Ring main and spurs

[Farmer Giles]

I've been trying to sort out the electrics at a property my son bought about

18 months ago for some time. The lighting circuit was condemned when an inspection was carried out by an electrician because there was no cpc. I've rewired all of that, and now want to tackle the ring main. The ring wasn't criticised when it was inspected, but I think it's an absolute abortion.

The cables are stretched tightly across the loft - which is not acceptable in itself because he is shortly going to install 200mm of insulation - where most of them feed round junction boxes with up to six cables coming out of them (how they manage to get so many conductors into the connectors I can't imagine!). The ring main itself supplies only about 6 sockets, the rest being supplied by a weird assortment of radials and spurs. I intend to rewire as much as I can in the time available - he can get his loft insulation done free under some scheme or other, but it has to be done before the end of May. I want to incorporate as many of the spurs into the ring main as I can, but there are some where the socket drops will be too arkward and disruptive to disturb. One in particular, in the kitchen, causes me some concern. It is the only socket on that particular spur, but it is the one he uses for his electric kettle. What are the rules on spurs - and would it be acceptable to leave this one as it is?

Any advice/suggestions would be greatly appreciated.

[ARWadsworth]

any use?

Have a read and ask if there is anything not covered there.

[Farmer Giles]

Reading through that there doesn't seem to be any problem with the kitchen spur, and not a lot with some of the others - although I will incorporate them into the ring where it is possible to do so without too much disruption.

Thanks, Adam - once again!

[tinnews]

Do you have spare space for an extra MCB in the consumer unit? If so I'd simply re-wire as two radial circuits rather than a ring. A radial on a 20 amp MCB can be branched as much as you like and you never have to worry about whether something is a spur or not.

[ARWadsworth]

I cannot see any problem with the kettle supur (and remember that a kettle is a short term load.) Just make sure there are no spurs from spurs. Your biggest danger (as you already know) is the insulation. What's your plans to avoid problems with that?

[Farmer Giles]

Just to rewire leaving sufficient cable for it to go on top of the insulation. For the lighting circuit I used 1.5mm clipped to the joists - I understand that can safely be covered by the insulation.

[Farmer Giles]

That's a very interesting suggestion - thanks for it. Yes, there are a couple of spare ways in the CU - I think one is 20 amp and the other 32 - so I might well look at that option.

[ARWadsworth]

It can be safely covered by the insulation when clipped to the joists. The installation methods and cable ratings are near the top of here.

(sorry to use the uk.d-i-y wiki again but it is so easy to link to).

Your 1.5mm T&E cable is installed as ref method 101 and can carry 13A.

It is worth mentioning that a 4mm 32A ring circuit CAN be installed using method 101. 2.5mm spurs are allowed from this ring circuit however these spurs would need to be installed with a method that keeps the current carrying capacity greater than 20A (eg methods A, B, C or 100). There is nothing to stop you using 4mm on runs that you need to and 2.5mm for the rest of the circuit.

[ARWadsworth]

It also brings with it two problems. The first is that you will still have to move the cables due to the forthcoming insulation, the second is that a

20A radial circuit is not suitable for kitchens or other higher loaded circuits.

[tinnews]

They're much better in all sorts of ways IMHO. As you seem interested I've dug out my (slightly out of date) IEE "On-site Guide" to check the details. A radial with a 20A MCB is one of their "Conventional Circuit Arrangement" specs along with 32A Ring Circuits and 32A Radials (wired with 4mm2).

An 'A3 Radial' is as follows:-

Protection device - Any type, 20 amp Conductor size - 2.5mm2 Maximum floor area - 50m2

(The maximum floor area for a 32 amp ring is 100m2)

There is no explicit limit on the number of sockets or on the number of branches in the circuit. Obviously though you should check that

20 amps is sufficient for the circuit (allowing for diversity etc.).

[ARWadsworth]

Where does it say you can use any type 20A protective device? I would expect that BS 3036 20A fuse wire and D type MCBs to be excluded.

[tinnews]

In my "On Site Guide", table 8A in Appendix 8 it says for "A3 Radial" in the "Overcurrent Protection Device" columns 20 amps for "rating" and Any for "Type". It's only for an "A2 Radial" (4mm2 wire) that it specifies a 30 or 32 amp "Cartridge fuse or circuit breaker"

The on-site guide doesn't seem to worry about different types of MCB, you're pretty unikely to get a D characteristic MCB in a domestic consumer unit anyway. ... oh yes it does, it gives the typical applications for B/1, C/2/3 and 4/D characteristics so I'd guess that "Any" in table 8A *does* include D type MCBs.

[ARWadsworth]

Most intersesting. My 16th OSG does not have a "Type" column. It does however give more details in it's following paragraphs. Interestingly the OSG contradicts itself. Mine says that (this is the bottom of pg 152 on my copy) "the current carrying capacity must be at least 27A for an A1 circuit when using a BS3036 fuse". Now that would permit a 2.5mm clipped direct cable to be used. However, table 7.1 precludes BS3036 from being used on a

20A 2.5mm radial.

The maths would stop a 2.5mm cable been used with a 20A BS3036 fuse. ie 20 divided by the 0.725 fuse correction factor gives 27.6A which is greater than 2.5 T&E in free air.

It is not actually relevant to the OP as he has a CU not a fuse box.

I agree that it is unusual to see a D type in a domestic CU, but I have seen it! He got it from a sparky at work for free:-)

[tinnews]

Mine is a 16th Edition OSG too! It is "including Amendments 1 and 2" though which probably explains the minor differences.