Installing ring main

Postby Peter Do It All » Mon Oct 27, 2008 10:29 am

I'm about to renew the ring main in my bungalow (solid floors). Even after splitting the load into two or more circuits, I still have a problem restricting the lengths of cable in each circuit to the required 50 metres. This is because all sockets need to be supplied from above. Is it acceptable to run a ring in the loft space, and take a series of spurs from it via junction boxes?
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Postby TheDoctor4 » Mon Oct 27, 2008 5:27 pm


Have you had a look at the DIY Projects area at all: especially the project titled "Electrical, extending a ring main". This may provide you with some assistance.

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Postby ericmark » Mon Oct 27, 2008 9:02 pm

Where did you get 50 meters from? I make is around 106 meters at 18mV/A/m and a volt drop of 5% this gives at 32 amp about 79 meters.
But it seems instead of working on the max at the centre of ring main we assume 20 amp at centre and the 12 amp even around the ring plus page 258 6.1 gives a correction for operating temperature. Using this one gets a little more giving full 106 meters for ring main I will try to find excel formula for you.
Last edited by ericmark on Mon Oct 27, 2008 10:13 pm, edited 1 time in total.

Postby ericmark » Mon Oct 27, 2008 9:55 pm

Nomial volts 230 = B1
Correct factor Ct 0.923 = B2 cal below (B1+B3-((POWER(B5,2)*POWER(B4,2))-POWER(B6,2)/POWER(B7,2))*(B3-30))/(B1+B3)
Max permitted operating temp tp 70 = B3 for thermoplastic
Rating factor for grouping Cg 1 = B4
Rating factor for ambient temperature Ca 1 = B5 from table 4B1 at 30deg
Design current for circuit Ib 26 = B6
Tabulated current-carrying capacity of a cable It 40 = B7 double 20 amp as ring
Orignal mV/A/m 18 = B8 for 2.5mm cable
Corrected mV/A/m 16.614 = B9 B8*B2
5% of 230V = volt drop of 11.5 = B10 230*5%
Volt drop per meter 0.431964 = B11 remember doubled up B9*B6/1000
Meters = 106.4903557 Remember times 4 as ring main B10/B11*4
So for a ring main 106 meters max. I have shown the squares used in excel so this can be repeated for other values. For example as a radial B6 and B7 both become 20 and it is not times 4 so max length 32 meters which is a good reason to use ring mains. Remember this is for volt drop only as all sockets now on RCD.

Postby sparx » Mon Nov 03, 2008 8:57 pm

Hi sorry for late reply as been on hols but I must point out a few facts obviously being overlooked with regards to domestic ring circuits.
The IET have stated that so long as a circuit serves no larger area than 100 sq. metres of FLOOR AREA no further calcs are required! End of story!! 2.5mm2 cable on 32A type B mcb, job done, repeat NO FURTHER CALCS REQD.!!!! Cable lengths irelevent!
If 101 sqM or more then 2 rings required with load spread as evenly as practicable between them.
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Postby ericmark » Fri Nov 07, 2008 12:16 pm

Historically 2.5mm² radial as been 50 m² and 4mm² radial 75 m² and 2.5mm² ring main 100 m² but since these are no longer stated as a “Rule of thumb” but have been re-classified as a historic item I would consider since July 2008 when the corrigendum was issued one today can no longer use these figures without verifying that the impedance and volt drop are within limits.
I would agree in most cases following the historic rule of thumb would mean it would comply but the one third rule on drilling beams has resulted in some premises of the 106 meter limit being exceeded although the 20 amp at centre of ring and 12 amp spread throughout it’s length is also what may be considered as “rule of thumb” and on a circuit where there is no fixed current draw it is hard to use any hard and fast rules. I have not bought the latest guides and these are as the name says only guides and we have all read to Electrical Safety Council’s guidance where they condone the continued use of lighting circuits without earths so long as certain percussions are taken i.e. use of class II equipment yet it quite clearly does not comply with 17th Edition. Clearly some common sense must be used but unless one can find a report from a recognised authority allowing you to do something without having some qualifications oneself it is hard to persuade figures of authority that what one has done is OK. It is far easier to just follow regulations.

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