Can you run electrical cable under a floating floor?

Yes. Twin and earth cable can run in the void beneath a laminate or engineered floating floor, provided it is protected from mechanical damage where it enters or exits through a hard edge, no joints are made in the concealed section, and the cable does not touch any heat source such as underfloor heating pipes.

What size cable is needed to run a socket under a floating floor?

For a standard 13 A socket on a ring final circuit, 2.5 mm² twin and earth is correct. For a radial socket circuit protected by a 20 A MCB, 2.5 mm² is also adequate. The cable size must be chosen for the circuit design current, not just whatever fits under the floor.

How to Run Cable Under a Floating Floor (UK)

Helpful video reference. We use the "Electrical Refurbishment 4: Floating Floor Cable Rodding" video as the reference here. Filmed as part of a UK domestic refurbishment series, it shows the cable rod technique clearly, including how to deal with the foam underlay that often blocks the run.

Before you start. Isolate any existing circuit in the room at the consumer unit and confirm dead before lifting boards or opening back boxes. A floating floor may have been laid over cables that are already live. Never assume the void is clear of live conductors until you have tested. If the floor sits on a concrete slab, check for underfloor heating before drilling or cutting anything.

1. Plan the cable route

Identify where the cable needs to start (usually a back box, junction box or consumer unit on the wall) and where it needs to arrive. The void under a floating floor is typically 8--15 mm deep: enough for 2.5 mm² twin and earth, but tight. If underlay is foam, you may need to cut a narrow slot in the underlay at the entry and exit to allow the cable rod to pass cleanly.

Check whether the floating floor has any cross-battens or expansion gaps that will block a continuous rod run. On large rooms you may need to lift a board to allow the rod to change direction.

2. Access the void

The easiest entry points are:

Behind a skirting board removed at the start of the run
A door threshold bar that can be unscrewed and refitted
A lifted board at each end where the cable needs to emerge at a back box

Floating floors are not fixed to the subfloor, so any individual board can be eased up from the cut end and slid free. If the board clips to its neighbours, work from the expansion gap at the room edge rather than forcing the locking tongue.

3. Rod the cable through

A fibreglass cable rod set is the right tool for this. Thread the cable through a loop or hook on the nose of the first rod, tape the cable securely to the rod with electrical tape so it does not separate mid-run, and push the rod into the void. Add sections as you go.

Push steadily and rotate the rod gently as you go. The foam underlay can grip the rod on long runs: if it stops moving, pull back slightly, rotate, and try again. Forcing it crumples the cable insulation.

Once the rod emerges at the exit point, pull the cable through and leave enough slack at both ends to dress into the back box without strain.

4. Protect the cable at entry and exit

Where the cable passes through the edge of the floor covering and enters or exits a wall, the insulation can be abraded by the hard flooring edge. Fit a PVC grommet or a short length of conduit at every point where the cable crosses from void to wall or ceiling. Without this protection, the insulation deteriorates over time as the floor moves.

At a back box, the cable should enter through a clean knockout, not draped over the edge of the metal. Use a grommet in the knockout to protect the insulation where it enters the box.

5. No joints under the floor

BS 7671 Regulation 526.3 is clear: every electrical connection must be accessible for inspection and maintenance. A cable joint concealed under a floating floor is not accessible and is not compliant. If the run is too short to reach, the answer is to run the cable in two sections with a junction box at an accessible point on the wall or skirting, not to bury a connector under the boards.

6. Test before relaying the floor

Before the boards go back down, carry out an insulation resistance test on the cable run at 500 V DC. Connect the tester between the line conductor and earth with the neutral and line linked together. A healthy new cable reads well above 200 MΩ. A reading below 1 MΩ means the insulation is damaged and you need to investigate before relaying the floor.

Also test continuity of the CPC. With the far end disconnected, link the line and CPC and measure the resistance at the supply end. An open circuit here means either the cable is broken or the CPC was not pulled through with the rest of the cable.

7. Relay the floor and finish

Relay boards from the same end you started from, clipping each board to its neighbour before moving on. Refit the threshold bars and skirting. Make good any small notches cut in the skirting for the cable with a filler or a short length of quadrant beading.

Once the circuit is connected, carry out a full polarity test and, for the finished circuit, an earth fault loop impedance test and RCD trip time where applicable.

Stop and call an electrician if: you discover existing cables buried in the void that are not in conduit and their condition is unknown, the insulation resistance reading is below 1 MΩ on the new run, you find the subfloor is a concrete slab with in-floor heating pipes whose positions are not marked, or the cable run crosses into a room where a different circuit is already live and you cannot safely confirm all conductors are dead.

When to call us

Running cable under floors and testing it correctly is part of any good first or second fix. In Sandwich, Richard carries out this kind of work on rewires, extensions and new circuits at a fixed quote, and issues the correct certification at the end.

Need a cable run in Sandwich?

Richard runs cables neatly under floors, through voids and along walls, and tests correctly before any circuit goes live.

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How to run cable under a floating floor