4mm ring

[sparkwright]

thankyou for all your comments!!
just a few comments to clarify
1. the 4 mm SWA is underbnearth the 150mm thick hardcore..sand blinded and running in parralells..its final depth will be 250mm underfloor, a full "as built" diagram will be part of the file I keep on the overall project of my renovation of this smallholding.
2. The cables are chased into the wall and have literally just now sunk in the metal backboxes, the cable will have the steel armour removed within 50mm of the back box, the insulator around the three cores will remain until inside the box, grommets have been fitted, this insulation will be stripped back when 2nd fixing.
3. Design, I havent really had a design done for this but feel I am following pretty standard guidelines...the barn is 7.8m long and 4.2 m wide, I feel that I can consider this a pretty smallish installation that would not need a overegg design, (but am open to ask here if my installation passess design criteria)

I know its easy to follow the advice that best fits the task I am trying to achieve, Plugwash you have given me confidence that there is nothing untoward about what I am doing, I hope others can agree.

I wouldn't have said it was good practice to have unearthed armour about the place, because if the cable did get damaged, the armour could become live.

Although it could be argued that metal capping and old mid century unearthed metal conduits in walls are just the same.

A pedantic type could also argue that there would be a section of cable where near the box where there is no actual outer sheath, just 'bedding'.

 

[EFLImpudence]

As things stand (i.e. as the regs actually are), the only reasons one might contemplate a 32A 4mm² ring are (a) if the installation mwthod of the 4mm² cable were such that its CCC was less than 32A or

Is that not the crux of the matter?

If the CCC of a 4mm² cable has to be derated because of the installation method to less than 32A, then you still cannot have a 32A OPD even if you do wire it as a ring.

Or can you? I don't think so.

 

[Murdochcat]

I, for one, have read Part P, the entirety of which says:

What bit of that do you believe it relevant to this discussion?

Kind Regards, John

The part which suggests the OP hasn’t got a grasp on what they are doing and I’m doing my bit to attempt to help DIYers understand what legislation is relevant to them

After all there are dumb sparks spending heaps of money keeping up with the unnecessary constant changes to the regs and paying fees to achieve compliance etc etc

That’s all

 

[ericmark]

The ring final allows the use of 2.5 mm² or with mineral cable 1.5 mm² cable where normally with a 32 amp overload one would need a thicker cable, but it also states the cable must be able to carry 20 amp, so using installation method 101 it would not be permitted to use 2.5 mm² twin and earth.

So to say it must be 2.5 mm² is clearly wrong, also we calculate the volt drop in the main considering the load as 20 amp at centre and 12 amp even distributed, so we use 26 amp as "Design current for circuit Ib" but this is just a normal method, it is not part of the regulations, however following that the limit is 106 meters of 2.5 mm² if the ring final exceeds 106 meters then one would need to use a heavier cable.

So with 4 mm² the limit would be more like 180 meters, and installation method 101 could be used.

If the work needs planning permission then to add the wiring would cost very little extra, but if you are using a scheme member electrician then to allow him to self certify he has to design the installation, one would hope to be a scheme member he could work out how to wire in 4 mm² and to say 4 mm² not allowed puts into question his ability to do the work.

So I would guess he thinks you want a larger than 32 amp MCB/RCBO/fuse and if you did then yes it would not qualify for the special dispensation with the use of ring finals. you could clearly still have a ring main, but that is very different to a ring final.

 

[skenk]

If the CCC of a 4mm² cable has to be derated because of the installation method to less than 32A, then you still cannot have a 32A OPD even if you do wire it as a ring.

Or can you? I don't think so.

Yes you can, a 32A ring cable needs to have a CCC of 20A, and 4mm becomes unsuitable for a 32A ocpd easily with correction factors and length

 

[JohnW2]

Dont think anyone said they was going to put it on a 40 or 50 amp protective device,

I'm sure they didn't, not the least because it would be contrary to the regs.

However, my (and EFLI's) point was that, unlike the case with 2.5mm² cable, the regs do not allow a ring using 4mm² cable to be protected by a device whose 'rating' (In) is greater than the CCC of the cable.

The regs do allow a cable with a CCC of 20A to be protected by a 32A device,, hence 1.6 times the CCC. The equivalent for a cable with a CCC of 32A (say Method C 4mm²) would be for it to be allowed to be protected by a device rated at 32 x 1.6 - which is just slightly above 50A. However, the regs do not allow that, since they only relate only to circuits protected by a 32/30A device.

Kind Regards, John

 

[JohnW2]

Yes, agreed, but there was no mention of using a higher OPD as I recall.

Exactly my point - if (unlike the case with 2.5mm²) one wires a ring with 4mm² (or larger!) cable, then one is not 'allowed' (by regs) to protect it with a device whose In is greater than the CCC of the cable - so there is no point in wiring it as a ring (rather than a radial), other than the exceptions I mentioned (4mm² de-rated to <32A or 'a VD issue').

