Why are ring finals split up/down not side/side?

[Harry Bloomfield]

Well one explanation given by someone in this forum is that the plan was you'd run the ring round the core of the house, and any sockets on the outside would be spurs. That way, the ring itself is relatively short for the area covered, and the spurs are no longer than branches off a radial would be.
Given this arrangement, I could well imagine the extra length of smaller cable needed to bring the other end of the ring back to the fuse would be more than offset by reducing the CSA of the "trunk" of the radial. There's also the factor, especially given some of the discussions regarding competence of some people, that the whole circuit can (for most installations) be done with one size of cable without any thought or maths needed on the part of the installer (who could just do the whole lot in 7/029 as long as he didn't exceed the guidelines for area covered).

I feel I should point out, that the early ring installs offered in most cases up to a medium sized home, just one single ring for the whole home. The ring run under the first floor, going up and down to sockets. It progressed from that to an upstairs and downstairs ring, then as we have now - a kitchen + ground + first floor ring. As my home is built on a concrete slab, all wiring is still in the ground floor ceiling.

My home progressed with rewires, from single ring with very few sockets, to a more extensive single ring, to what I have now - an upstairs ring, downstairs ring, loft/workshop radial and a garage radial. To the basic scheme I have added numerous outlets, as they have been needed and opportunity presented itself. I have never counted the number of sockets, but I would guess at something in excess of 50. I have no substantial loads at all, biggest are washer and dishwasher, but I do have lots of tiny loads of a few watts.

I don't have a separate ring for the kitchen, with no plans to install one - loading is so light on my present ring.

 

[EFLImpudence]

Do you mean because you think that a 30A radial would then have been possible - hence no need for a ring?

No, I just meant nowadays - or since MCBs.

Had the 2.5mm² cable been a tiny bit bigger then the ring final would disappear with BS3036 fuse boxes.

 

[JohnW2]

No, I just meant nowadays - or since MCBs. Had the 2.5mm² cable been a tiny bit bigger then the ring final would disappear with BS3036 fuse boxes.

Oh, fair enough. I think that would have to be a bit more that 3.0 mm² to be able to support a 32A radial protected by an MCB (if that's what you mean) - the graph I presented above suggests that it would need to be around 3.2 mm² (in practice, if anyone ever made it, I imagine that it would probably be 3.5 mm²).

Kind Regards, John

 

[EFLImpudence]

There were 30A MCBs.

Would two 25A radials not be better?
(Although, as keeps being said, everyone else manages with 16A radials.)


Another thing is - the special ring final circuit could use 1.5mm² cable with 32A MCBs were it not for the explicit prohibition because BS3036 fuses are still included in the regulation - or rather MCBs have simply just been included in the BS3036 regulation.

 

[JohnW2]

Well one explanation given by someone in this forum is that the plan was you'd run the ring round the core of the house, and any sockets on the outside would be spurs. That way, the ring itself is relatively short for the area covered, and the spurs are no longer than branches off a radial would be. ... Given this arrangement, I could well imagine the extra length of smaller cable needed to bring the other end of the ring back to the fuse would be more than offset by reducing the CSA of the "trunk" of the radial.

I somehow doubt that was the original thinking behind the concept of the ring final - since I don't think that spurs were much of a feature of the early rings (and I'm not even certain that spurs were even allowed back then).

However, it's obviously the case that the relative amounts of copper used (by radial or ring) will depend upon the arrangement of the sockets ...

In the (geometrically) simplest situation (albeit not a realistic one), if one has a string of sockets in a straight line ('radiating from the CU'), then a 'ring' circuit will, with currently available cables, use more copper than the corresponding radial. If the furthest socket is L mm from the CU, then the volume of copper (for L or N conductors) is ...

4.0 x L mm³ for a 32A 4 mm² radial
and
2.5 x 2 x L = 5.0 x L mm³ for a 32A 2.5 mm² ring final.

Moving from that extreme situation, the saving of copper (if any) by using a ring (without spurs) will depend on the geometrical arrangement of sockets, and what wiring routes are used, so one cannot generalise. I've been trying to work out whether there is a theoretical 'best case' (in terms of copper saving), but am not convinced that there is a general answer to that.

Kind Regards, John

 

[JohnW2]

There were 30A MCBs.

I don't doubt that there were, but they clearly are not 'readily available' now.

Would two 25A radials not be better?

Well, for those with high requirements, 50A is obviously more than 30A or 32A.

I think that most of us are agreed that the 'benefits' of ring finals (if any) are minimal, particularly today.

(Although, as keeps being said, everyone else manages with 16A radials.)

