Back to basics with connections - help with clarifying this

[henry1]

Consider a socket outlet on a ring mains. Let's just look at the live connections and forget neutral for the sake of simplicity.

There'll be one wire in and one out to complete the ring. But there's also a third connection out for the current, that being for the socket itself. Current flows fromt that junction within the socket to the socket itself.

So there should be no difference in the connections and flow of current if I were to join the two live cables from the ring circuit together and then add a 3rd cable to carry current to the socket.

I could join all 3 cables together into a junction box. Then I can do a similar connection with the neutral cables and join all the earths together.

So all this should be the same in terms of connections and current flow as the usual socket being connected directly to the ring with one in and one out.

But hang on, isn't that set-up with the junction box called a spur? Why can't I add a second spur when I would have been able to add one to the socket directly? In the case of a spur directly off a socket, there is current flowing out into 2 sockets but the junction box will have current flowing out into only one socket and no more spurs allowed.

Can anyone clarify my confusion?

 

[PrenticeBoyofDerry]

I think we have to look at the loading of the cable, as RFC are generically installed using cable with a maximum of 27A capacity and a double outlet on a spur could demand 26A, where adding a further socket to the spur would/could raise that demand to over 27A and with a 32A protective device in please, there is a value where the device will not operate but the cable will be under excessive current.

 

[conny]

I think we have to look at the loading of the cable, as RFC are generically installed using cable with a maximum of 27A capacity and a double outlet on a spur could demand 26A, where adding a further socket to the spur would/could raise that demand to over 27A and with a 32A protective device in please, there is a valve where the device will not operate but the cable will be under excessive current.

Should the word 'valve' in your post read 'VALUE'?

 

[PrenticeBoyofDerry]

Should the word 'valve' in your post read 'VALUE'?

Indeed it should, well spotted.
Happy New Year conny.

 

[henry1]

I think we have to look at the loading of the cable, as RFC are generically installed using cable with a maximum of 27A capacity and a double outlet on a spur could demand 26A, where adding a further socket to the spur would/could raise that demand to over 27A and with a 32A protective device in please, there is a value where the device will not operate but the cable will be under excessive current.

Which cable will be under excessive load? it can't be the ring circuit as it can power 2 sockets if one is taken as a spur from the other.

 

[PrenticeBoyofDerry]

The spur cable.

 

[henry1]

When there is one spur at the JB, all is fine. When adding a second spur from the JB, are you saying the load on the spur cable will become excessive?
But the spur cable is connected from the ring circuit to the socket only. It is unaffected by what other spurs off the JB are doing.

 

[rjm2k]

Probably just one of those things, spurs are not considered to be great practice anyway so why make up a special rule for your case of considering the internals of a socket as a spur in its own right?

It's easier to just say "only 1 spur off anything" than "one spur off a socket and 2 off a junction box" I guess that way if your JB (possibly a terminal block in a blanked off backbox) is ever changed to a socket, you don't have to worry about a potential extra spur to get rid of.

I guess another way to look at it is that spurs are about cables not accessories, so considering the internals of a socket as a cable isn't a valid thing to do?

 

[PrenticeBoyofDerry]

But hang on, isn't that set-up with the junction box called a spur? Why can't I add a second spur when I would have been able to add one to the socket directly? In the case of a spur directly off a socket, there is current flowing out into 2 sockets but the junction box will have current flowing out into only one socket and no more spurs allowed.

We will take a step back to this comment as I may have misunderstood the question you were actually asking.

Adding a second spur at a junction box on a true ring, is not the same as adding a spur to a socket on a true ring, adding two spurs at the socket would be.
Adding one spur at each would be the same, so what is your point?

To look at this into more depth it would be a matter of design and product standards of the accessory.

 

[rjm2k]

Adding a second spur at a junction box on a true ring, is not the same as adding a spur to a socket on a true ring, adding two spurs at the socket would be.

He's trying to treat the internals of the socket as a (very short) piece of wire in its own right.

So on an RFC socket (unspured) you have 1 x "in", 1 x "out" and 1 x bit of metal to the plug prong - ie 3 paths. If you add a spur, you then have 4 paths. At a junction box, you are only allowed 3 since you can only have 1 x "in", 1 x "out" and 1 spur.

 

[PrenticeBoyofDerry]

He's trying to treat the internals of the socket as a (very short) piece of wire in its own right.

I would say that the contacts would be considered negligible.

 

[henry1]

But hang on, isn't that set-up with the junction box called a spur? Why can't I add a second spur when I would have been able to add one to the socket directly? In the case of a spur directly off a socket, there is current flowing out into 2 sockets but the junction box will have current flowing out into only one socket and no more spurs allowed.

We will take a step back to this comment as I may have misunderstood the question you were actually asking.

Adding a second spur at a junction box on a true ring, is not the same as adding a spur to a socket on a true ring, adding two spurs at the socket would be.
Adding one spur at each would be the same, so what is your point?

You can only have one spur off a JB or a socket. But the way a socket (not spur) is connected to the ring is like a spur already:

There'll be one wire in and one out to complete the ring. But there's also a third connection out for the current, that being for the socket itself. Current flows fromt that junction within the socket to the socket itself.

Yet you can add a spur off it, so in a way it's 2 spurs from the junction where the socket connects to the ring. How come the JB is only allowed one spur then?

Or in your words then, why is this not true:

Adding a second spur at a junction box on a true ring, is not the same as adding a spur to a socket on a true ring

 

[rjm2k]

Yet you can add a spur off it, so in a way it's 2 spurs from the junction where the socket connects to the ring. How come the JB is only allowed one spur then?

Because nobody else considers the internals of a socket to be a spur, maybe they could, but why bother making more and more exceptions to rules when it's considered bad practice anyway?

 

[EFLImpudence]

Well, I suppose he is right.

The flex from the plug to appliance would be a spur.

However, that does not change the rules regarding spurs from 'one point' on the ring.
Whether it is detrimental electrically to have two spurs from one point has been debated before.

 

[henry1]

He's trying to treat the internals of the socket as a (very short) piece of wire in its own right.

Yes, that's correct. I was just analysing all this by drawing circuits on paper. Would have been much easier to expain that way.

At a junction box, you are only allowed 3 since you can only have 1 x "in", 1 x "out" and 1 spur.

Yes, that's what I would like to know why, especially given that the socket is akin to a spur off the ring.

Unless all this is simply down to what you said earlier - simplicity.