Cables in Screed

[Barnman]

I am renovating a barn and need to run cables under the floor as the roof is open to the eaves and it is single story. I think the options are pvc conduit in the insulation layer below the screed which makes it deep enough to prevent damage or possible running it along the walls near the roof. Any suggestions and comments if these are legal. I intend to get the local authority to inspect under a building notice but want to be sure of howe to approach the work.

 

[jj4091]

See the wikki for safe zones

 

[luminaire]

If you are putting cables into pipe under screed, make the pipe big enough so that you can also put in a pull rope in case you want to add further cables in the future.

 

[ban-all-sheds]

Plan B - use the shallow trunking that is usually used to supply flush floor boxes?

 

[Barnman]

Thanks - all good advice. Still not sure if it is OK to put cables in conduit in insulation later. There does not seem to be many restrictions apart from running in walls or floors - i.e. depth and zones. So I assume conduit in insulation is OK, and also running in roof above plaster board and insulation in a vaulted ceiling.

 

[ban-all-sheds]

Cables in conduit in insulation are OK, provided you derate them appropriately.

 

[Barnman]

By derate I assume this is making sure the conduit is of sufficient cross sectional area to cater for heat. The tables in the John Whitfield book give the factors to apply.

The shallow trunking used for flush floor boxes - does this lie in the screed? With underfloor heating this would make the cable even hotter. Can you reccommend the best place to buy this. I have sen some large trunking that has compartments for the different cable types.

I am begining to think running the cables in the top of wall zone may be the easiest.

 

[ban-all-sheds]

By derate I assume this is making sure the conduit is of sufficient cross sectional area to cater for heat. The tables in the John Whitfield book give the factors to apply.

If you mean conduit factors, they are about getting the right sized conduit.

You still need to apply all the current-carrying capacity correction factors, which could well include grouping ones with conduit, and with conduit buried in insulation will certainly include a significant thermal insulation factor. I reckon that overall you could lose at least 50% of the nominal capacity of the cables, which means no ring finals, 4mm socket circuits on 16A breakers etc.

The shallow trunking used for flush floor boxes - does this lie in the screed?

Yes.

With underfloor heating this would make the cable even hotter.

Yes.

Can you reccommend the best place to buy this.

No - sorry.

I have sen some large trunking that has compartments for the different cable types.

They are often quite wide, with compartments for different cable bands.

I am begining to think running the cables in the top of wall zone may be the easiest.

Could be....