Understanding fused spur

[SUNRAY]

The question here is has anyone yet come across a melted leg(s) of a ring due to imbalance?

https://www.diynot.com/diy/threads/...s-is-this-good-pic.483672/page-9#post-3927461 #124

 

[robinbanks]

maybe what im saying is being misunderstood

Draw a picture then

Sounds like you're talking about this:

a fixed double gang socket can only take 13amp ? across both its points not 13amp on each point

They can certainly pass more than 13A per socket

 

[JohnW2]

https://www.diynot.com/diy/threads/...s-is-this-good-pic.483672/page-9#post-3927461 #124

Yes, we know that story of yours

For a start, it's hardly a typical domestic situation, with a welder drawing 40 A (with peaks of 200 A) from a socket 2m from the end of a ring protected by a D32!

However, as I'm sure I've said before,I still don't see why 2.5mm² cable should come to any harm (let alone 'melt') in that situation, since we know from a combination of the regs and the characteristics of MCBs that a cable is deemed to be able to carry 1.45 times its 'rated current' continuously for an hour without coming to harm. Method C 2.5mm² cable should therefore be OK with up to about 39A continuously for an hour.

 

[markus2025]

Draw a picture then

Sounds like you're talking about this:
View attachment 397213


They can certainly pass more than 13A per socket

Yeah this one is kind of correct but its not coming out the back of a socket on the ring its coming from a break/juction box in the ring

So you cant break into a ring run a single lenght of 2.5mm to a new double socket the spur off that socket with fcu

 

[pete01]

Yeah this one is kind of correct but its not coming out the back of a socket on the ring its coming from a break/juction box in the ring

So you cant break into a ring run a single lenght of 2.5mm to a new double socket the spur off that socket with fcu

No the fcu would need to be before the first socket, if you wanted more than one.

 

[robinbanks]

So you cant break into a ring run a single lenght of 2.5mm to a new double socket the spur off that socket with fcu

A double socket is a break in a ring/there is no difference between a JB and a socket

Come off the ring in 4mm, or to a fused outlet then put as many sockets as you want downstream of it (you keep talking about coming off to a socket; put a fused outlet where the socket is and put the socket downstream of it)

The whole idea is that you can't cook the wire.
* If you come off the ring in 4mm you won't succeed in cooking the 4mm before you blow the breaker for the entire ring.
* If you come off straight to a fuse you won't succeed in cooking the wire downstream of the fuse, before you blow the fuse.
* If you come off in 2.5mm with no fuse you may only have one double socket, which cannot accept enough appliances to cook the wire; even if you the plug two 4 way extension bars into it, and plug in 8 toasters and turn them all on, all you do is blow the fuses in the plugs of the extension bars

There always needs to be a fuse/protective device upstream of the load, that blows before the wire melts (the wire must always be able to carry more current than the protection device)

 

[SUNRAY]

Yeah this one is kind of correct but its not coming out the back of a socket on the ring its coming from a break/juction box in the ring

So you cant break into a ring run a single lenght of 2.5mm to a new double socket the spur off that socket with fcu

 

[plugwash]

Hi guys can anyone answer a question im intrigued by

I feel the need to start from the beginning here.

You should not design installations you expect to be overloaded. However, unanticipated usage of the installation, simple bad luck with timing, or faulty equipment can lead to overloading. The possibility of overloading cannot be completely eliminated in virtually any real-world installation.

To mitigate this we have fuses and/or breakers selected to provide protection in the event of overload (these fuses and breakers also serve to protect against faults in the wiring but that's a seperate subject). The normal rule is that the overload protection device should have a rating not more than the cable rating. The cable ratings include enough wiggle room that the fuse/breaker should trip before there is any significant damage to the cable.

Over-current protection devices are not intended as load-limiting devices. If a fuse is blowing or a breaker tripping regularly then the cause should be investigated and appropriate changes made to either the installation or the practices of it's users.

Back to the rule about breaker ratings verses cable ratings, there is an exception to this rule for 32A ring circuits supplying BS1263 accessories. This rule requires three things.

* The cable must be at least 2.5mm²
* The cable must have a current rating of at least 20A (whether 2.5mm² will have this rating or whether you need to upsize will depend on the installation conditions)
* The circuit must be designed such that "sustained" overloading of the cables is "unlikely".

you can spur off any part of the ring with only 1x double socket

The "normative" part of the regs does not explicitly say how many sockets are allowed on a spur, but there is guidance in an appendix that specifies one single or double socket. There is an implied assumption here, that it is "unlikely" that both sides of a double socket will have large loads presented at the same time. The test requirements for sockets in BS1363 have a similar assumption.

however i believe its permitted that you can add a 13amp spur unit from the spurred socket and then run as many sockets as you want from that being protected by the 13amp unit

To comply with the guidance, the fused connection unit must be located such that it protects *all* the sockets on the spur.

Once the fuse is added, you are essentially no longer relying on the ring circuit dispensation. The 13A fuse protects the spur in the event of overloading.

However, we still have to consider the first point that we should not design installations we anticipate will be overloaded. So we should think about the anticipated loads before installing sockets on a fused spur.

 

[SUNRAY]

Yes, we know that story of yours

For a start, it's hardly a typical domestic situation, with a welder drawing 40 A (with peaks of 200 A) from a socket 2m from the end of a ring protected by a D32!

However, as I'm sure I've said before,I still don't see why 2.5mm² cable should come to any harm (let alone 'melt') in that situation, since we know from a combination of the regs and the characteristics of MCBs that a cable is deemed to be able to carry 1.45 times its 'rated current' continuously for an hour without coming to harm. Method C 2.5mm² cable should therefore be OK with up to about 39A continuously for an hour.

