4mm² T&E in masonry wall - ref methods?

[echoes]

Hi, I've been trawling the internet for the appropriate reference method for cables buried in masonry and de-rating 4mm² T&E cable accordingly.

I would like to supply my long, narrow kitchen with 2 socket radials: there would be a washing machine, tumble dryer and 3 sockets down one side, and a dishwasher and 4 sockets down the other. The sockets will be used for toaster/kettle/microwave, that sort of thing.

Now installation methods 57 and 58 both refer to method C if I am not mistaken. My question is whether buried oval conduit is classed as 'mechanical protection' or 'conduit' when buried in plaster
(I appreciate that SURFACE trunking or conduit would be method B), and common-sense says that buried oval conduit is closer to capping than surface conduit or trunking, but common-sense is not always reliable(!).

Grateful for your time and input as always...

 

[PrenticeBoyofDerry]

The use of conduit would be considered as mechanical protection providing it was metallic and not plastic.
What are your reasons for mechanical protection on the circuits, is it because the routes are outside permitted safe zones or are you trying to avoid the use of RCDs?
//www.diynot.com/wiki/electrics:installation_techniques:walls
It's clear you have been studying the BS7671:2008 requirements, so I hope you are also aware of compliance to part p of building regulations and notification of work in the kitchen.
//www.diynot.com/wiki/electrics:part_p

 

[echoes]

Thanks PBofD. All cable runs will be in safe zones, and the circuit will be RCD protected. Have notified.

Your answer implies that buried oval conduit and plastic capping would be ref method B then? Which means that the CCC of 4mm² T&E would be 30A, so a 32A MCB would be unacceptable to the tune of 2A.

The 4mm²/32A radial final is limited to quite specific installation methods it seems.

<edit>
So to supply a 32A socket circuit, must use a 2.5mm²/32A ring final or clipped-direct 4mm²/32A?
</edit>

 

[ban-all-sheds]

Now installation methods 57 and 58 both refer to method C if I am not mistaken. My question is whether buried oval conduit is classed as 'mechanical protection' or 'conduit' when buried in plaster

Conduit - look at the drawings. Conduit goes all the way around, with capping the cable is in contact with the wall.

Clipped direct is C.

Buried is C.

Buried with capping is C.

In conduit on the surface is B.

In conduit buried is B.

There's no B in 4D5, you'll have to use 4D2 - 4mm² is 30A. You might justify saying it's 31A because of the slightly higher ratings for T/E compared to generic multicore.

So to supply a 32A socket circuit, must use a 2.5mm²/32A ring final or clipped-direct 4mm²/32A?

Or a 4mm² ring final or a 6mm² ring final or a 6mm² radial.

And of course there is no compulsion for socket circuits to be 32A.

 

[ricicle]

And using some engineering judgement in the fact that the load on the cable will be cyclic then a 32A OPD will be fine on a 30A rated cable.....

 

[echoes]

Now installation methods 57 and 58 both refer to method C if I am not mistaken. My question is whether buried oval conduit is classed as 'mechanical protection' or 'conduit' when buried in plaster

Conduit - look at the drawings. Conduit goes all the way around, with capping the cable is in contact with the wall.

Clipped direct is C.

Buried is C.

Buried with capping is C.

In conduit on the surface is B.

In conduit buried is B.

There's no B in 4D5, you'll have to use 4D2 - 4mm² is 30A. You might justify saying it's 31A because of the slightly higher ratings for T/E compared to generic multicore.

That's great clarification b-a-s - thank you very much.

So to supply a 32A socket circuit, must use a 2.5mm²/32A ring final or clipped-direct 4mm²/32A?

Or a 4mm² ring final or a 6mm² ring final or a 6mm² radial.

And of course there is no compulsion for socket circuits to be 32A.

Yes, but for a kitchen I would prefer not to use 20A socket radial(s) due to the high power stuff used in there, not necessarily distributed nicely among circuits.

... and 6mm² for socket terminations is going to be either impossible or next to impossible...

 

[echoes]

And using some engineering judgement in the fact that the load on the cable will be cyclic then a 32A OPD will be fine on a 30A rated cable.....

I think that could be justifiable in this case
... or if not I've plenty of capping left over from a previous 'weekend'.

Thanks both for your time & replies.

 

[PrenticeBoyofDerry]

So what is the reasoning behind the use of conduit?
Is it so you can replace cables in the future or a means of protection?
You are now aware that it can lead to de-ratings of the cables current carrying capacity.
So is there a problem with running the cable without conduit and just installing it to be plastered over, this will keep the safe current carrying capacity of the cable to it's maximum.

 

[echoes]

So what is the reasoning behind the use of conduit?
Is it so you can replace cables in the future or a means of protection?
You are now aware that it can lead to de-ratings of the cables current carrying capacity.
So is there a problem with running the cable without conduit and just installing it to be plastered over, this will keep the safe current carrying capacity of the cable to it's maximum.

Reason is only to hold cables in place while plastering over. I find oval conduit easier to fix than capping, and chase is narrower. Unlikely that the cables will be replaced anytime soon.

I was always aware that current-carrying capacity depends on installation method, and that ref method C is the only one tabulated for this circuit in the OSG, but was unsure which ref method applied since there is no explicit rating for multicore with CPC in buried conduit (as per BAS's reply.)

No problem whatsoever using just clips or capping and plastering over.

Thanks again.

 

[ban-all-sheds]

Why use capping over chases?

And if you're chasing, ones for cables rather than oval conduit are narrower still. Cables can easily be held in chases prior to plastering with a few dabs of adhesive or filler.

 

[echoes]

And if you're chasing, ones for cables rather than oval conduit are narrower still. Cables can easily be held in chases prior to plastering with a few dabs of adhesive or filler.

Point well taken - neither capping nor conduit are necessary. I tend to use use oval conduit for switch drops, generously sized, just in case woman wants more 2 way switching somewhere and new cable can be drawn down.

There won't be any such tweaking with the kitchen sockets/switches/FCU's.