Spurs and heat in cable.


Postby gollum » Tue Aug 31, 2010 10:12 am

I have jst bought old house where an extension was completed in 1975. Now I have started to hack plaster off I can see they have a spur from a spur from a spur.
Now from what I understand this is incorrect as only 1 spur at a time is allowed but you can have as many spurs as sockets on the ring . Correct so far .? .

So it seems to me there must be an awful lot of houses that have wiring such as this and I just wanted to understand why the limitation is 1 spur.
I have had an explanation from a friend of how loose connections can vibrate under load and then heat up. But is the 1 spur limitation purely due to heat building up due to amps through the 1 piece of 2.5mm cable. ? I mean are there any figures for the safe amp load on 1 cable of 2.5mm before it heats to a critical level where it may catch fire.?
Rgds.
Gollum.
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Postby ericmark » Tue Aug 31, 2010 6:14 pm

2.5mm cable is rated at approx 20A and with a ring main it is fed from both sides so total rating is 40A but there is a chance that the current is not even in each leg so we reduce to total to 32A.

When we spur off a socket we use just one length of 2.5mm cable so we need to keep it down to 20A and by limiting it to one socket then then total is 13A or 26A with twin socket. Since the likelihood is that both plugs will not draw the max then we have kept it to the 20A max and we rely on the 13A fuses in the plug to limit the current.

There is also a limit of 3 meters regulation 433.2.2 from the point of current carrying capacity change which is the socket it is spurred from. To the point where the fuse is fitted socket it supplies if is installed in such a manner as to reduce the risk of fault to a minimum.

However if the ring was supplied from a 20A fuse/MCB then you could have as many spurs of spurs as you liked without danger and where it is found that spurs are taken off spurs that is one method of removing the hazard although not strictly following regulations.

The other method is to fit spur connection units which technically means you are making a new circuit but Part P seems to have different definitions to the IET and does not class it as a new circuit.

The BS 7671:2008, Corrigendum (July 2008) issued by the IET is a free download and shows in pictorial form what is allowed. Google "theiet.org/publishing/wiring-regulations/updates/" if the syops let it through. This shows the other method of wiring sockets as well called a radial and this can be with 2.5mm and 20A fuse or 4mm or 6mm can be used and a 32A fuse as with the ring main. Hear again when 4mm is used you can spur with 2.5mm for one socket. When 6mm is used there is just not enough room to add another cable. I have found where 4mm has been used and some one has not realised it was 4mm and has extended the radial with 2.5mm cable thinking it was a ring. However although easy to make mistakes that is no reason why 4mm radials should not be used.

However it is not the size of cable but the current carrying capacity that matters and although as a surface cable the 4mm is rated 37A with method #100 it is only rated at 27A the only way would therefore be to use cable rated at 90 degs C instead of 70 degs C so with 90 deg cable it is rated 45A and then de-rated by multiplying by 0.78 giving 35.1A which can be protected by a 32A MCB or 30A fuse. However it is so easy to get 70 deg and 90 deg cable mixed up this can really cause confusion unless using something like Ali-tube cable which is then obvious as to type.

The same of course applies with a ring main and one could use 1.5mm cable if using mineral insulated type. The only place where this is used is old stone cottages so one can hide the cable in the mortar. With the cable able to run at very high temperatures the cable will not be damaged but of course other items against the cable may not have the same resistance to heat so mineral insulated cable is not used much now. In the main the Ali-tube has taken it's place.

Does this explain why? So to recap where spurs are taken from spurs either use FCU at origin or replace the main fuse for 20A if buried or 25A if surface. Do remember only flex has a fixed amp rating all other cable varies according to how it is run.
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Postby sparx » Tue Aug 31, 2010 10:49 pm

hi,
current rating only part of the story, but buried in a wall it will be less than 20A.
You say there must be many such installations but in my experience very few with more 2 on a spur.
The point being that all circuits must be 'designed' to comply with regs.
to save repeated calcs. to show compliance the IET set out some 'standard circuits' one being a domestic ring circuit which if we stay within those design parameters no further calcs are reqd.
The basics being;
One loop (un-bridged)not to exceed a floor area of 100M2.
No more as spurs than on ring.
Unfused spurs to only feed one single or double outlet
Fused spurs can feed unlimited number of outlets and can be in 2.5 or 1.5m2 if volt drop due to load accounted for.
Thats it, so you have a non standard circuit.
it can either be corrected by alteration or by inserting a fused spur (FCU) at the first spur point, thus limiting full load on the multi spur to 13Amax continuous .
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Postby gollum » Thu Sep 09, 2010 9:47 am

Hi and thanks for your responses. Apologies for late reply but have been dealing with some roof issues. It is very helpful because I realise I want to go with a fused spur.
1 question is though the only fused spurs I have seen have a surface switch type or just plain fuse type with no switch. Is there such a thing as a fused spur that also has a socket incorporated(I doubt it but just trying to save space) .?

My other issue is I need to install wall cabinet lighting for kitchen which will be LED's fed from a transformer. Are the principles the same for lighting circuits with fused spurs .?

One other question is that my boiler is plugged into a normal switched socket What should I change it to.?

And last question is it allowed to take a spur from a switched cooker junction.?

Many Thanks
Steve.
gollum
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Joined: Tue Aug 31, 2010 9:59 am


Postby gollum » Thu Sep 09, 2010 11:24 am

Hi and thanks for your responses. Apologies for late reply but have been dealing with some roof issues. It is very helpful because I realise I want to go with a fused spur.

My other issue is I need to install wall cabinet lighting for kitchen which will be 12v LED's fed from a transformer. ? And how best to do this , would it be to place transformers in void behind base level units then feed up the thin 12v cable in the wall to the underside of wall cabinets ?

One other question is that my boiler is plugged into a normal switched socket What should I change that to.?

And last question is it allowed to take a spur from a switched cooker junction.?

Many Thanks
Steve.
gollum
Posts: 3
Joined: Tue Aug 31, 2010 9:59 am


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