That's true but, as I've often said, I'm not too keen on the concept of even a 20A multi-socket radial - it just feels a bit 'odd' to have a multi-socket circuit that theoretically cannot service even two 'maximum' (say 3 kW) plugged-in loads.

Another thing is - the special ring final circuit could use 1.5mm² cable with 32A MCBs were it not for the explicit prohibition ...

Very much so. A cable with a CCC of at least 20A is a cable with a CCC of at least 30A, so the prohibition of 1.5mm² seems very odd - and even more odd that, despite that, 1.5mm² pyro is allowed. It's hard to think of what the reasoning might be.

Kind Regards, John

 

[Harry Bloomfield]

1.5mm² pyro is allowed. It's hard to think of what the reasoning might be.

That I think is obvious, Pyro will not be so easily damaged by heat and dissipates its heat more easily.

 

[JohnW2]

That I think is obvious, Pyro will not be so easily damaged by heat and dissipates its heat more easily.

The tabulated CCC of the cable, together with any applicable de-rating factors, is meant to take that into account. In other words, if the current exceeds the CCC (after application of any de-rating factors), then the cable is deemed to not be in a 'safe' situation as regards possible thermal damage.

Hence, if 1.5mm² pyro is associated with de-derating factors which reduce its CCC (from 25A in free air or 23A Method C) to 20A (which is still compliant with 433.1.204), then it should be no 'safer' from the thermal point of view than is 2.5mm² T+E when its CCC is reduced to 20A by de-rating factors (again, compliant with 433.1.204). I therefore cannot really understand why pyro is treated differently by this reg.

Kind Regards, John

 

[winston1]

I somehow doubt that was the original thinking behind the concept of the ring final - since I don't think that spurs were much of a feature of the early rings (and I'm not even certain that spurs were even allowed back then).

I'm not sure you are right. I grew up in a 1952 new build. The ring had 3 sockets on the ring all upstairs and 3 spurs all dropped from the 3 upstairs sockets. Six sockets in total.

 

[JohnW2]

I'm not sure you are right. I grew up in a 1952 new build. The ring had 3 sockets on the ring all upstairs and 3 spurs all dropped from the 3 upstairs sockets. Six sockets in total.

You might be right, since my experience is limited. I'm pretty sure that I never even saw a ring until after the 60s.

Kind Regards, John

 

[ericmark]

The spec for ali-tube cable is nearly the same as mineral insulated, but you can use 1.5 mineral but not 1.5 ali-tube on a ring final.

I think the 1/3 rule on drilling beams did change the way a house needs to be wired, if wired between ground and first floor to have two rings following same route one with cables going up to sockets and other with cables going down to sockets seems daft, splitting side to side saves so much cable and reduces the loop impedance and volt drop, does not matter is ring final or radial side to side split seems the best option, yet not done very often, which was the question WHY?

 

[ericmark]

You might be right, since my experience is limited. I'm pretty sure that I never even saw a ring until after the 60s.

Kind Regards, John

Dad's house built 1954 that had a ring, think he said 6 sockets, three times the number in my granddads house, he had two on in hall one on landing, anything else you plugged into light socket.

 

[JohnW2]

.... splitting side to side saves so much cable and reduces the loop impedance and volt drop, does not matter is ring final or radial side to side split seems the best option, yet not done very often, which was the question WHY?

Does your 'side-to-side' split include 'front-to-back' split?

I ask because very many houses, including all those I've lived in prior to my present house, are not wide enough to have significant rooms on both 'sides' - the rooms therefore tend to nearly all be on one 'side' (with the staircase on the other side). A literal 'side-to-side' split would therefore have virtually all rooms on one ring, the other ring serving little more than hall and landings (and, physically speaking, often a bathroom, but that would/should not have sockets).

As for 'why?', I think the safety issue that has been mentioned is certainly valid. 'Upstairs' and 'Downstairs' is totally unambiguous, in a way that left/right (and front/back) will often not be in many houses. It's quite common to have three rooms behind one another - would the middle one be 'front' or 'back'?

Kind Regards, John

 

[ericmark]

Yes front back split as with side side means less cable so wins all around except for labels. My sisters house was wired side to side, and when I realised what had been done, I thought what a good idea, why aren't all houses wired this way.

I suppose the same goes with central heating, putting zones which mean bathroom either cold at night or day is crazy, the bathroom should not be on either dormitory or living zone, as used day and night, but often they are grouped with dormitory zone.

When asked why, answer is often we always do it that way.

 

[SimonH2]

... we always do it that way.

Isn't that a good enough reason
bear in mind that some plumbers are only now fitting zone valves and room stats because the regs say they have to. I suspect many would be happy to fit on-TRV valves and no controls if they could.