Answer this one!

[Robbo]

Right. Imagine a ring final circuit all wired in 2.5mm twin and earth.

Now if you need to spur from this circuit you can do so. Providing you wire from the chosen socket to your spurred socket/fused spur in 2.5mm twin and earth. Correct?

So if you find within a distribution board in a ring final circuit 32amp rated mcb, along with the two cables for the ring in there, there is also another 2.5 twin and earth cable. Turns out its supplying the boiler. From the board to the boiler in 2.5mm where it is then connected into a 3A fused spur and the boiler is then fed from that etc etc.

So in theory that boiler is spurred from the ring. Its not spurred from a j.b or back of a nearby socket but from the consumer unit. But its still a spur.

Nothing wrong with that so far?

So is this deemed ok by the majority here? Only what happens should a fault develop? Isnt 2.5 only suppose to take 20 odd amps before getting a little hot? But theres a 32amp breaker protecting it, not a 16 or 20?! The ring would be fine but what about the single 2.5mm cable?

So if you come across a situation like this where theres no room in a consumer unit to wire the new boiler on its own dedicated 16A circuit and some mush has wired it in with the ring? Isnt it technically just a spur and safe? Or potentially dangerous?

Or have I had too much Stella for a Sunday night.

Discuss.

 

[Andy™]

short circuit protection can be done by the 32A.

Overload protection is by the SFCU at the end, which can draw a max total of 13A

A spur can be taken at any point from a ring - including the MCB terminal

 

[SLIPSHOD & SLAPDASH]

The On site guide shows this method ( a spur taken from a breaker) as acceptable. Only older NICEIC sparks have argued this is not allowed and when i throw the regs book at them, they run away

 

[Apache]

Good point!

24A is the rating that the cable can take continuously for many many years. I would bet it could easily handle twice that for a period of time. If there is a suitable fuse in the fused spur the only thing to go wrong is the fused spur (before the fuse) or the cable itself - which seems unlikely.

Is there an RCD?

 

[Andy™]

Is there an RCD?

ah yes. the RCD. the answer to everything!

 

[NozSpark]

I'll have a go at this....

You've got a 2.5mm final ring circuit protected by a 32A MCB and a 2.5mm spur to a single socket.

Now as 2.5mm is only rated at just over 20A it is under rated for the circuit, however the load on it is limited by the fuse in the plug that you plug in (max 13A). There is a reg that allows for overload protection at the final point of the circuit and not the origin... and this is what it comes under.

Hope that helps..

 

[SLIPSHOD & SLAPDASH]

The other day i was correcting faults brought up in a PIR a colleague carried out. One of these 'faults' he had coded as a 2 was the boiler spur (fitted with a 3A fuse) connected to the cooker isolator via a .50mm 3 core flex (about 200mm long) The cooker supply was via a 40A rewirable fuse. Although i didnt like it, for the reasons Nozsparks mentioned i dont think this was codable. However i did change the cable for a bit of 2.5 Tw&e.

 

[Andy™]

but would the 0.5 flex be able to take out a 40A fuse incase of fault, before the 0.5 flex acts as a fuse itself? unless it can, i would agree with code 2

(it may be able to, but cant really be bothered to work it out)

 

[NozSpark]

I didn't think you were mean't to change cable sizes in a circuit without having a fused spur at that point in the circuit.

 

[SLIPSHOD & SLAPDASH]

(it may be able to, but cant really be bothered to work it out)

Neither could i, so thats why it got changed

 

[Andy™]

I didn't think you were mean't to change cable sizes in a circuit without having a fused spur at that point in the circuit.

you can, providing that the smaller cable can take a short circuit. i.e you could take a spur from a 2.5 ring on 32A breaker, in 1.5mm to a SFCU

Either that, or my memory isnt working too good tonight

 

[Andy™]

Neither could i, so thats why it got changed

for 200mm.... much easier to replace than work it out... and then replace

 

[SLIPSHOD & SLAPDASH]

I didn't think you were mean't to change cable sizes in a circuit without having a fused spur at that point in the circuit.

I was taught that as well. Its never been a reg, just good workmanship. However in the real world when there is a 10mm cable feeding a cooker switch with nothing else electrical on that wall i fitted a bit of 2.5mm and am not going to loose sleep over it.

 

[betty swollocks]

in the red book theres a diagram of a spur being taken direct from the ring main in 1.5mm without any fused connection

 

[dougal]

Right. Imagine a ring final circuit all wired in 2.5mm twin and earth. Now if you need to spur from this circuit you can do so. Providing you wire from the chosen socket to your spurred socket/fused spur in 2.5mm twin and earth. Correct?

So if you find within a distribution board in a ring final circuit 32amp rated mcb, along with the two cables for the ring in there, there is also another 2.5 twin and earth cable. Turns out its supplying the boiler. From the board to the boiler in 2.5mm where it is then connected into a 3A fused spur and the boiler is then fed from that etc etc.

So in theory that boiler is spurred from the ring. Its not spurred from a j.b or back of a nearby socket but from the consumer unit. But its still a spur.

Nothing wrong with that so far?

So is this deemed ok by the majority here? Only what happens should a fault develop? Isnt 2.5 only suppose to take 20 odd amps before getting a little hot? But theres a 32amp breaker protecting it, not a 16 or 20?! The ring would be fine but what about the single 2.5mm cable?

So if you come across a situation like this where theres no room in a consumer unit to wire the new boiler on its own dedicated 16A circuit and some mush has wired it in with the ring? Isnt it technically just a spur and safe? Or potentially dangerous?

Or have I had too much Stella for a Sunday night.

Discuss.

the spured cable is protected from shorts by 3A fuse.

right?

 

[Andy™]

the spured cable is protected from shorts by 3A fuse.right?

no. the 3A fuse cannot protect a cable from shorts before it. but it will protect against overload

 

[SLIPSHOD & SLAPDASH]

Wrong, the feed to the spur is only protected by the MCB

 

[Robbo]

Yep as slipshod said. The feed to the spur (2.5mm t&e) is only protected by a 32A breaker.

 

[ian932]

in the red book theres a diagram of a spur being taken direct from the ring main in 1.5mm without any fused connection

Not in my red book there aint, Appx 15 page 362 ??

You need to run 2.5mm to the FCU then you can take 1.5mm to the rest of the radial points

 

[ian932]

I'll have a go at this....You've got a 2.5mm final ring circuit protected by a 32A MCB and a 2.5mm spur to a single socket.

Now as 2.5mm is only rated at just over 20A it is under rated for the circuit, however the load on it is limited by the fuse in the plug that you plug in (max 13A). There is a reg that allows for overload protection at the final point of the circuit and not the origin... and this is what it comes under.

Hope that helps..

Is it not 27A if installed as method 'C', 20A is method 'A' conduit in insulated wall