Rule of thumb v calculations

[ericmark]

We were told in the guides that if we limit a ring main to 100 sq meters we don’t need to calculate but is that still true? As an apprentice my mentor told me no more then one role of cable that being then 100 yards and slightly thicker than 2.5mm² so when we went to metric I suppose that was no longer true?

So looking at calculations seems we can consider a ring main feed from a 32A MCB as having a “Design current for circuit” (Ib) of 26A. It seems we consider 20A drawn from centre point and 12A spread even so even that is a rule of thumb.
We are given the formula Ct = (230 + tp (ca²cg²-lb²/li²)(ip – 30))/(230 – tp) see page 258 in BS7671:2008 and by using this we can modify the 18 mV/A/m to 16.52 mV/A/m using that modified figure to get a volt drop of 11.5V we can use 106 meters approx of 2.5mm² cable.

Looking at ELI we can use with same calculations 101 meters of 2.5mm² cable.
So 1.504 is max ELI that equates to a volt drop of 11.5 volts.

Although a short circuit to earth may open the RCD a short circuit line to neutral still needs the 5 x 32A to flow to open a B type MCB using the magnetic part of the MCB.
I would still look for 1.44 ohms line to neutral to ensure this is the case. Yes I know the earth wire is thinner so this will be around 1.7 ohms line to earth which is higher than the 1.5 ohms to get correct volt drop but use a C type then 0.87 ohms earth to line or 0.72 line to earth is max and many RCBO’s are C type.

So we do a inspection report and we measure the ELI at mid socket with a C32 RCBO and it is above 0.87 ohms. Is that flagged as unsatisfactory? If so what then? Can the owner ask the installer to come and correct free of charge? It this why we have a 10 year test so the builders guarantee has expired before we find out the installation did not comply?

If we miss calculate or use the 100 sq meters rule and some clever clogs comes along and tests and finds we used too much cable what then. Changing to a 25A MCB is the only option if we can find one to fit the board.
Now if we say the Ze was 0.25 when we installed and now it is 0.35 can we get away with it? The DNO is only required to give a 0.35 ELI with a TN-C-S supply. Yet of course with a factory and a 300A supply this would be silly we would expect well below the 0.35.

Today we have to issue a schedule of test results, which of course works both ways protecting us in we can prove some one altered it and them by showing it was OK or it failed. But which socket did we read the earth loop impedance on? There is only one box and we all know we should measure it at all sockets and enter the highest reading. But how many of us do? And unless we can show a list of all socket readings how can we prove this was the worst reading?

And does it really matter anyway? With RCD protection do we really need a MCB to still have a magnet part to trip it within 0.1 seconds or is 100 seconds good enough? Because of the magnetic part a MCB will trip at same current in 0.1 seconds or 5 seconds but in real terms cable takes much longer to heat up and if it trips within 100 seconds then unlikely there will be any danger but that’s not what the regulations say.

So should we worry about cable length in ring mains and in view of above would radials be a better idea?

 

[dingbat]

As an apprentice my mentor told me no more then one role[sic] of cable that being then 100 yards and slightly thicker than 2.5mm² so when we went to metric I suppose that was no longer true?

1000 sq ft, was the rule. The domestic ring was based on the floor area of a three-bedroomed council house in the 1940s. When we went decimal it became 100 sq m.

So looking at calculations seems we can consider a ring main feed from a 32A MCB as having a “Design current for circuit” (Ib) of 26A.

No. Ib is 32 A.

So should we worry about cable length in ring mains and in view of above would radials be a better idea?

Yes and yes.

 

[ericmark]

As an So looking at calculations seems we can consider a ring main feed from a 32A MCB as having a “Design current for circuit” (Ib) of 26A.

No. Ib is 32 A.

I must admit the idea of working on 20A at centre and 12A spread even so giving an average of 26A seemed a bit odd to me. However this was how it was explained at an IET lecture. The guy from one of the scheme operators was head guy in area checking on other peoples work so assumed he knew what he was talking about!