No it's not typical of a domestic property... But

What is typical it a ring covering the whole house, yes even now there are many houses like this - a neighbours CU is in the back of a kitchen base unit, in the first part of the ring is the multiswitch serving: dishwasher, washing machine, tumble drier, fridge/freezer etc, immediately followed by three double sockets for: kettle, toaster, microwave and airfryer etc all within the first 2 metres and yes it's not uncommon for a significant overload to occur, when they had RCBO tripping problems (dishwashers mains filter) their electrician placed data logger clamps on both legs and there were frequent instances of 50A on one leg which I assume was the short.

 

[ebee]

For a Ring Final Circuit (often wrongly referred to as a Ring Main) then following basis as so well illustrated by EFLImpedance needs to be followed.
The other considerations mentioned by other contributors also needs to be followed.
That way all should be well in most cases.

A few rules and a little thought should suffice.

If you do follow all the rules and regs then all should be well - if you do not then you might have problems - The Ring Final Circuit can be an excellent circuit to have and can have advantages over a Radial Final Circuit. A Radial Final Circuit can have some advantages over a Ring Final Circuit.
There probably is not one overall winner between the two except that a Ring Final Circuit can have more chances of errors causing problems if done incorrectly. swings an roundabouts really.

Stick to all of the rules and guidance and you should not be able to go far wrong.

Any point on the ring or any junction box on the ring should not be spurred to any more than 1 only of the following twin/single socket/fused connection unit (whether that FCU is switched or unswitched type).

 

[markus2025]

ok i understand now so your not suppose to add a fcu to a socket that has already been just ran off on a single 2.5mm cable and is not part of the ring


the way to do it would be change the socket that has been ran off the single length of 2.5mm to a fcu then add further sockets after that


just another question i have single socket in my loft that is running off a fcu that is in my living room which come off from a socket


can i ask would i be able to change the socket in the loft to a double pole fcu with a 3a fuse to be able to run my extractor off


so it would be socket off the ring going to a 13amp fcu on 2.5mm which then would run to a second double pole fcu with a 3amp fuse which would then run to a pull cords and then to my extractor on 1mm twin and earth

 

[EFLImpudence]

ok i understand now so your not suppose to add a fcu to a socket that has already been just ran off on a single 2.5mm cable and is not part of the ring
the way to do it would be change the socket that has been ran off the single length of 2.5mm to a fcu then add further sockets after that

Yes, work out how much current could possibly be in the cable and if it is more than the cables rating then it is not allowed - except of course the guidance that only one single socket may be on an unfused spur - we wouldn't want everything to be totally clear.

just another question i have single socket in my loft that is running off a fcu that is in my living room which come off from a socket


can i ask would i be able to change the socket in the loft to a double pole fcu with a 3a fuse to be able to run my extractor off

Well yes but there is no point having two FCUs - and are they not all double pole nowadays?

so it would be socket off the ring going

Surely a socket ON the ring.
Such is what lead to confusion earlier.

to a 13amp fcu on 2.5mm which then would run to a second double pole fcu with a 3amp fuse which would then run to a pull cords and then to my extractor on 1mm twin and earth

Change the first FCU to 3A

 

[JohnW2]

ok i understand now so your not suppose to add a fcu to a socket that has already been just ran off on a single 2.5mm cable and is not part of the ring

Correct.

the way to do it would be change the socket that has been ran off the single length of 2.5mm to a fcu then add further sockets after that

Correct again.

just another question i have single socket in my loft that is running off a fcu that is in my living room which come off from a socket ... can i ask would i be able to change the socket in the loft to a double pole fcu with a 3a fuse to be able to run my extractor off

Ifthe socket in the loft is fed from an FCU, you don't need a second one. Since that socket is already downstream of an FCU, you can connect any number of additional sockets (or anything else) to that socket.

 

[SUNRAY]

just another question i have single socket in my loft that is running off a fcu that is in my living room which come off from a socket


can i ask would i be able to change the socket in the loft to a double pole fcu with a 3a fuse to be able to run my extractor off


so it would be socket off the ring going to a 13amp fcu on 2.5mm which then would run to a second double pole fcu with a 3amp fuse which would then run to a pull cords and then to my extractor on 1mm twin and earth

I believe you currently have this

Essentially the simple rule is an unfused spur from a ring, be it from a socket or junction box etc may only be connected to one of these accessories: single socket or double socket or fused connexion unit. After the fuse in those accessories almost anything may be added.

 

[JohnW2]

No it's not typical of a domestic property...

Quite so ....

But ... What is typical it a ring covering the whole house, yes even now there are many houses like this - a neighbours CU is in the back of a kitchen base unit, in the first part of the ring is the multiswitch serving: dishwasher, washing machine, tumble drier, fridge/freezer etc, immediately followed by three double sockets for: kettle, toaster, microwave and airfryer etc all within the first 2 metres ....

That is a pretty crazy (and non-compliant) design. If all of those loads were actually AT 2 metres from origin of the ring, then the short leg of Method C 2.5mm² cable could never become 'overloaded' (provided the total load on the circuit did not exceed the design current of 32A) if the total length of the ring were at least about 11 metres.

... their electrician placed data logger clamps on both legs and there were frequent instances of 50A on one leg which I assume was the short.

That obviously means the the circuit was loaded far beyond its design current.

However, no matter what load there is (even if it is enormous), and where, I still do not think that Method C 2.5mm² cable should come to any harm (let alone 'melt') if protected by a 32A MCB - after all 32A is not much higher than 27A.