A bathroom may measure 6m², but a bath, shower enclosure, toilet and fitted vanity unit can reduce the available floor to 3m². Choosing a cable based on the full room size leaves too much cable for the space. The result is often an unsafe layout and an expensive installation delay.

For refurbishment projects, make a scaled sketch before ordering. Include the position of permanent items, the thermostat, the floor sensor route and the cold-tail connection point. This simple planning stage is particularly valuable where a cable will need to work around several obstacles.

Recommended layout for an even floor temperature

Begin by marking the heated boundary and keeping the recommended clearance from walls, fixed fittings and other heat sources. The exact perimeter distance varies by system, but the cable should not be forced tightly into corners or run against unheated zones. Follow the supplied fitting instructions for the required edge clearances.

Loose cable is generally laid in a back-and-forth pattern, secured to the floor using the fixing strips, clips or mesh supplied with the system. Keep each run parallel and use the calculated spacing throughout. It is sensible to start at the connection end and dry-lay the route first, ensuring the full cable length reaches the intended area without tension.

The heating element itself must never cross, touch or overlap. Crossovers concentrate heat in one small point and can permanently damage the cable. If the layout does not work, stop and review the chosen cable length, rather than trying to make it fit.

Pay close attention around WCs, pedestals and narrow returns. A loose cable is often the right choice for these spaces, but the bends still need to be gentle. Do not kink the cable or bend it more tightly than the manufacturer allows. Maintain the same centre-to-centre distance around the turn as far as practical.

Keep the sensor in the right place

The floor temperature sensor should sit centrally between two adjacent cable runs, never touching the heating cable. It should be installed inside a suitable conduit so it can be replaced if required. Avoid placing it near an external wall, doorway, direct sunlight or another heat source, as these locations can give an unrepresentative reading.

A correctly positioned sensor allows the thermostat to regulate the floor accurately. It protects sensitive floor finishes and helps avoid unnecessary energy use once the room has reached the set temperature.

Spacing, heat output and floor finish

Closer cable spacing does not automatically mean a better system. The appropriate output depends on the room’s heat loss, insulation level, floor finish and whether electric underfloor heating is being used as primary heating or simply for comfort.

Bathrooms and well-insulated small rooms often suit electric systems very well, especially where tiled floors benefit from a responsive heat source. Larger or poorly insulated areas may need a proper heat-loss assessment before an electric system is selected as the main source of heat. In some projects, water underfloor heating is the more economical long-term choice.

Floor finish affects how the heat is felt. Tile and stone transfer heat efficiently, while timber, laminate, vinyl and carpet introduce more thermal resistance. Where a softer finish is planned, check the combined tog value of the floor covering and underlay, as well as the maximum floor temperature permitted by the flooring manufacturer.

Insulation beneath the cable is equally significant. Without suitable insulation boards or an approved insulating layer, a proportion of the heat can be lost into the subfloor. The floor may still warm up, but it will take longer and cost more to run. Good insulation improves response time and helps the designed cable output reach the room rather than the structure below it.

Common cable spacing mistakes to avoid

The most frequent problem is ordering before calculating the free floor area. A cable cannot be trimmed at the end of the installation, so an incorrect size can turn a straightforward job into a replacement order.

Another mistake is changing the spacing halfway through because the layout appears to be running short or long. A small correction may be possible only within the manufacturer’s approved range, but it should be planned, not improvised. If a cable has been laid with a noticeably different gap in one section, the temperature across the floor can vary.

Avoid placing cable under fixed units, running it across expansion joints, covering it with materials not approved for underfloor heating, or installing it where it could be punctured by later fixings. Photograph the completed cable layout, including the sensor position and resistance readings, before covering it with adhesive or levelling compound. This record can be extremely useful during future work on the room.

Electrical testing is also essential. The cable’s resistance and insulation resistance should be checked and recorded before installation, after laying, and once it has been covered. Any reading outside the stated tolerance should be investigated before the floor finish is fitted. Final electrical connection and testing must be carried out by a competent electrician in line with current UK wiring regulations.

When to ask for a cable layout

A simple rectangular room can often be planned from accurate measurements and the system instructions. More complex rooms deserve specialist input, particularly where there are multiple fixed fittings, a timber subfloor, unusual floor build-up heights, or a requirement for the electric system to provide the main heat.

The Underfloor Heating Company can help identify a suitable loose cable kit, output and accessory package for the room. Having a measured plan, floor construction details and intended floor finish ready will make that advice more precise.

Take the time to set out the cable before mixing adhesive or levelling compound. A carefully measured, consistently spaced layout is what turns a flexible electric cable into quiet, even and dependable warmth underfoot.