exactly why is a spur off a spur dangerous?

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was thinking about this last night.
Is the problem overloading the section of spur closest to the ring?

If you connect a spur off a spur the max load from the furthest socket is 13A, therefore the max load off the first bit of spur is therefore 26A.

If you connect the spur using 4mm cable then it can carry 25A safely.
If the main trip that protects the ring that the spur comes off is a 16A MCB, surely you cant overload that section of cable.

Surely this is the same principle as a radial. (just fed from a ring rather than a CU).
Also in theory isnt the main supply to your house a ring, and therefore if you connect a radial circuit to a your CU you are connecting a spur off a spur (albeit with a protecting MCB)
 
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dabaldie said:
was thinking about this last night.
Is the problem overloading the section of spur closest to the ring?
No - the problem is overloading the spur cable.

If you connect a spur off a spur the max load from the furthest socket is 13A, therefore the max load off the first bit of spur is therefore 26A.
Which is more than 2.5mm² can safely carry

If you connect the spur using 4mm cable then it can carry 25A safely.
If the main trip that protects the ring that the spur comes off is a 16A MCB, surely you cant overload that section of cable.
If if if..

1) People don't use 4mm² for spurs, they use 2.5mm² (if you're lucky :confused: )

2) Rings are protected at 30/32A, not 16A...

Surely this is the same principle as a radial. (just fed from a ring rather than a CU).
Not quite, as a radial socket circuit has to have overload protection provided by its fuse/MCB, as there is nothing else to provide that (i.e. if you have 10 on-gang sockets on your circuit, you can draw 200-250A for an hour or more based on the overload protection provided by 13A BS1362 fuses. A one-socket spur is protected from overload by the plugtop fuses in the appliances plugged in. You also need fault-current protection, and a 30/32A device will adequately provide this for a 2.5mm² cable, provided it meets the disconnection time requirement in the first place.

Also in theory isnt the main supply to your house a ring, and therefore if you connect a radial circuit to a your CU you are connecting a spur off a spur (albeit with a protecting MCB)
Not sure what your point is....
 
Quote:
Also in theory isnt the main supply to your house a ring, and therefore if you connect a radial circuit to a your CU you are connecting a spur off a spur (albeit with a protecting MCB)

Not sure what your point is....
my point is that a radial is just a series of spurs off spurs off spurs off spurs...
therefore connecting to the main supply at the CU (the incoming ring)... with a radial is connecting a spur off a spur off a spur etc.. off a ring.
:eek:
 
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dabaldie said:
Also in theory isnt the main supply to your house a ring
No, not that it makes any difference.

my point is that a radial is just a series of spurs off spurs off spurs off spurs...
therefore connecting to the main supply at the CU (the incoming ring)... with a radial is connecting a spur off a spur off a spur etc.. off a ring.
:eek:
So?

It's all about overload and fault current protection, not topography. If you start your spur from a ring with an FCU, thus providing upstream protection to the spur cable you can have whatever tree structure and as many sockets as you like on the load side of it.
 

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