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ring circuit length.

Postby sparx » Wed Dec 29, 2010 8:54 pm

question for ericmark and others.
please tell me where this 106m max length of 2.5 on a ring comes from?
It's raised its head a few times recently but as I've only been wiring rings since 1963 I seem to have missed it!
In all that time the rules have been about square area covered by circuit not exceeding 1000sq.Ft/100sq Mtrs.
equal balance of outlets on each leg, no more spurs than on ring etc.
The rules have always said if these criteria are followed no other calcs are reqd. so which reg states max length please.
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Postby ericmark » Thu Dec 30, 2010 11:24 am

My first consideration as to ring main length calculated maximum length well below the 106 meters voiced in the IET lecture on 17th Edition and I had to visit a second lecture to get the answers.

The first consideration, which I had not originally considered is the loading for a 32A MCB protected ring main is calculated as 20A from central point and 12A spread evenly around the ring so 26A is the figure taken as average current in a ring main.

Since 2.5mm cable is rated at 21A and we are using 2 lengths the total rated value is 42A so we are not drawing anywhere near the full capacity Page 258 of BS 7671 gives the formula to correct for operating temperature.

I remember when shown this I got the 106 meters but when I tried to re-calculate I am coming out with 100 meters so some where I have entered a wrong figure maybe you can spot it?

Values used Measured Ze 0.35 standard TN-C-S supply guaranteed figure.
Max Zs 1.4375 (46/32 as given page 49 right hand margin)
Line conductor 2.5mm² earth conductor 1.5mm²
Ambient temp 30ºC Nomial volts 230 Max permitted temp (tp) 70ºC rating factor for grouping (Cg) 1 Rating factor for ambient temp (Ca) 1 design current (lt) 26 Tabulated current carrying capacity (lt) 42 (ref method 100) original mV/A/m 18 corrected mV/A/m 16.51973 this gives a cable length of 100.84654 Meters but volt drop is only 10.8287 volts and we are allowed 11.5 volt
So where a RCD is used the ELI can be increased and the Zs value is only important to keep the volt drop within limits so the Zs can be increased to 1.504 ohms and that gives a volt drop of 11.49089 volts and cable length of 107.01325 Meters.
If I was using a calculator and correcting to 4 s.f. then 106 meters would be answer.
Correction factor is 0.9177627

Sorry the maths is a tad complex I used excel so I could change the figures entered and work it out for 4mm for example.

If you want spread sheet email me my call sign at hotmail and com not uk.

73 Eric GW7MGW

PS don't think I would want to take the chance that cable is spot on size and think one role rule of thumb is way to go. By time trimmed to length around 90 meters!
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Postby ericmark » Thu Dec 30, 2010 11:35 am

Regulation wise Appendix 12 voltage drop in consumers installation and Table 41.3 max ELI.

It was considered that 100 sq meters would need less than 100 meters of cable to wire but with 1/3 rule for drilling beams now easy to exceed 100 meters. I note some now wire a house side to side rather than up-stairs and down stairs as this reduces the cable for each ring main so reduces the ELI.

Not sure I like the idea as easy to be caught out if one does not prove dead and although electricians may prove dead not so sure about the DIY guy!
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Postby ericmark » Fri Dec 31, 2010 7:50 am

There has been some debate over the IB and I had 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.

I found the calculations as I said a bit much for fag packet and used Excel on my PDA. And the figures quoted are from that program I wrote a few years back. My worry is that some time in the future some clever clogs will start to test the prospective volt drop to drum up work and we could be called back to old jobs where we did not exercise enough care ensuring the volt drop was within limits.
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Postby sparx » Fri Dec 31, 2010 4:04 pm

Hi Eric,
usual thorough answer from you,
nice maths but not sure correct.
Issue is using table for voltdrop with 2.5/1.5 over 106 meters, surely its either 5/3mm2 as doubled up effectively in parallel or the ring is being treated as a radial ie only one end connected at supply?

Anyway my point was that by definition all circuits must be 'designed' eg if intending to install a swa submain to another dist board then we would need to consider as you say all parameters such as load, distance, protective device, voltdrop installation conditions etc.
However the IET state that to save us reinventing the wheel each time, the 3 'standard circuits' they publish in appendix 15 if followed correctly 'need no further calculations' since all domestic situations are covered by their criteria & will comply with regs.

As to the routing of cables the 'centre 1/3' rule has always been in force for the depth of joists, the only change being the span width location where it used to be as near as possible to each end or support wall, now plumbing is from support out to 1/4, electrics between 1/4 & 1/3 of span.
which uses little more cable than before imo,

Thanks for interesting reply, no doubt not the last one,

73's Mac. G6NLZ
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Postby ericmark » Fri Dec 31, 2010 10:50 pm

I do agree it seems daft to do a load of calculations every time but we have for years used B32 MCB's and know without looking we must get an ELI reading of 1.44 ohms or less.

Appendix 15 lists 4 bits it does not cover electric shock, thermal effects, overcurrent, and selection and erection of equipment.

It says regulation 433.1.5 can generally be achieved by it does not say it is achieved by.

The adopted limit of 100 sq meters is a historically thing it does not say this is still the case.

So good guide but it no longer says if we follow it then calculations are not required.

I know of a few jobs I got wrong. Not dangerous but still not fully complying. And after accident to my hand can't even cross my fingers any more!
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