How deep into the PIR should I recess 20mm conduit for in-floor plug sockets?

 

Any tips on cutting the PIR? I’m assuming I cut a square channel with a thin layer of PIR over the top of the conduit just so the UFH pipe isn’t directly on the conduit.

How deep is the PIR?

What is the build up of the floor upto and including the PIR?

Could you run the cable below the PIR and then bring it up to the location of the floor sockets?

 

I have 2 floor sockets. The cables were run between the joists and then a hole made to bring the cable up vertically to where the floor sockets would be.

1 hour ago, BotusBuild said:

How deep is the PIR?

What is the build up of the floor upto and including the PIR?

Could you run the cable below the PIR and then bring it up to the location of the floor sockets?

 

I have 2 floor sockets. The cables were run between the joists and then a hole made to bring the cable up vertically to where the floor sockets would be.

Concrete beam & block, 150mm PIR, UFH in 75mm screed, LVT to finish

So your approach seems the only option 😉 

 

As for cutting the PIR, a reciprocating saw (I'd use a hand held one for control), and you'll have to break small chunks out as you go. Messy but a hot knife won't work.

 

Use the chunks to wedge the conduit down from the UFH.

I would put them as deep in the PIR as you can.

 

What you've described doesn't quite fit any of the standard installation Reference Methods in BS7671 but from first principles, surrounded by insulation and overlaid by heating pipes is about the worst possible case.

 

Slightly counterintuitively on a heated floor "not covered by thermal insulation" really means putting it on the cold side of the insulation e.g. at the bottom.

If you can get the conduit touching or at least nearest the concrete then I'd say it's similar to Reference Method A.

 

Be aware that the rating for 2.5mm flat twin & earth in method A is exactly 20 amps at an ambient temperature of 30 degrees.

 

Depending on what temperature you're heating the floor to, and what temp you expect below the beam and block, your ambient at the conduit might be above 30° - you might want to model that temperature gradient to check (or design the circuit to operate at higher temp e.g. by using a 16A radial).

Sorry no clue to the real answer, but don't sockets have to be a  minimum given distance from the floor, so wheel chair users etc can use them? So are you allowed floor sockets?

1 hour ago, JohnMo said:

Sorry no clue to the real answer, but don't sockets have to be a  minimum given distance from the floor, so wheel chair users etc can use them? So are you allowed floor sockets?

I believe the Part M document refers to "wall-mounted sockets" in relation to the minimum distances. AFAIK, floor-mounted sockets are still allowed, but always best to check with your BCO for their guidance.

>>> Messy but a hot knife won't work.

 

Why not? Otherwise I would suggest a Winons hot knife.

A hot knife is good for polystyrene. They just dont cope with PIR. Someone may be along to explain why. Most guidance for cutting is sharp knife, jigsaw or handsaw

3 hours ago, JohnMo said:

Sorry no clue to the real answer, but don't sockets have to be a  minimum given distance from the floor, so wheel chair users etc can use them? So are you allowed floor sockets?

As long as there are wall sockets in the vicinity that comply with the height guidance then floor sockets are allowed

I'm thinking the duct/cable should be at least 50mm deep if in a floor and of course RCD protected. 

On 21/03/2026 at 07:55, Onoff said:

I'm thinking the duct/cable should be at least 50mm deep if in a floor and of course RCD protected. 

Would it need to be under a steel plate as well, or is that for <50mm.

On 21/03/2026 at 07:55, Onoff said:

I'm thinking the duct/cable should be at least 50mm deep if in a floor and of course RCD protected. 

 

16 hours ago, SteamyTea said:

Would it need to be under a steel plate as well, or is that for <50mm.

 

Technically speaking there are no specific requirements for cables in a floor apart from where they're passing through joists. The 50mm depth or steel plate (or other mechanical protection) is to protect against damage by fixings into the joists.

 

Between joists, or in cases like this where there are no joists, there's just a general requirement that the cable is installed so that it isn't likely to be damaged by contact with the floor or its fixings. That's a risk assessment you have to make based on the nature of the building/installation.

 

So if e.g. you have a floating floor over the PIR then there's fairly limited risk of anyone fixing down through that. And the PIR itself and the space within the duct will provide a fair bit of protection against the cable being crushed by weight on the floor.

 

So I'd think steel plate would be overkill, especially if 50mm or deeper.

 

SELECT have quite a good guidance note on cables concealed within building structures https://select.org.uk/common/uploaded files/Technical/FreeDownloads/30270_PG_InstallationOfCables.pdf

On 20/03/2026 at 15:54, garrymartin said:

I believe the Part M document refers to "wall-mounted sockets" in relation to the minimum distances. AFAIK, floor-mounted sockets are still allowed, but always best to check with your BCO for their guidance.

They are allowed as you say - confirmed by both my BCO and electrician (I got the idea from another build locally which installed floor sockets).