cable rating on a spur?


Postby skin » Sat Aug 30, 2008 10:58 pm

hi all
as a practising part p electrician, i needed clarification on a issue i have regarding double sockets that are on a spur off a ring main socket... the rules state that the number of spurs are limited to one off each socket on a ring. what would happen if you plugged in 2 3kw heaters into that spur socket? as this spur is on a single 2.5 cable which is only rated at about 19 amps. the current that the 2 heaters would draw would be 26 amps. now the mcb will not trip but the cable will overheat and surely melt and maybe catch fire. how can you cover yourself from this kind of possibilty? maybe use 4mm cable?
thanks for reading
skin
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Joined: Fri Jun 01, 2007 1:07 am

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Postby BLAKEY1963 » Sun Aug 31, 2008 12:31 pm

[quote="skin"]hi all
as a practising part p electrician, i needed clarification on a issue i have regarding double sockets that are on a spur off a ring main socket... the rules state that the number of spurs are limited to one off each socket on a ring. what would happen if you plugged in 2 3kw heaters into that spur socket? as this spur is on a single 2.5 cable which is only rated at about 19 amps. the current that the 2 heaters would draw would be 26 amps. now the mcb will not trip but the cable will overheat and surely melt and maybe catch fire. how can you cover yourself from this kind of possibilty? maybe use 4mm cable?
thanks for reading[/quote]

SKIN
The easiest way to protect the 2.5 mm2 cable against overloaded
appliances , if the twin socket is used for the spur is to
fit next to it a fused spur unit , so a maximum load of only 13amps
can be drawn


BLAKEY1963
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Joined: Wed Jun 13, 2007 10:57 pm


Postby mephistico » Sun Aug 31, 2008 1:48 pm

It one of those anomalies of the regs. You have to take diversity into account. What are the chances of someone using a 26A load on a double socket?
Your argument is perfectly sound but if you have ever calculated the maximum demand on a house it comes out at 160A (typical) and your service fuse is 60 or 80A.
It based on not everything been used at the same time.
mephistico
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Joined: Sun Aug 31, 2008 6:23 am


Postby ericmark » Sun Aug 31, 2008 2:29 pm

Hi Skin nice to see a thinking man.
You are of course correct you could draw 26 amp and 2.5mm² 70°C flat twin and earth is rated between 13.5 and 27 amp. Reading Appendix 15 it seems 2.5mm² to BS6004
Electric cables. PVC insulated, non-armoured cables for voltages up to and including 450/750 V, for electric power, lighting and internal wiring Insulated cables, Polyvinyl chloride, Sheathed cables, Electric cables, Power cables, Single-core cables, Multicore cables, Electric conductors, Cable sheaths, Cable cores, Lighting systems, Electrical properties of materials, Mechanical properties of mat

However 90°C is up to 33 amp clipped direct either as twin and earth or Ali-Tube and the latter complies with BS 8436.

So
1) Twin and earth could be OK for 26 amp if cable installed correct.
2) Reading IET forum seems double sockets are only rated to 20A.
3) And there are 2.5mm² cables able to handle the extra current.
4) It is very unlikely that two items taking 13 amp for any length of time will ever be used in the same pair of sockets.
Therefore it is not completely against the rules but very much on the edge.


Eric
ericmark


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