I expected a bigger response. I hear so many times how people believe if within 100 sq meters (sorry not into metric yet still use miles where I live.) then you did not have to calculate which goes against the grain with me.

 

[Risteard]

No. Ib is 32 A.

I use 26A.

 

[dingbat]

No. Ib is 32 A.

I use 26A.

No. Ib is 32 A.

 

[dingbat]

However this was how it was explained at an IET lecture. The guy from one of the scheme operators was head guy in area checking on other peoples work so assumed he knew what he was talking about!

Which tells you all you need to know about experts.

 

[ebee]

The rule of thumb has stood the test of time and is quite a sensible approach.
Having said that you must realise that a rule of thumb is just that - a rule of thumb and not an exact calculation. So care must be taken. Modern homes perhaps draw more than the older ones did for a start and a lot depends on the likely usage.
Whe I do a ring I keep well within the 100sq m area served and less than 88m total length for a start (one of the previous OSGs give figures around 88/90ish for ring lengths so it seems a sensible maximum).
Balancing the ring for even loading and the balancing of rings to attain equal usage is a good starting point too

 

[NotHimAgain]

No. Ib is 32 A.

I use 26A.

No. Ib is 32 A.

32A - are you certain ?

Does that means I can design a ring final circuit protected by a 30A BS 3036 fuse to supply an assumed maximum load of 32A - oh goody .

 

[ericmark]

I had also considered that the design current for circuit (Ib) should be 32A however if one takes a figure of 32A instead of 26A then there are a lot of changes. Pre BS7671:2008 when we had 9.2 volt drop (4%) we would have been limited to an ELI of 1.2 ohms and cable length of 68 meters and this would mean many houses did not comply.
Even with the 11.2 volt (5%) now permitted we are looking at a 1.29 ohm rather than 1.44 ohm for earth loop impedance and maximum length of 85 meters.
Again many houses would fail and as I said the problem is customers could insist we correct these problems.
Looking at the whole purpose of the design current for circuit (Ib) it does seem reasonable to use the 26A figure as it will in reality as stated maximum of 20A at central point and 12A even spread. It is very unlikely one could ever draw 32A at a central point on a ring main. And we are only looking at volt drop so no overload will happen even if 32A was drawn at the mid-point.
I will therefore accept the figure given at the IET lecture of 26A. It makes sense.
This is of course only true with a British ring main system for radials supplying a single item this would not apply.

And this is where the problem lies it only works with a 32A supply and as we reduce the supply overload size we still retain the 20A at centre point on a ring or end on a radial so swapping the 32A MCB for a 20A MCB only reduces the design current for circuit (Ib) by 6A not 12A.

So with a 2.5mm² 20A radial our maximum ELI is 1.1 ohms and the maximum length is 32 meters which is very easy to exceed. It also points out how to split a ring into 2 radials one is very likely to exceed the volt drop limit.

Even at 16A the ELI is only 1.29 ohms and the maximum length is 42 meters so a 106 meter ring can’t be split into two radials. Even with an old 16th Edition house only if the split is central could one likely get away with splitting a ring into two radials.

The question is still the same do we check all sockets for earth loop impedance and if any are over 1.5 ohms do we consider that it tagged as unsatisfactory as the volt drop is likely over the 5% permitted. Also do we test the line / neutral loop impedance or prospective fault current and ensure it is better than 1.44 ohms or 160A?

The IET sample test sheet only asks for ELI so I will guess in many cases the neutral / line figures are not tested. To me the two tests quickly confirm if ring is complete. Assuming we have original figures to compare with. And in commercial premises where records were kept taking the ELI and PFC were really all that was required on a periodic inspection as we were not looking for DIY work.

Domestic is of course a different case with the DIY guy figure of 8 ring mains and spurs galore.

Having said that in commercial the 100 meter radial to 32A socket was common so quite often one finds undersize cable was used when electricians have not done their maths first and once fitted to tell boss sorry I need to do that one again is not easy. Likely flagged at next inspection though so soon corrected not 10 years like domestic.

