Interesting take on a ring circuit

[John Rumm]

Looking at a mates ring circuit wiring, the original builder (1950's bungalow) has wired the only ring final circuit up from the CU into the loft space, done a lap of the loft and then back to the CU. At each socket position there is a junction box, which takes off a single 2.5mm sq unfused spur and drops it down a conduit to the socket 5' away.

The question is, is this style of wiring still permissible? It violates the guideline of having no more spurs per circuit than there are actual outlets directly on it, but I can't find anything in the regs that explicitly forbids it.

(For various reasons, he is keen to rewire with as little disruption as possible and is happy to accept the limted numbers of sockets etc he currently has - so being able to reuse the conduit drops to the sockets would be ideal, but they are only large enough for one cable, unless the circuit is wired in singles and conduit added everywhere).

Background for those that are interested:

Got a call from my friend the other day to say that he just had a new washing machine delivered, and his house nearly managed to electrocute the delivery bod! Anyway we traced the problem to a disconnected earth on the socket in question, capacitive filters on the appliance input, and earthed pipes. Anyway we fixed that easy enough.

However a look at the general state of the wiring leads us to suspect that it could probably do with a rewire PDQ. So I thought it prudent to do some tests on it.

There are three/four [1] circuits in total, connected to to three rewireable fuses. All the cable is rubber or PBJ insulated (inner and outer), unearthed on the lighting circuit. Separate steel earth wire on an ex-cooker point radial (this was the one that was disconnected), and T&E construction on the ring circuit cable.

The Earth fault loop impedance was actually not bad considering (no worse than 0.25 ohm in most places, rising to 0.7 on the end of several cascaded 4 way trailing leads!). TN-S supply.

Insulation resistance on the lighting and ex-cooker point circuit was also ok at >200M Ohm @ 500V (surprisingly!). The ring circuit however was another matter. The best isolation between any pair of conductors being 40K ohms! The (inner) cable insulation was visibly disintegrating and would fall off if the wire was bent about much.

[1] The lighting circuit seems to have two cables terminating at the CU. There is an open circuit between them, leading to the conclusion there may in fact be two separate circuits (12 light fittings - so that would also suggest more than one). For some reason however they are terminated on one fuse, in spite of there being three spare ways in the CU complete with unused 5A fuses!

[Andrew Gabriel]

Yes.

The reason behind that is that rings normally have no spurs when initially installed, and are extended with spurs. When you get to the point where it's doubled in size with added spurs, then it's time to reevaluate the adiquacy of the original design.

Given that this ring was not installed that way, then you need to use your own judgement to decide if it's time to consider if the ring is too big and needs splitting or has been hacked around so much that it could do with reinstalling.

One issue with that would be you can't get two connections to each socket for separate earth paths, not that this is required for just a washing machine in any case.

That's one reason why extension cords are bad things, and daisy chaining extension cords and/or multi-way socket blocks is a very bad thing.

Rubber when it was in good condition actually has a higher insulation resistance than PVC.

[Owain]

He might be, but IMHO it would be irresponsible to install a circuit which would not be sufficient for the purpose by today's standards.

Would it be acceptable to go *down* one conduit to a socket, horizontally along the wall (either chased or mini-trunking) with intermediate sockets, to socket at another conduit and then back *up*?

If minitrunking, a bit of slack would offer the possibility of chasing in at a later date. Horizontally between accessories is a permitted zone.

An alternative is to use radials, not rings. Then any additional sockets required can be spurred off existing socket locations, without having to do down-along-up to preserve continuity of a ring.

Another thought is MICC cable might be thin enough to get 2x down a conduit. IIRC rings only need 1.5mm minimum in MICC, not 2.5mm? Would involve some faffing and making good at the socket locations though to accommodate the glands.

Owain

[John Rumm]

Personally I would tend to agree, and would try to guide him toward a slightly more elaborate setup (the whole house currently only has about

10 sockets (mostly doubles) in total). Having said that, I can also see why he is keen to do the minimum possible since he only owns one third of the house (was left to him and his sisters) so recouping the cost of any work whe he sells is harder, and the sisters will just cop a bigger CGT bill.

