ring design and voltage drop

[Guest123]

Hi lenny, im with i = p/u. Can we see you full calc
Cheers

Ok, VD will limit the length of your RFC before any other factor. So if we use our max permitted VD of 11.5V in our calc for finding the max permissable length we 'should' arrive at a value within the 5% tolerance.


So (4 x 11.5 x 1000)/(Ib x mV/A/M x Ct) = max permissable length.

= (4 x 11.5 1000)/(26 x 18 x 1) = 98.29M.


As I said it can be engineered further to give a longer length but I personally use the above.

 

[esc]

Cheers for your help spinlondon, things are getting clearer. Have always taken appendix 15 as is, with no explanation of the design behind these circuits. Radial are fine, the ring threw me, so thanks for your help in this matter
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[esc]

Thanks for the detail in your post malcomsanford, thats a great help.
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[esc]

Thanks for you explanation on max permissable length lenny. Things are making sense
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[i=p/u]

yes good post lenny &co

 

[i=p/u]

nice sums malcom, but can you say lets say you have values for r1 r2 rn... if you designing or longest possible

That is how I work out the length and volt drop in a Ring.

Length

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 if you have a measured value of say

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

Volt Drop

Divide the length by 4 = 19.57 x 18 x 26/ 1000 = 9.16v

Bs 7671 pg 258, 6.1 correction for operating temp.

This calculation give you a ct of .923, x 9.16 = 8.45v

or

4 x 8.45v x 1000/26/18 = 72.22 / .923 = 78.24m

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

average = (32 + 20)/2 = 26 which is the value that Lenny posted.

 

[i=p/u]

so is the moral of the story, dont go over 100m meteres of 2.5mm on a A1 ring circuit???

 

[Deleted member 26818]

I believe it's 10m for 2.5mm² protected by a 32A B type MCB.
That's according to Table 7.1 in the OSG.

 

[i=p/u]

kooool spin london was there all along, knowing the sums is good tho

 

[Deleted member 26818]

Sorry that should be 106m not 10m.
Don't seem to be able to edit it for some reason?

 

[i=p/u]

knew what you ment .... and that 106m meter from table 7.1, does this include the approriate disconnection time???

 

[i=p/u]

yes sorry