Why can't you take a spur from a spur?

[daneaston]

Don't worry, I'm not going to do it, but I got into a YouTube hole earlier and have become intrigued by electrical circuits! But this is confusing me.

I get that, on a ring circuit, you can run a cable from a socket to create a new socket. But you can't then run a cable from that new socket to create another new one. Why? What are the consequences of doing this? Why is it ok to have (say) 10 sockets on a ring main, but not 8 sockets on a ring main and then 2 spurred off of one of the original sockets?

And how is that different to a radial circuit which, unless I'm mistaken, is sockets running in series?

 

[Andy78]

It's to do with the current carrying capacity of the cable and the protective device that protects the cable from overload.
Difficult to explain without going into circuit design but essentially a single point from a spur from a ring circuit using standard cable sizes is permitted as the potential current drawn won't exceed that of the cable. Multiple points would and in a ring circuit the overcurrent protective device would not be coordinated to protect the cable in that arrangement.

 

[SparkyChick]

A typical ring final is wired in 2.5mm T+E Cable and is protected by a 32A MCB. 2.5mm cable at best has a current carrying capacity of around 27A, so in the ring on the left, it's balanced and well within the current carrying capacity of the cable.

The ring on the right is still balanced, and the cables that make up the ring itself are not overloaded. But spurs are usually wired in the same cable as the ring and as such, you should be able to see that it's possible with spurs on spurs to overload a cable segment. The circuit breaker may take a while to trip, in the meantime the cable would heat up which will cause damage to the insulation.

In a typical radial circuit using the same cable, we would limit the circuit breaker to 16A or (at most) 20A, thus the cable is protected from an overload.

If you want to have lots of sockets on a spur off a ring, you can use a fused connection unit to create the first spur from the ring. This would limit the current to 13A maximum and thus protect the cables after it from an overload.

Hope that clarifies it a little

 

[littlespark]

Very detailed, @SparkyChick but I think the maths have let you down.
13A split across 2 legs would be 6.5 each...

Basically, OP, its the current carrying capacity of the cable.
Yes,come t up as much as you like, it’ll work, but the more load added to the circuit, then there’s the danger of cables overheating.

 

[SparkyChick]

Very detailed, @SparkyChick but I think the maths have let you down.
13A split across 2 legs would be 6.5 each...

Basically, OP, its the current carrying capacity of the cable.
Yes,come t up as much as you like, it’ll work, but the more load added to the circuit, then there’s the danger of cables overheating.

Thank you, much appreciated

 

[daneaston]

View attachment 56071
A typical ring final is wired in 2.5mm T+E Cable and is protected by a 32A MCB. 2.5mm cable at best has a current carrying capacity of around 27A, so in the ring on the left, it's balanced and well within the current carrying capacity of the cable.

The ring on the right is still balanced, and the cables that make up the ring itself are not overloaded. But spurs are usually wired in the same cable as the ring and as such, you should be able to see that it's possible with spurs on spurs to overload a cable segment. The circuit breaker may take a while to trip, in the meantime the cable would heat up which will cause damage to the insulation.

In a typical radial circuit using the same cable, we would limit the circuit breaker to 16A or (at most) 20A, thus the cable is protected from an overload.

If you want to have lots of sockets on a spur off a ring, you can use a fused connection unit to create the first spur from the ring. This would limit the current to 13A maximum and thus protect the cables after it from an overload.

Hope that clarifies it a little

A-ha! Ok, that makes sense. And that also suddenly explains the use of a fused spur - so if the load gets too much, the fuse pops. So, technically, you could spur off a spur as many times as you liked so long as each one is a fused spur...?

I've genuinely become quite fascinated by this in the last 24hrs and all from reading that an electrician can check whether a socket is on a ring or a spur with a gadget. I thought that didn't seem possible, so started researching.

 

[SparkyChick]

A-ha! Ok, that makes sense. And that also suddenly explains the use of a fused spur - so if the load gets too much, the fuse pops. So, technically, you could spur off a spur as many times as you liked so long as each one is a fused spur...?

Not exactly... you would connect the ring to the supply side of the spur, put a 13A fuse in it and then from the load terminals you could connect 1, 2, 3.... sockets, and you're safe in the knowledge you can't overload the cables.

The regulations don't really specify much about a ring final circuit other than it should be designed in such a way so as to ensure the cables used are not overloaded.