electrics
- Thread starter janb
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B
breezer
one, twin maximum
This is then called a spur, you are not allowed to spur off a spur.
I also have to mention you can only spur off the main ring
This is then called a spur, you are not allowed to spur off a spur.
I also have to mention you can only spur off the main ring
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Its down to what the cable can take.
One twin socket = one 3kw fire plus lamp, second one could be plugged in the same, too much load on the cable(?), it's not the way to do things these days.
I personally, if possible, when running one cable into the loft would run two and extend the ring main, if you're going to chase the wall for one cable an extra 3/4" of plaster over the second is no hardship and then you can add as many sockets as you want (theoretically).The way I did it was to find a convenient double ring main socket, remove one set of wires, go into the loft, come back down and added another twin socket next to the original I broke away from, connected to this and reconnected the old ring main to the new socket. Reason being, a socket is not much more than a junction box and at least you can use it for something (hair drier, curling tongs, whatever turns you on) !!
One twin socket = one 3kw fire plus lamp, second one could be plugged in the same, too much load on the cable(?), it's not the way to do things these days.
I personally, if possible, when running one cable into the loft would run two and extend the ring main, if you're going to chase the wall for one cable an extra 3/4" of plaster over the second is no hardship and then you can add as many sockets as you want (theoretically).The way I did it was to find a convenient double ring main socket, remove one set of wires, go into the loft, come back down and added another twin socket next to the original I broke away from, connected to this and reconnected the old ring main to the new socket. Reason being, a socket is not much more than a junction box and at least you can use it for something (hair drier, curling tongs, whatever turns you on) !!
From reading the original note ...I think JANB is putting a ring circuit in by running 2 x 2.5mm T/E cables. JANB if you are running these from a seperate MCB / Fuse in the Concumer unit then you can put in as many sockets as you like likewise if you are 'breaking in' and extending a ring circuit as Scoby_Beasly says
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A bit more info about the question of why you should not spur off of a spur etc:
2.5 T+CPC cable can carry about 24 amps. In a ring, that effectively doubles it to 48 amps - it is protected by a 30Amp fuse/MCB, therefore safe.
If you spur from a spur and so on (i've seen people run lots of spurs from a spur), then that leg (which is basically a radial circuit wired from a ring circuit) has only a 24Amp cable carrying capacity yet is still backed by a 30A fuse. Therefore risk of cable overload. The theory is that you can only plug so much into one spur, so the risk is minimized.
Other snippets - you should only run one spur maximum from each socket on the ring, and the number of spurs must not exceed the number of sockets on the ring (which could happen if you use junction boxes to wire spurs.
If you do do any work on a ring circuit, it is advisable that you check the ring continuity when you are done, to check the ring is still intact
Cheers
Paul
2.5 T+CPC cable can carry about 24 amps. In a ring, that effectively doubles it to 48 amps - it is protected by a 30Amp fuse/MCB, therefore safe.
If you spur from a spur and so on (i've seen people run lots of spurs from a spur), then that leg (which is basically a radial circuit wired from a ring circuit) has only a 24Amp cable carrying capacity yet is still backed by a 30A fuse. Therefore risk of cable overload. The theory is that you can only plug so much into one spur, so the risk is minimized.
Other snippets - you should only run one spur maximum from each socket on the ring, and the number of spurs must not exceed the number of sockets on the ring (which could happen if you use junction boxes to wire spurs.
If you do do any work on a ring circuit, it is advisable that you check the ring continuity when you are done, to check the ring is still intact
Cheers
Paul
Interesting...
Sorry to be a pain,but better safe than sorry!
So when a cable overload what happen,does the wires start to melt or does it trip out at the CU first?
It is the live wire melt first then it trip out when touching the neutral + earth? It is the whole cable melt or certains parts.If the CU tripped why does the fire start ?
How does the cables overload ? I would've thought if the cables can't cope with the demands it won't get enough current thru' it or does it draws too much current thru' it.
This can wait so no rush anwering this.
Sorry to be a pain,but better safe than sorry!
So when a cable overload what happen,does the wires start to melt or does it trip out at the CU first?
It is the live wire melt first then it trip out when touching the neutral + earth? It is the whole cable melt or certains parts.If the CU tripped why does the fire start ?
How does the cables overload ? I would've thought if the cables can't cope with the demands it won't get enough current thru' it or does it draws too much current thru' it.
This can wait so no rush anwering this.
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In an overload the wires will heat up, the higher the overload the hotter they will get. Eventually the insulation will melt and/or catch fire (or catch fire to something adjacent to it).
The Fuse or MCB will go when the wires touch because the insulation has melted, or if the wires touch earthed metalwork because the insulation has melted.
