DIY a new ring main

[flameport]

with 20 amp radial in 2.5 mm² 32 meters, so single radial is out, even with 16 amp, 42 meters still not good enough, so would need to use 4 mm² so then at 20 amp 56 meter

Some of those figures might be valid for a circuit with 100% of the load at the end of the circuit.
They are not applicable to a circuit with distributed loads, or a circuit where the total load is less than the maximum rating of the protective device.
Suggestions for maximum volt drop are not compulsory either.
The OSG is a guide, not a design specification.

I would be happy to be guided by the electrician on whataver way they feel is best.

Appropriate if they are actually going to consider what's suitable for a particular installation, and are not someone who just slings a ring in because the old ways are not only the best, it's the only way they know.

 

[ericmark]

Some of those figures might be valid for a circuit with 100% of the load at the end of the circuit.

The guide was to consider two sockets fully loaded to 20 amp at the centre or end or ring or radial, and then any left over i.e. 12 amp with a 32 amp overload even spread, using that clearly with a 20 amp radial the load is simply 20 amp.

I will agree the whole idea of working out the volt drop based on a 26 amp load (Design current for circuit Ib) can be questioned, one could consider 13 amp at furthest point and 19 amp even spread, so Ib = 22.5 amp with a 32 amp supply, so with a 16 amp radial it would be 14.5 amp as Ib instead of 16 amp, but 42 meter to 47.5 meters is hardly worth writing about. I am including the Correction factor (Ct) in my calculations.

But there is nothing so annoying to find after laying a cable, that the volt drop is too high, only had it once, and lucky for me I had expressed my doubts about the volt drop before installing, and my foreman was unlucky that the shrink wrap machine was so voltage dependent, but lucky we could hid the expense of replacing the 4 mm² cable with 10 mm² so no one who could cause a problem was aware of our error.

I wrote the program in java script as I was worried with the PIR as it was then, now called an EICR that some one would find out if under size cable was used, volt drop wise, but the program pointed out that a loop impedance meter showing 1.42 Ω is not really that accurate, repeat the measurement and one can get 1.40 to 1.44 Ω the same at the origin so 0.33 to 0.37 Ω so once one adds the +/- into the calculations one really needs to be way out before some one can really point the finger.

So most electricians who have measured the prospective short circuit current or line - neutral loop impedance on a regular basis get to know how much they can get away with without working it all out first, it is the DIY person who has the problem, as they don't have the experience. So they need to work it out first, and can't really have a reasonable guess as to what the likely result will be.

But again today most equipment is designed to work on around 200 to 254 volt, and in the main our supply is near the higher mark, although officially 230 volt, more like 240 volt in most homes, so we can have a 40 volt volt drop and equipment still work, where the limit is 11.5 volt.

However easy for an electrician even if potential volt drop is over the limit to disregard it, as the EIC does not have a place to record the line - neutral loop impedance at the end of the circuit, so any error will not show on the paperwork. As long as he gets 1.38 Ω so trip will operate on the magnetic part, with a B32, he's OK, even if with a 0.35 Ω incomer he should show 0.94 Ω line - neutral centre of the ring to be within the volt drop, since he does not need to record it on the form, they get away with excessive volt drop.

However getting away with it, does not make it correct.

 

[Ragnar_AT]

The beauty of a radial is that you don’t have to jump from socket to socket but can branch out in multiple directions, even from the CU. In this case I‘d probably run one cable for the hall sockets and another to the lounge. That means the lengths don‘t simply add up, it’s the length of the longest branch that counts.

 

[JohnW2]

With a ring final with 32 amp MCB allowed 106 meters so that's OK, with 20 amp radial in 2.5 mm² 32 meters, so single radial is out ...

Your 'not allowed' is misleading. The only thing which is 'not allowed'(by BS7671) is a voltage drop so high that the impairs 'safe functioning' of corrected equipment, and I can think of very very few things whose 'safe functioning' would be impaired by even a massive voltage drop. The equipment might not work (satisfactorily or at all) if the VD was very high, but there would still be nothing 'unsafe' about it.

Even in terms of the guidance which leads to your so-often-repeated comments about the 106 metre (or whatever) maximum would only apply if the circuit was maximally loaded (e.g. 32A load on a 32A circuit) which I imagine would be a very rare situation in a domestic environment.

 

[Taylortwocities]

I was planning to leave all the channels and boxes open, conduit in place

You would need to do that, but you must do this:

You need to contact the electrician before you start so that, if willing, he can supervise your work throughout.

You’ll be lucky to find anyone interested in doing anything retrospectively.
But think about the landscape: electricians in the UK are busy busy busy. They are able to pick the jobs that they want to do. Doing half a job for DIYers is not high on the want list.

 

[pete01]

Doing half a job for DIYers is not high on the want list.

It just wouldn't be worth the bother.

 

[Harry Bloomfield]

No it's purely cost.

I've spent a decade or more over the years working on sites doing carpentry, groundworks, some plastering and all sorts so I'm more than capable of doing the physical chasing a fixing the back boxes in, I'm just being mindful of the fact that I may to it all perfectly well and still not get an electrician willing to come near it.