Could be in some cases...

Yup, I may suggest that for some areas.

I did think of that, but you don't gain much in this case since I don't think you can get 4mm^2 cable down the conduit either, you would still be limited to 2.5mm^2 conduit drops. So extension would still have to be via a new drop rather than from an existing socket.

Would probably be simpler to go for singles in the conduit in that case I would have thought?

[John Rumm]

This place is like a little island in time wrt wiring. Original install, with practically no mods made to it since (one, possibly two extra sockets added, and one cooker point converted to feed a socket). Everything else seems much as it was when installed. (even the MK socket faces which have four securing screws - two at the top and two at the bottom).

I was quite supprised to find that a couple of the circuits were basically ok even after all this time. I would guess that the story may change if any of the cables were moved about too much or stepped on (currently run loose in the loft - not even clipped to the joists - even the JBs are not screwed down).

[Christian McArdle]

It is likely to be a perfectly OK design.

Forget about any "more spurs than sockets" rules of thumb. They are just guidelines for mentally challenged electricians. The important things to determine are:

(a) That the end of the spurs have sufficiently low earth loop impedence for a rapid disconnect on an earthlive fault. (Plus voltage drop calcs if required.)

(b) That the ring itself is reasonably balanced, so that sockets and spur connections are evenly placed around the rings, with sockets likely to have heavy loads either moved off ring (preferably) or distributed around the ring.

Christian.

[usenet]

Doesn't have to be 4mm^2, you can wire radials with 2.5mm^2 and use a

20amp MCB. This is what I have done for a lot of new circuits to the study and places like that in our house. It's much easier in many ways than a ring.

[John Rumm]

Yup, sorry I ommited part of my reasoning there... there is the same conduit issue getting the wires from the CU to the loft as there is for the socket drops. There is currently room for at best I would guess four

2.5s and it struck me that a 32A radial would be more use than a pair of 20A ones.

Thinking about it though, you plan might work if we did one ring for the kitchen (only room with any sizeable appliances) and then a couple of

20A radials for the rest of the house.

[John Rumm]

Yup, ta for the confirmation... I was pretty sure it was only a guidline, I just wanted to make sure it did not have some obscure regs "heritage" that I had not spotted!

Based on my tests using the present wiring that should not be a problem. (The incoming supply is also newish and has a well made earth connection to the sheath - so Zs should be nicely low)

Again the house layout is also conducive to a nicely ballanced circuit. The kitchen is also at the other end of the house from the CU, so you get the load nicely spaced round the middle of the cable runs without any bunching toward one end.

[Owain]

You may run into trouble both with conduit capacity factors and also derating the cables for grouping.

If this guy is doing the place up to sell then

(a) the number of sockets must be adequate - inadequate electric sockets is the sort of stating-the-bleeding-obvious to be picked up on survey. If the job can't be done to current expectations of number of sockets, perhaps better to leave it for the buyer to modernise. (b) the place probably needs the decoration freshening anyway, whether he wants to or not - so better to bite the bullet, chase in the cables and re-emulsion. IIRC you said the sockets had old mounting holes, so you'll probably have to break out the old back boxes from the conduit anyway. (c) if the sisters are getting 2/3rds of the profits from sale, perhaps they should be prepared to chip in 2/3rds of the cost of the upgrade.

Owain

[John Rumm]

Possibly, but the one(s) with most load (CU to loft) are only 1m long

Nope, he plans to stay there for the time being IIUC.

I did suggest that, and expect that in reality one could add more with very little disruption in some places (like the kitchen etc). There is no point saving 500 now if it causes a future buyer to bargin you down 3K.

Given that he may be there for another 10 years, I would not be keen to hang about with insulation in that sort of state though.

Decoration was done recently alas (before mom departed)...

Sockets are on standard metal back boxes from what I have seen - so no problem with them. The light switches however have wood ones that will not fit current switches, so some delicate work with the SDS will be needed for them!

I think they may be prepared to cough up some contribution (although they probably figure that letting brother live rent free in their inheritance contributes a way toward it!). I don't think he was that bothered by the cost, but just wanted to avoid too much disruption, and hates the smell of paint!