The live and neutral will heat up at the same time. The live and neutral alway have exactly the same current in them in normal use.
The fire may start even after the fuse has gone because the wire may already be melted and have caught something alight before the wires shorted and blew the fuse.
If too much equipment is pulling too much current down a cable, the cable will keep trying to pass the current until the copper melts. The insulation will melt first though, it takes a lot of juice to melt the copper itself. Your house will have burned down long before the copper melts.
None of this will happen in a correctly wired house. Or one where someone hasn't got a double socket feeding loads of four-way trailing leads!
Hope it answers your query, feel free to ask more..
Paul
The Fuse or MCB will go when the wires touch because the insulation has melted, or if the wires touch earthed metalwork because the insulation has melted.
The live and neutral will heat up at the same time. The live and neutral alway have exactly the same current in them in normal use.
The fire may start even after the fuse has gone because the wire may already be melted and have caught something alight before the wires shorted and blew the fuse.
If too much equipment is pulling too much current down a cable, the cable will keep trying to pass the current until the copper melts. The insulation will melt first though, it takes a lot of juice to melt the copper itself. Your house will have burned down long before the copper melts.
None of this will happen in a correctly wired house. Or one where someone hasn't got a double socket feeding loads of four-way trailing leads!
Hope it answers your query, feel free to ask more..
Paul
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I don't understand how any spur can be safe. If 2.5mm cable is capable of taking 24Amps and a double socket is rated at 2 x 13Amps. In theory it is capable of drawing 26Amps, more than the cable rating.
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yes, but this is not a major risk, 2A above rated current - in some cases the cable is rated to that anyway, I just use 24A as an average figure - a current rating of a cable depends on how it is installed, for instance the rating is lower if buried under plaster, higher if clipped on the surface etc, i think the theoretical maximum is 29A in the right conditions. There are many factors which influence how much current a cable can carry.
The risk is that if you have two or more spurs on a branch of a circuit, you can go significantly over the cable rating.
A spur is safe. It only gets a bit hairy in my view when people decide to plug two four-way trailing leads in and run thier entire kitchen from it!!!
That said, in an ideal world, every socket should be on the ring, not just because of the current-carrying capacity, but for other reasons too. We don't live in an ideal world tho....
Cheers
Paul
The risk is that if you have two or more spurs on a branch of a circuit, you can go significantly over the cable rating.
A spur is safe. It only gets a bit hairy in my view when people decide to plug two four-way trailing leads in and run thier entire kitchen from it!!!
That said, in an ideal world, every socket should be on the ring, not just because of the current-carrying capacity, but for other reasons too. We don't live in an ideal world tho....
Cheers
Paul
Thats a bit short sighted of you isnt it?
Not you paul ,,, the other guy
If a ring main has say 5 double sockets on it , in theory you could try and draw 130 amps from the ring. Only a Complete idiot would even think of doing it though. 2.5 mm twin and earth can carry hundreds of amps (but Not safely ) at 24 amps it is quite safe. Even 26 amps would barely heat up the wires...
Not you paul ,,, the other guy
If a ring main has say 5 double sockets on it , in theory you could try and draw 130 amps from the ring. Only a Complete idiot would even think of doing it though. 2.5 mm twin and earth can carry hundreds of amps (but Not safely ) at 24 amps it is quite safe. Even 26 amps would barely heat up the wires...
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Well said Gees,
Our old friend Diversity - ie, well designed circuits should never run at full load.
As an interesting (perhaps) aside, based on an overloaded installation I looked at a few years ago (with warm cables, MCBs, the lot), I reckon a 2.5mm cable will start to get warm at about 31-35 Amps depending on how it is installed.
Have a good one..
P
Our old friend Diversity - ie, well designed circuits should never run at full load.
As an interesting (perhaps) aside, based on an overloaded installation I looked at a few years ago (with warm cables, MCBs, the lot), I reckon a 2.5mm cable will start to get warm at about 31-35 Amps depending on how it is installed.
Have a good one..
P
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Granted - I was being a bit pedantic
However you would expect the regulations to stack up.
The point is though that the ring main is protected by the fuse in the consumer unit which is rated below the theoretical current handling limit of the ring-main, however is rated higher than the theoretical current carrying limit of the spur. The spur is only protected by the fuses in the plugs that are inserted into the double socket which could in theory alow a total of 26 Amps to flow.
However you would expect the regulations to stack up.
The point is though that the ring main is protected by the fuse in the consumer unit which is rated below the theoretical current handling limit of the ring-main, however is rated higher than the theoretical current carrying limit of the spur. The spur is only protected by the fuses in the plugs that are inserted into the double socket which could in theory alow a total of 26 Amps to flow.
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