Find your electrician willing to let you do some of the work, let him/her design the installation, based on your spec., including marking where he needs chases, boxes chopped in, and floor boards lifted - then you do all of that, under his supervision.

 

[JohnW2]

It just wouldn't be worth the bother.

I think that probably 'depends'. Although TTC says that electricians are "busy busy busy", I know of at least a few ('good ones') in my area who are struggling to find as much work as they would like/need - so 'anything which pays' might be fairly attractive for people like them.

 

[ericmark]

Even in terms of the guidance which leads to your so-often-repeated comments about the 106 metre (or whatever) maximum would only apply if the circuit was maximally loaded (e.g. 32A load on a 32A circuit) which I imagine would be a very rare situation in a domestic environment.

Since I stated how the Design current for circuit Ib was calculated and with a 32 amp MCB on a ring final this is 26 amp, clearly selectively read.
And I have already said how volt drop is often ignored, but appendix 12 says:-

It does not matter if I agree with the allowed volt drop, this is what the regulations state.

You can argue if the 20 amp at centre (ring final) or end (radial) and 12 amp even spread is the correct method to use, but not as to the 5%, we did I seem to remember have around the 80 meters as the max for a ring final before 2008, as the volt drop was 4% but in 2008 it changed, to as shown, and I tried to work out where the 106 meters came from, so at the IET lecture on the subject I asked the question. It was then I was told how calculated, and once I entered the correct figures I also got the 106 meters.

 

[pete01]

I think that probably 'depends'. Although TTC says that electricians are "busy busy busy", I know of at least a few ('good ones') in my area who are struggling to find as much work as they would like/need - so 'anything which pays' might be fairly attractive for people like them.

Good point, I suppose if you can see how the cables have been run then theres not a problem.

 

[JohnW2]

Since I stated how the Design current for circuit Ib was calculated and with a 32 amp MCB on a ring final this is 26 amp, clearly selectively read.
And I have already said how volt drop is often ignored, but appendix 12 says:- ..... It does not matter if I agree with the allowed volt drop, this is what the regulations state.

You don't seem to get the difference between regulations and guidance.

Furthermore, as i said, when taking note of the guidance one can/should also apply some intelligent judgement (aka common sense) As I said, extrapolating from the 'maximum voltage drop' figures suggested in the guidance and the 'maxim cable lengths' you quote assumes a fully-loaded circuit, so I think it's perfectly reasonable for a designed to make a judgement as to how likely that situation is to arise.

Before you come back with a "the designer can't be certain" comment, I would remind you that, in relation to ring final circuits, a designer is only required to render it "not likely that any part of the circuit's cable will be overloaded for long periods" - so there is regulatory precedent for such judgements.

 

[ericmark]

OK I will admit I can look at the solar panel software and see my consumption for the whole house and it is rare not only for the ring final to exceed 5.688 kW the first peak shown as the batteries started to recharge, but the whole house has not exceed that load, only when the shower is used (8.738 kW) do I see a load greater to what a ring final could supply.

However I can condone when we want to modify a circuit which already exists that the volt drop may be exceeded, but here we are starting from scratch, to suggest one designs an installation with excessive volt drop by using a radial, when it would be so easy to install a ring final which would be within the limits, is beyond believe.

 

[JohnW2]

OK I will admit I can look at the solar panel software and see my consumption for the whole house .... and it is rare not only for the ring final to exceed 5.688 kW the first peak shown as the batteries started to recharge, but the whole house has not exceed that load, only when the shower is used (8.738 kW) do I see a load greater to what a ring final could supply.

Quite so.

You may recall the below that I have posted a few times before. It relates to the entire demand of my (very large) house over a period of about 1.35 million minutes (about 2.5 years), and shows that the total demand of the whole house only exceeded 30A for 0.11% of those minutes, and exceeded 35A for only 0.025% of those minutes. On that basis I think one can very reasonably say that any one ring circuit in the house was "not likely" to be loaded to 32A (or anywhere near that) for appreciable periods of time.

However I can condone when we want to modify a circuit which already exists that the volt drop may be exceeded, but here we are starting from scratch, to suggest one designs an installation with excessive volt drop by using a radial, when it would be so easy to install a ring final which would be within the limits, is beyond believe.

I don't personally think that the fact that one is designing a circuit from scratch makes any difference to the concept that one should utilise 'intelligent judgment'and common sense in heeding and applying 'guidelines' (provided one remains compliant with actual regulations).

 

[FrodoOne]

There have been many comments in favor of one Ring circuit and others in favor of one Radial circuit.
I would be in favor of two Radials - roughly, on on each side of the diagram in Post #22.

(Now, please see

which explains how a Ring Circuit is a "poor design"
and
which has extra expenses of testing - at the time of installation
and
"in the future".)

With two Radials
(and two associated RCBOs)
you would have "diversity",
and
the ability to use other outlets (with an extension lead)
in the remote event of a fault occurring on one of them
at little (if any) extra cost..

You would also have a maximum capacity of 2 * 20 A instead of 32 A, although this is unlikely to be necessary.

 

[Ragnar_AT]

Yes, two radials would certainly be an option, although in all likelyhood oversized for the expected demand. They‘d probably use less cable than a ring.