[Owain]

Very delicate, if you want to sell them on Ebay afterwards :-)

Owain

[John Rumm]

Is there a big aftermarket for (flush mount) wooden back boxes?

[Owain]

There is a smallish market for old electrical collectables, and I doubt many wooden back boxes make it to Ebay, so you could make some sad anorak happy, earn yourself a beer voucher, and reduce landfill.

Owain

[John Rumm]

Spose I could throw in the old switch as well ;-))

[jim_in_sussex]

2 aspects come to mind:

Firstly, heritage is what it is.

15th ed IEE & (IIRC 14th & maybe earlier) had explicit rules for ring circuits. These were moved to OSG after 16th ed published, so they are now guidelines. The rule is now in OSG appendix 8 - Spurs page 149 in my vintage 1998 yellow cover copy.

IIUIC you can't easily be fingered (part pee aside) if you follow OSG to the letter, but you can depart if you are prepared to justify the design.

Secondly, the original post said the circuit dated from the 50s. If so it would most likely have been cabled in imperial 7/029 (earth conductor (CPC)is 3/029 IIRC) - that is approx 3mmsq with 1.25mmsq earth. 7/029 has a rating of about 30A on the same basis that modern

2.5mmsq has 27A. The whole installation would also have run off a conventional (probably wired) fuse box.

In very broad terms, and leaving aside any derating for the cable being run down the wall in conduit, each socket would have had the full nominal 30A ring current available. Nowadays spur ratings are fiddled. After maximum derating a 2.5mmsq spur cable is held to have a current rating of 20A and the socket at the end, whether single or double 13A is held to have a maximum demand of 13A.

Now that RCDs are in wide use and we have precise instruments to measure fault currents and we are very aware of things like earth loop impedances and so on, there would be a major quibble with a 3/029 (ie

1.27mmsq) CPC. It is a tad too small - in the 'standard' fault condition this could leave a fault greater than the 50V maximum - limiting the maximum cable run length. (Standard 2.5mmsq cable uses 1.5mmsq CPC & avoids the problem. Modern 4mmsq, with a 1.5mm CPC, suffers from the same fault - which limits its popularity.

One cynical way of complying with OSG is simply to insert ghost 13A sockets between each drop in the attic, though that might be queried by pundits pointing to the requirement to evenly spread load around a ring.

HTH

[John Rumm]

Yup, page 150 Annex 8 in the current one, with the specific bit on spurs on page 153 now.

Tis what I figured. I could not find any technical / moral reason for not using the same layout as is used currently, but thought I would check the collective memory banks to see if there was a reg I had missed! ;-)

Yup certainly imperial sizes (and stranded). I did not actually measure the cross section of the wire other than approximately by eye... did not seem much point since it all needs to go really.

It would be a problem on a long spur, but as a short "finger" from a ring it probably still copes.

That and it is harder to work with anyway...

[jim_in_sussex]

or perhaps not, IIRC when I tried out various designs for this place (sometime ago now), ISTR that I discovered suggested/standard circuits in OSG are generally limited by voltage drop. Interestingly no 4mmsq circuits are listed.

When you do the calcs using 4mmsq you find that (depending on open loop supply impedance and cicuit length) the potential fault voltage at the point where there is a live to earth fault can be over 50V. This is due to the small dia of the CPC. SFAIUI 50V is generally considered the max safe permissable voltage at a fault point, as you might touch an earthed item near the fault. ie the length of a 4mmsq cct in those circumstances is limited not by voltage drop, but by fault voltage.

Without doing the calcs my gut feeling is that 7/029 is likely to give similar results as 4mmsq, as the CPC dia is less than that of 2.5mmsq cable. IIRC the conductor & CPC sizes in 2.5mmsq FT&E means volt drop just scrapes home as the premier limiting length factor.

IMHO even if the new ring circuit uses 2.5mmsq, it should be checked by calculation for 2 or 3 typical spurs drawing 30A - in varying proportions - between them. The calc should be a doddle with a spreadsheet.

HTH