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4mm spur off ring


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6 replies to this topic

#1 m4tty

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Posted 08 December 2009 - 07:58 PM

Hi,

Is this allowed ??

To spur off the 2.5mm ring using 4mm cable. i.e. creating radial circuit from ring.

I thought spur off spur is not allowed??

Thanks

#2 kme

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Posted 08 December 2009 - 08:02 PM

Although the 4mm can handle the current, you could be drawing from a point in the ring where you`d prospectively cause overload in one of the legs of the ring.

It`d be better to take the 4mm back to the CU, or feed 1 socket only; thereby negating the necessity for 2.5

HTH

KME

#3 choccy

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Posted 08 December 2009 - 08:03 PM

a spur from a spur is a no no but you are asking can you take a spur from a ring and this is allowed.:D

#4 ian932

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Posted 08 December 2009 - 08:59 PM

I would say that you should only feed the max of 1 double unless you come off the output side of a FCU sat on the ring.

While in theory the 4mm could take more points, what happens if the ring is spilt into 2 radials down the line, this will leave your bit plus whats left of the ring feeding more than 50m2 floor area and as kme said if your not in the mid point of the ring then current will not split equal each way (path of least resistance to origin and all that good stuff)

#5 Andy™

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Posted 08 December 2009 - 09:49 PM

a ring is designed with a max draw of 13A at any point (or something like that). so although the 4mm can safely take the full 32A, you would be overloading a point on the ring, and if its close to one side, you could easily overload one leg of the 2.5

#6 spinlondon

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Posted 09 December 2009 - 03:34 AM

If you spur off of a ring in 4mm T&E, then you will not require an FCU.
You are allowed one double socket outlet per spur when using 2.5mm T&E, because the both the cable and the double socket outlet are rated at 20A.
There is some credence to the suggestion that you may overload part of the ring if the spur is not taken from the mid point. However it is not a real concern for most domestic installations.

#7 batty

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Posted 09 December 2009 - 11:10 PM

My thoughts are its better to take a spur of ring in 4 mm than extend ringmain again and again. I have been to properties that have been extended where ringmain has been extended so much readings are of the scale. Although when I am doing extensions I put new circuits in unless its a real problem.
Batty




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