Discuss Ring finals and fused spurs in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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Pretty Mouth

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Evening all. The OSG says "As a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.".

Okay, so what is the maximum length for a fused spur then? I don't seem to be able to find the answer in the OSG or BBB. I assume the limiting factor is voltage drop, but how is this determined?
 
So based on that information how far can the furthest point of the ring be from the spur?
 
I think we're driving down the wrong road here Charlie. The OSG states that 1/8th rule is for unfused spurs, I want to know the max length of fused spurs. I assume there is a difference, otherwise why would the OSG say 'unfused spurs' rather than just 'spurs'?
 
Ahh sorry didn’t realise you mentioned fused spurs..
Yes as you say it would be down to volt drop for the length of the cable and current
 
That's the problem, determining the volt drop for a spur from a ring. How do you do it? Calculating the voltage drop for the spur bit I can do, but not sure about the ring part.
 
You can also do it using Zs tables and R1+R2 values..

By this method do you mean
L = R x 1000 / millohmpermeter

And then Vd calculation?

Which I presume would suitably work if conductors were the same csa?
 
That must be total length of cable then. But for sure the OSG also states an A1 RFC is maximum 100sqm floor space.

For a 100sqm room you could be using any length of cable from 20m to over 100m in length, dependant on the route the cable takes etc.
 
for VD on a spur, I'd estimate the length of cable from one end of the rfc and add the spur length, then use mV/A/m. this would give a pessimistic figure as the VD on the rfc part would be less.
 

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