50 metre limit on ring circuits

[jcp]

Hi

Can anyone tell me how to extend the ring main into my new kitchen/utility extension. The current ring main (installed by contractors 18 years ago) is at ceiling level, and drops down to sockets above ground floor skirting boards (and goes up to sockets at first floor level). The sockets are NOT spurs. Two rings cover the house (North and South wings), and each is about 70m long. I now need to extend the ring for the kitchen, but the length is already over 50m. I could put in a whole new ring for the kitchen alone, but just getting to the kitchen from the consumer unit takes up 10m each way, with another 8m each way to get out to the utility room. Surely there must be a way of exceeding the 50m limit.

Any help would be much appreciated

John

 

[Adam_151]

1) where did 50m come from? max length for a RFC in 2.5mm² is 80m-ish IIRC , should be in the OSG... max length for ELFI and VD considerations, using worst case Ze

2) Good idea to have kitchen on its own rin anyway

3) Are you familiar with the requirements of the building regulations regarding electrical work?

 

[jcp]

Hi

I am not fully conversant with building regs and electricity, but I know I need the work certified (or carried out by a self certifying electrician). However, I am a competant DIYer, and did the second fix of my house rewire (following major rebuild) 18 years ago.

For the current kitchen extension I have just had a self-certifying electrician round to discuss my ideas/needs. I already knew of area limits, but he mentioned the 50m cable length limit. Subsequently I looked up a few websites, and found this on http://www.diydoctor.org.uk/projects/ringmain.htm

"The maximum area for a ring main is 100 square meters....The cable itself can be up to 60 meters long if it is protected by a cartridge fuse and 50 meters long if protected by an MCB."

I found similar statements elsewhere, saying the length limit included the ring and any spurs.

The two main rings in my house (laid out by proper contractor who did the first fix) are both about 70m long (totting up approximate distances between sockets and making some allowance for cable routes through joists etc). I was concerned that they would not be allowed now as they are, let alone extended. My consumer unit has integral MCBs and RCD.

I wondered if the 50m was an approved basic approach, but longer lengths could be used if supported by proper design calcs (in usual building reg style).

Also, what about the recommended socket height (450mm). This seems very high. The mid point of most of my current sockets (outside the kitchen) are what I would call a conventional 300mm height above the floor.

So I just want to know what I can ask the electrical contractor to do!

Cheers

John

 

[pcboffinuk]

jcp,

According to table 7.1(pg 42) of the On Site Guide (which you should consider buying if you want to know anything about how to DIY electrical), the maximum length for a 32A ring on a type B breaker(the most common arrangement) is 84 m so that should give you something to work with.

Having the kitchen on a separate ring is definitely a good idea, especially if you have washer/dryer & dishwasher in there.

The table mentioned above is a guide only, and means that you do not need to do all the calculations - In most domestic situations it probably won't help to do all the calculations to try to get a few extra metres though.

The socket height is a building regs requirement and is intended to make reaching them easier for a disabled or elderly person (with the aging population this will be more an issue in years to come) - for the same reason the recommended switch height is lower than most existing houses.

However, the rules only apply to new builds and complete renovations. As you say you have an extension it may be down to your local building control department as to whether they enforce it or allow you to match the rest of the house. Certainly there is no requirement to raise the existing ones, even when replacing them.

Hope that helps, if not then feel free to ask further.

Gavin

 

[Adam_151]

And if you want to go further than that, you can up the cable size, its a bit late now but I could work out what you get for 4mm² if need be, I'm not sure whether the limit of 84M for 2.5 is constrained by ELFI or VD, but if the former then measurement of the Ze value rather than assuming worst case for supply type could allow you to take it a bit further

I wondered if the 50m was an approved basic approach, but longer lengths could be used if supported by proper design calcs (in usual building reg style).

Thats very much how BS7671 and the OSG opperate, chuck in a standard circuit arrangement and you're laughing, in a more unconventional situation design calculations come into play and can prove compliance of much more unusual arrangements, etc (which is why periodic inspections need to be done by people who know the regs inside out - expecially if the design calculations have gone missing... though I suppose more exotic arrangements could just be marked down as 3.... but I digress)

Building regs for electrical (the new part P) don't actually require BS7671 to be followed, just that it needs provisions to be made for safety from fire and shock (rather vague isn't it!), the official guidence recommends BS7671 or other EU standard. BS7671 is what you do it to unless you have a good reason not to tho...

Also building regs require it to be notified to the council, not a problem if they are already involved with inspecting the structure, etc tho should be no extra charge

 

[pcboffinuk]

Good points by Adam as always...

If you are already paying LABC to certify the building work then check they don't charge you extra for certifying the electrical installation - they are specifically prohibited from doing so but most seem to do it since they normally sub contract to a spark. My local council's charges are based on the value of the work, but add £140ish for electrical work.

(If they are only certifying electrical work then of course they are allowed to charge)

There is a circular somewhere that states this if they get funny about it and a search on here will probably find it.

Gavin

 

[jcp]

Thanks all - that's very useful.

I actually have 4 MCB slots on the RCD side of my consumer unit. Beside the two rings I have mentioned, 1 slot is wired to another ring for my attached garage with TV/playroom above, and the last slot is wired to an outdoor 'terminal' that I never continued to a shed (I was worried about earthing arrangements). I think I will wire the new kitchen as a separate ring on that last MCB (and I have now been told I could always wire the shed as a fused spur off the garage ring).

It seems from what I've seen elsewhere that the earth arrangements of the supply are important. My supply is from a PME overhead cable.

Note that I wanted north & south rather than up & downstairs rings originally to limit interference from the kitchen machines on my Hifi. I like the idea of a separate kitchen ring.

Thanks again

John

 

[lampstart]

HI When I built my extension 3 yrs ago the building inspector informed me

that the new minimum heights for sockets did not apply to extensions,only

to complete new build.I will be interested to see the replies to the ring cable

length query,on a big bungalow it would take more than 50 m to coplete a

ring.

 

[ebee]

The 84m to 88m total ring length limits are based on worst case scenario for your incommimg supply and 100 sq m area served based on likely loading.
The ring length might be increased slightly based on actual measurements taken and calculations being made. One thing often forgoten is to sensibly arrange points on the ring so the load is evenly spread (or near to midpoint) to avoid one leg of the ring taking a disproportionate share of the total load. A seperate kitchen ring is often a sensible approach (A washer and dryer alone might take most of the rating and a kettle might take it over - small overloads of short duration are usuall and acceptable, large overloads should trip the fuse/breaker and medium overloads of long duration should be avoided because it make take over two hours to trip and the conductor might be more than a bit warm)

 

[securespark]

HI When I built my extension 3 yrs ago the building inspector informed me

that the new minimum heights for sockets did not apply to extensions,only

to complete new build.I will be interested to see the replies to the ring cable

length query,on a big bungalow it would take more than 50 m to coplete a

ring.

Rings are limited to 100m²....