This is really only about regulations, not electrical considerations. The latter would say that a Method C 4mm² ring protected by a 50A MCB ought to be just as acceptable as a ring of 2,5mm² cable de-rated to 20A on a 32A MCB - but the regs don't recognise that!

Kind Regards, John

 

[JohnW2]

If the CCC of a 4mm² cable has to be derated because of the installation method to less than 32A, then you still cannot have a 32A OPD even if you do wire it as a ring. Or can you? I don't think so.

Why not (provided it is not de-rated to less than 20A)?

It's still a cable with a CSA of ≥2.5mm² and a CCC ≥20A - so what part of the reg (allowing a 32A OPD on a ring final) would it not satisfy?

Kind Regards, John

 

[JohnW2]

The part which suggests the OP hasn’t got a grasp on what they are doing and I’m doing my bit to attempt to help DIYers understand what legislation is relevant to them

Maybe but, if that is your aim, you're expressing yourself very badly ... what bit of the (one sentence of) "Part P2 "suggests the OP hasn’t got a grasp on what they are doing" ?

After all there are dumb sparks spending heaps of money keeping up with the unnecessary constant changes to the regs and paying fees to achieve compliance etc etc

The (one sentence of) Part P has nothing to do with 'the regs'. I think you may need to try to express yourself a little more clearly/correctly.

Kind Regards, John

 

[JohnW2]

The ring final allows the use of 2.5 mm² or with mineral cable 1.5 mm² cable where normally with a 32 amp overload one would need a thicker cable, but it also states the cable must be able to carry 20 amp, so using installation method 101 it would not be permitted to use 2.5 mm² twin and earth.
... So to say it must be 2.5 mm² is clearly wrong,

Indeed, it would be 'clearly wrong' - which is perhaps why it does NOT say that?

It says "a minimum of 2.5mm²" - so, as I've recently been writing, the reg would still allow a 4mm² cable de-rated to <32A (but not to below 20A!) in a ring final to be protected by a 32A device. I would think that would be about the only reason for considering a 4mm² 32A ring (since 'the VD issue' being extremely improbably)

Kind Regards, John

 

[Adam_151]

I don't get what all the issue is about, a 4mm ring is pretty common in commericial work, and while there was probably no need to go to 4mm in the OPs case, its not wrong to do so.

Volt drop is often the reason they are specified, but often very cautious design assumptions are made, such as assuming the loading is 20A in the middle of a ring and then extra 5A load at 1/4 and 3/4 around. When the acutal load is a couple of PCs and less than 5A......

Grouping factors are another thing that might result in 4mm rings

Might also be necessary to meet Zs on longer rings, espeically with C type MCBs and no RCD

 

[EFLImpudence]

Why not (provided it is not de-rated to less than 20A)?

It's still a cable with a CSA of ≥2.5mm² and a CCC ≥20A - so what part of the reg (allowing a 32A OPD on a ring final) would it not satisfy?

Yes, of course you (and others) are correct.

I think I was confusing two different things.

 

[russellsmiler]

regarding the gland issue can I say wether this is going to be ok? as this is what I have done!!
1. I have used 3 core SWA as this cable came to me as a freebee from an electricasl company who had to provide temporary power to a site ( the cable is unblemished and was only in place for 3 weeks, )
2. I will use brown live, black ( blue tape at terminals) for neutral, and grey (yellow and green tape at terminals) for earthing
3. The cable is buried beneath harcore interbnally in the barn, the metal socket boxes have been cahsed into the brick work so as the cable, the steel armour as been removed to within 50mm of the socket box, and as stated the secondry insulator teminates just inside the grommet protected knock out. the cable is to be mortared in and the lower walls are to be rendered.

One tip i hope can give (if allowed) I drilled and plugged the brickwork at the back of the chasing, one hole low level ond say 100mm below backbox, then screwed through the back of a heavt duty cable tie thus attaching it to the wall, fitted cable and pulled cable tie tight...it really pulled the cable in nice and tight and worked well low down to get a tight radius coming from the horizontal to the vertical) I found the cable clips for SWA really fiddly so looked for an alternative, there held really good for me now to mortar in!!

 

[Adam_151]

Standard practice is to use grey as neutral (L3/previously blue) and black as earth (L2/previously yellow). Obviously sleeve appropiatly

How are you earthing the armour as it sounds like its not glanded? / Why not just gland off into the knockout boxes, use a coupler if worried about space

 

[russellsmiler]

thank you adam will do as said standard practise.

bit concerened that I will need to earth the armour!! its stripped back completely from the terminals and is not used as the earth as in 2 core.
can I ask naively does it need earthing, and if so..if I connect it with a coupler so it is continuous would an earth rod suffice??