Containment for island electrical points

[JohnW2]

Because it is closest to the cable being installed in a duct under the kitchen floor

It's a little odd that BS7671 does not explicitly include installation methods for cables in conduit under/within floors.

As I've just written to Sunray, I don't think we know for certain that it is a concrete floor, nor, if it is, to what extent it is 'insulated' - I would not have thought that it would have thermal insulation both above and below it, which would be essentially what you are assuming by using Method A.

Thermal insulation aside, cable in conduit within masonry (presumably including concrete) would be method B, which would give 2.5mm cable a CCC of 23A.

""insulated stud wall cable NOT touching plaster board"" would require 6mm²

That is true, but you are then referring to a situation in which the cable itself is totally surrounded by thermal insulation. If cable (or conduit) is buried in concrete, there will be no intervening thermal insulation between the cable (or conduit) and the concrete, so I would imagine that there would be a lot of scope for heat to be transferred to the mass of the concrete, even if there were insulation above or below the concrete.

The bottom line of all this is that the calculations presented rely on both guesses and some uncertainties in the regs. As a minimum, we should at least obtain clarification of the installation method before attempting any calculations.

Given the number of posts from Winston1 that were somewhere between "unhelpful comment" and "in-accurate advice" it would improve the forum if his postings were vetted before appearing in public. Especially when a dangerous situation could be created if his "in-accurate advice" was taken to be accurate and was used by a DIYer in a project.

I doubt whether the mods would be inclined to expend that much energy in looking in detail at all his posts, particularly if calculations and/or consultation of reference materials were necessary. I think that, as you are illustrating, the forum membership is quite capable of 'policing' these situations satisfactorily, with the added benefit that, if done 'in public', others will soon deal with "incorrect corrections" (which sometimes happen!).

Kind Regards, John

 

[bernardgreen]

The bottom line of all this is that the calculations presented rely on both guesses and some uncertainties in the regs. As a minimum, we should at least obtain clarification of the installation method before attempting any calculations.

In designing any installation there will be many variable and situations that have not been covered by the regulations.

Sunrays calculation using Cleveland Cables calculator will give results that are specific to the properties of their cable.

One could assume that all 2.5 mm² PVC twin and earth cables have the same thermal characteristics no matter which company manufactured them

That may not be the case, in which case the regulations should be set for the worst known case of thermal characteristics.

My bottom line is that oversizing cables is safer than going for a cable that just meets the regulations for the circumstances of the installation.

 

[padstar]

Hi the cables are run within a traditional timber floor clipped to the joists with PIR insulation above but not in contact with the cables. They are then run up pvc conduits to final connection points at various heights.

If the electrcian splits the kitchen and the island on 2 rings they would cover;

Kitchen & Utility Circuit - 4nr double sockets, washing machine, tumble drier, fridge, freezer and extract hood

Island Circuit - Wine fridge, Quooker hot tap, food disposal unit, dishwasher, 1nr double socket

Ovens and hob on separate circuits.

 

[winston1]

Hi the cables are run within a traditional timber floor clipped to the joists with PIR insulation above but not in contact with the cables. They are then run up pvc conduits to final connection points at various heights.

If the electrcian splits the kitchen and the island on 2 rings they would cover;

Kitchen & Utility Circuit - 4nr double sockets, washing machine, tumble drier, fridge, freezer and extract hood

Island Circuit - Wine fridge, Quooker hot tap, food disposal unit, dishwasher, 1nr double socket

Ovens and hob on separate circuits.

OK. Two rings in 2.5mm then as I said before all the arguments.

 

[Taylortwocities]

OK. Two rings in 2.5mm then as I said before all the arguments.

Gone off the idea of 1.5mm then?

 

[winston1]

Gone off the idea of 1.5mm then?

No. The recent amendments suggest 1.5 can be OK.

 

[bernardgreen]

How often has Winston1 told us

The wiring regulations are wrong.

 

[JohnW2]

In designing any installation there will be many variable and situations that have not been covered by the regulations.

There will, but it is, in my opinion, to even start one's calculations when one doesn't know how the cableis to be installed (but now we do - and it seems that my doubts {that they were necessarily in an 'insulated concrete floor were very justified ) is pretty foolish!

Sunrays calculation using Cleveland Cables calculator will give results that are specific to the properties of their cable. .... One could assume that all 2.5 mm² PVC twin and earth cables have the same thermal characteristics no matter which company manufactured them .... That may not be the case, in which case the regulations should be set for the worst known case of thermal characteristics.

Whilst that may be theoretically possible (I'm not familiar with the Standards for cables),I have never in my life heard anyone come up with that suggestion/argument. To the best of my knowledge, everyone assumes that the tabulated CCCs in BS7671 for a particular type of cable of a particular size installed by a particular Installation Method applies regardless of the manufacturer (which I strongly suspect will be the case by virtue of Standards).

I struggle to understand what you are suggesting. You surely are not saying that no-one should take any notice of the countless tables of CCCs in the Appendix of BS7671 because they might not be correct in relation to a particular manufacturer's cable, are you? If that were the case, what would be the point in the existence of all those tables?

Were used to your ultra-cautious approacxh to many things, which is fair enough, but I think this one may well deserve some prize

Kind Regards, John

 

[JohnW2]

Hi the cables are run within a traditional timber floor clipped to the joists with PIR insulation above but not in contact with the cables. They are then run up pvc conduits to final connection points at various heights.

Thanks. That's what I suspected might be the case, and I don't know where all the talk about "within an insulated concrete floor" came from.

... which I think takes us back to 2.5mm² cable, after all the distractions

Kind Regards, John

 

[JohnW2]

Gone off the idea of 1.5mm then?

No. The recent amendments suggest 1.5 can be OK.

Not OK, even under Amendment 2 - 1.5mm cable in conduit only has a CCC of 16.5A

Kind Regards, John

 

[padstar]

Thanks for all the help. Sorry to have caused so much trouble.

2 rings both of 2.5mm to split the load between 1) kitchen/utility and 2) island. Hopefully this will have all bases covered.

 

[JohnW2]

Thanks for all the help. Sorry to have caused so much trouble.

You're welcome, and I don't think that you have 'caused any trouble' at all!

Kind Regards, John

 

[SUNRAY]

Thanks for all the help. Sorry to have caused so much trouble.

2 rings both of 2.5mm to split the load between 1) kitchen/utility and 2) island. Hopefully this will have all bases covered.

Far from it, you have done nothing wrong, I'll go further and say you've done the right thing seeking advice/second opinion.

Thanks. That's what I suspected might be the case, and I don't know where all the talk about "within an insulated concrete floor" came from.


Kind Regards, John

I'll make the apology for assuming, however my assumption is based on not encountering a timber ground floor for a long time in a new build/alteration.

If there will be no access to under the floor when finished I'd personally provide conduit/duct/trunking as appropriate to allow pulling in additional cables later.