At the risk of making myself look a bit of a.......

[Sparky83]

....Pratt!

I was having a disscusion with a mate of mine and radials (like you do)

and we disagreed on on feeding a 2.5mm t&e from the 32a mcb of a ring ciruit.

I thought that you could only spur off the ring from a point on the ring itself like a socket, but he said you could take it straight from the mcb itself. To me this then becomes a radial circuit sharing a mcb with a ring main?

What would happen if the ring was removed and the radial circuit left, on a 32amp breaker, that would be right, so what makes it ok when a ring shares the mcb?

I've had a look about and even on this forum but some say its ok some dont. Im happy to admit im wrong, which is how it would seem it is but i would like a good explenation as to why its ok.

Cheers
Shawn

 

[Sparky83]

Addlights just saying that you can't add a 2.5 radial from a 32amp breaker protecting a ring.

A spur is one thing, a radial is something else

Is it still a spur when taken directly from the mcb? or is it a radial and a ring sharing a mcb???

In my other posts ive said radial but i guess it should read spur really.

 

[Amp David]

One socket taken from the MCB is a spur.

To me a spur is one socket say and a radial is a number of sockets. This is where our confusion is i think.

 

[La Poste]

But why is it OK to spur off a ring with 2.5 T and E?
A ring is used at 32A, when you spur off it you are using a 20A cable which because it is spurred off a ring is fused at 32A.
So why can you use a 20A cable fused at 32A?

 

[Amp David]

A socket (single or double) and FCU are rated at 13 amps max.

 

[Sparky83]

One socket taken from the MCB is a spur.

To me a spur is one socket say and a radial is a number of sockets. This is where our confusion is i think.

Would it still be a spur if it was one FCU on the end of it?

As surely this would be a radial if fed from the consumer unit like a supply to a boiler/immersion heater?

 

[Amp David]

Think you're taking the P now

I would sayit a spur if its sharing an MCB with a ring. On it own its a radial

 

[La Poste]

A socket (single or double) and FCU are rated at 13 amps max.

Fuses are there to protect the cable, it matters not what the fuse at the socket is.
A 20A cable fused at 32A, it's dodgy.
That's how I understand it.
Maybe I'm wrong.

 

[Amp David]

Against overloading, which with an outlet a 13amps can't happen

 

[Sparky83]

Think you're taking the P now

I would sayit a spur if its sharing an MCB with a ring. On it own its a radial

Im sorry you feel that way, im in no way taking the P*** im trying to get something in my head that i dont understand.

But all im trying to find out if that if you have a 2.5mm circuit that feeds a boiler (or any fix appliance really) via a FCU for arguments sake and spare ways in your DB you would stick it on a 16a mcb. Yes?

So what then makes it ok to put that same circuit into a 32a mcb instead which feeds the ring and then call it a "spur"?

I get it if done from a point within the ring but not at the origin.

That is what i dont get!

 

[Addlight]

"An unfused spur may be connected to the origin of the circuit in the distribution board" (433.1.5) - so it's okay by regs, but I don't like it when it comes to testing, as you have three cables at the MCB - the ring and the 'spur'. You can only properly test these as two circuits - do all you usual ring tests, then treat the spur as if it were a radial with one socket. Then you're faced with how to record it on the results sheet ... I'd rather try to find a way of putting the single socket on the ring as a 'proper' spur, or add it directly into the ring - or move to it's own MCB if there's enough ways.

 

[Amp David]

Im sorry you feel that way, im in no way taking the P*** im trying to get something in my head that i dont understand.

But all im trying to find out if that if you have a 2.5mm circuit that feeds a boiler (or any fix appliance really) via a FCU for arguments sake and spare ways in your DB you would stick it on a 16a mcb. Yes?

So what then makes it ok to put that same circuit into a 32a mcb instead which feeds the ring and then call it a "spur"?

I get it if done from a point within the ring but not at the origin.

That is what i dont get!

Only joking sparky83

 

[Sparky83]

Only joking sparky83

No worries

 

[Deleted member 9648]

It has always seemed a bit odd that a 2,5mm wired as a spur from a ring circuit can be fused at 32A while a radial circuit supplying 1 twin socket wired in 2.5mm would have to be fused at 20A max......I think this is a case of the IEE simply accepting that it has been done this way since the dawn of time so it's best left alone.

 

[keniff]

i personally would not spur a socket direct from the fuseboard on a 32a its just a bit diy, but, even a spur direct from a 32a breaker into a double socket the max it could draw is 26 amps (13 amps per socket) if T+E is rated at 27 amps at best, the cable is not going to be overloaded. but say for example mr diy comes along and spurs 5 more sockets from that spur then you have a problem!!! just break into the ring or put it on a 20a mcb just makes sense to me!

 

[Amp David]

Unfortunately, the regs are written and aimed at the competent person. No electrician in the world can plan for what the DIYer decides to do at a later date.

One reason why if we get questions on a forum asking 'how to do this and that' from a DIYer, then if they don't already have an idea, they should leave well alone.