working length of circuit from resistance for volt drop

[brucelee]

Hi all

could someone please be so kind to explain how i work out the length of a existing ring final circuit 2.5/1.5 T/E

and in turn Volt Drop

i know to work out length its L= R x 1000 divided by m/ohm/m but would i use the r1 measurement eg 0.58ohms
rn was 0.58ohms
r2 was 0.96
or from the highest measured value from step 2 cross connection of line and Neutral which is 0.33 ohms which was from a spur in the loft

any help would be appreciated think i am just confusing myself lol


to work length of circuit
is it
0.58x 1000 divided by 7.41
and then volt drop
0.58 x 2 x 1.2 x lenght divide 1000
cheers

 

[malcolmsanford]

2.5mm T&E ring.
1 conductor 2.5mm CSA @ 7.41mohm/m
1 conductor 1.5mm CSA @ 12.10mohm/m
{from page 166 OSG}

so your values give..
r1 @ 0.58/7.41 x 1000 = 78.27m
rn @ 0.58/7.41 x 1000 = 78.27m
r2 @ 0.96/12.10 x 1000 = 79.33m


That is how I work out the length Karl.

The Volt Drop mate is

Divide the length by 4 = approx 20 x 18 x 26/ 1000 = 9.36v

Bs 7671 pg 258, 6.1 correction for operating temp.

This calculation give you a ct of .923, x 9.36 = 8.36v

or

4 x 9.36 x 1000/26/18 = 98.3 / .923 = 85m

As I used approx 20 metres in the VD calc to round it up Karl you see it is ball park and if I used the percentage length of 78.27 it would have been closer

It assumes 20 amp at the furthest point and 12 amp distributed evenly around the ring so,

average = (32 + 20)/2 = 26

 

[brucelee]

mate your a gent and should be a tutor lol as none i have learned with have explained this and the college course i am doing at minute for lv3 is the worst course i have done

i have been racking my brains to work it out can it be done from the measured impedence as the regs say
was using 32A as my Ib which you have corrected kindly most all sockets were around 0.29ohms apart from the spur to loft which was 0.33 for R1 +RN
thanks malc and tel who replied by pm

 

[brucelee]

Malc jsut had a look at the relevent page you have mentioned in the regs

I wouldnt have been able to work it out from that mate lol that is one calc i am lost on i will copy your aswer for future reference

i had a feeling to work lenght from the resistance of the line ie r1 but didt kow to only allow 26A as my design current