 

[ericmark]

The rule of thumb has stood the test of time and is quite a sensible approach.
Having said that you must realise that a rule of thumb is just that - a rule of thumb and not an exact calculation. So care must be taken. Modern homes perhaps draw more than the older ones did for a start and a lot depends on the likely usage.
Whe I do a ring I keep well within the 100sq m area served and less than 88m total length for a start (one of the previous OSGs give figures around 88/90ish for ring lengths so it seems a sensible maximum).
Balancing the ring for even loading and the balancing of rings to attain equal usage is a good starting point too

I would agree with you and I have always worked on no more than one role of cable which by time the tails are trimmed is likely less than the 88m you talk about. Since twin and earth does not have meter marks on it had to work out how much used unless one starts with new role.

I think the modern home draws less current than the ring was originally designed for. Post war it was thought we would use electric heating and the 13A socket was designed to supply heaters in every room. With the central heating even electric because of storage radiators the demand on the ring main has dropped. Now we use loads of low power equipment rather than a few high powered items.

The change in plug design with the plastic sleeves on the line and neutral pin has to my mind reduced the ability of the plug to supply a full 3Kw for extended time and now the main current users washing machine, kettle and tumble drier don't draw 3Kw for as long as old three bar fires did. And the max power seems to have dropped from 3Kw to about 2.2Kw for most items.

The problem with the 100 sq meter rule is the introduction of the 1/3 rule on beams. Get a room where supporting walls are 3 meters and you use 2 meters going back and to from hole another 2 up and down wall so every socket needs 4 meters plus of cable just for drop put 6 sockets in a room and it is easy to use 30 meters of cable in just one room.

One housing estate I know of had the new beams in first few houses with knock outs for cables and this reduced cable used and first house was wired and tested before cables covered and it was well within limits. Then the builder cut costs and went back to plane wood beams and three houses were wired before the tester found the extra cable required had resulted in too high of an ELI. Those 3 had 25A MCB's fitted then rest had extra ring main.

And lets face it with an estate one wires each house the same and you don't re-design for each house. So when builders alter designs without informing the office of the electrical contractor mistakes are made. One does not employ super sparks to house bash on estate work, it's all pre-planned.

 

[ebee]

Good point about the heaters, yes 3KW used to be common and got plugged in a lot in cold weather.

A roll of 100m less tails trimmed - I'd start to feel slightly uncomfortable.
88 less tails would be better - actualy I try to make them a lot shorter anyway but just occasionally creep up towards this length and that's when I keep a close eye on it

 

[OwainDIYer]

....Get a room where supporting walls are 3 meters and you use 2 meters going back and to from hole another 2 up and down wall so every socket needs 4 meters plus of cable just for drop put 6 sockets in a room and it is easy to use 30 meters of cable in just one room.

And when rings were first in use post-war there were far fewer sockets per room. Although rings were intended to allow more sockets at less cost than radials, there were fewer appliances. Bedrooms might have had 2 sockets (bedside lamp and dressing table hairdryer) and the lounge 3 or 4.

On the other hand we have better circuit protection than rewireables now.

Maybe one step would be to stop using reduced size CPCs and use the same size CPC as the circuit cables - I think they do in France.

 

[dingbat]

32A - are you certain ?

Does that means I can design a ring final circuit protected by a 30A BS 3036 fuse to supply an assumed maximum load of 32A - oh goody .

That would actually work forever and a day - you'd never get disconnection on overload. A 30A BS 3036 fuse will allow 50A to pass for almost three hours!

Fair enough though. I should have said that the design current is the same as the rating of the OPD.

 

[dingbat]

And this discussion is yet another great reason for abandoning rings altogether.

If it was a simple and reasonable circuit design, none of that confusion would arise.

My general rule of thumb is that if an electrician thinks 'sockets' and then immediately thinks 'ring' he should consider retiring or re-training.

 

[NotHimAgain]

A BS 3036 fuse will actually allow 2 * In for up to 4 hours - we used to call it coarse excess-current protection .

Mind you I can't remember climbing over too many bodies killed by ring mains - can you?