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# Ring finals and fused spurs

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

#### 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?

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#### Charlie_

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Arms
So what’s the maximum length of a ring?

#### Dom123

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I’m pretty sure for a 2.5mm RFC protected by a 32a is maximum of 100sqm floor space.

#### Pretty Mouth

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OSG says for 2.5mm² on a 32A type B with RCD for disconnection times = 106m long

#### Dom123

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That must be total length of cable then. But for sure the OSG also states an A1 RFC is maximum 100sqm floor space.

#### Charlie_

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Arms
So based on that information how far can the furthest point of the ring be from the spur?

Arms
~15Meters?

Arms

#### Pretty Mouth

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If spurred from the board, 53m

#### Charlie_

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Arms
So based on that the maximum length would be 53/8

#### Pretty Mouth

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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'?

#### Charlie_

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Arms
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

#### Pretty Mouth

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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.

#### Charlie_

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Arms
You can also do it using Zs tables and R1+R2 values..

#### H

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Arms
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?

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