one for the pro's to debate...

[ColJack]

it depends on how you read the wording of the reg that allows 2.5 rings..

it says that the RING must be in 2.5... it doesn't mention what size the spurs are, fused or unfused..

they COULD have meant that a 2.5 ring with 4mm unfused spurs comply...

 

[RF Lighting]

I think appendix 15 shows the intention of the regulation.

 

[GaryMo]

Whatever arguament is put forward for not supplying a single accessory from a 32A protective device seems to me to fall down when you start looking at it from the point of view a bog standard spur from a ring final circuit.

Especially when you can originate the spur directly from the 32A protective device!

 

[Paul_C]

But there is a rule stating that the spur conductors must be no smaller than the ring conductors, which rather implies that equal sizing is acceptable in at least some cases.

Whatever the manufacturers of sockets list as the maximum combined current for a double, the fact is that somebody could quite easily load the double socket to 26A, so shouldn't the spur cable be rated accordingly? 2.5 T&E clipped direct would be O.K., but run in other conditions, not.

But if - under the right conditions - a 2.5 spur from a 32A ring MCB is permissible, then how can you argue that the same thing run as a radial isn't? It's exactly the same arrangement as far as the radial/spur is concerned, the only difference being whether the MCB also feeds something else.

 

[martinxxxxxx]

Whatever arguament is put forward for not supplying a single accessory from a 32A protective device seems to me to fall down when you start looking at it from the point of view a bog standard spur from a ring final circuit.

So you have it all decided then?

I kinda agree with your thinking, but I am sure there will be some one brighter will be along soon to come up with an angle which we have not thought of.

The point about the ring being already loaded does not seem wholly convincing as it may not necessarily always be partially loaded.

Maybe it is just a discrepancy, an anomaly in the regs, which FRC are anyway really.

Anyway we await education on this.

Martin.

Nothim again prob has a view, as does ban.

 

[GaryMo]

it says that the RING must be in 2.5

Must? Pyro!

 

[ColJack]

it says that the RING must be in 2.5

Must? Pyro!

oh don't be awkward..

 

[GaryMo]

I know, I know

 

[Paul_C]

The point about the ring being already loaded does not seem wholly convincing as it may not necessarily always be partially loaded.

You shouldn't be deciding anything about the spur and its protection based on an assumption that the ring is partially loaded in my book.

And if you take the case where the ring has no load whatsoever, then the protective device and spur will behave exactly as if the ring weren't there at all, making it no different from a simple radial circuit.

(I suppose you could take a 2-inch length of wire and connect both ends to the fuse/MCB in the board so you can then say it's a ring circuit and there's no argument about the "real" circuit then being a spur!)

 

[mfarrow]

More likely it is allowed by 433.3.1 (ii).

The maximum loading a double socket can take is 26A (2x13A), probably less as most are rated lower. 2.5mm2 t&e cable, clipped direct is rated at 27A.

Most double sockets are rated at 20A, because diversity is taken into account which ensures that you can operate the circuit with 2.5mm T&E. This would have been the thinking behind 433.1.5, otherwise electrically there would be nothing safely permitting the installation of double socket spurs on ring mains.

If we apply that diversity to our double socket and then use 433.3.1 (ii), then we can use most of the installation methods listed for 2.5mm cable and get away with a single ended feed fed from any practicable (noting fault current) protective device.

 

[ban-all-sheds]

it says that the RING must be in 2.5... it doesn't mention what size the spurs are, fused or unfused..

Actually it says the ring must (apart from pyro) be in a minimum of 2.5mm², and that the cable must have an Iz of at least 20A. You might need 4mm² or 6mm², or possibly even 10mm² if you have an unfortunate combination of derating factors (can't be rsed to work out if it could get that bad), to get that.

 

[ColJack]

hang on... weren't you on the other side in that debate?????

 

[ban-all-sheds]

No - I was the one saying that you could not use a cable rated at less than 20A and drop the rating of the OPD to suit. Ring finals can only be 30/32A.

Possibly you could safely do a 20A one with 16A cable, but you'd have to justify your design (might be tricky with sockets on it), and you'd have to list it as a departure.

You could safely do a 45A one with 30A cable, but again, not allowed.

 

[Spark123]

Whatever arguament is put forward for not supplying a single accessory from a 32A protective device seems to me to fall down when you start looking at it from the point of view a bog standard spur from a ring final circuit.

So you have it all decided then?

I kinda agree with your thinking, but I am sure there will be some one brighter will be along soon to come up with an angle which we have not thought of.

The point about the ring being already loaded does not seem wholly convincing as it may not necessarily always be partially loaded.

Maybe it is just a discrepancy, an anomaly in the regs, which FRC are anyway really.

Anyway we await education on this.

Martin.

Nothim again prob has a view, as does ban.

Yep, I am decided that it is fine.

One for you to think about, can a 2.5mm RFC protected by a BS3036 comply?

 

[Lectrician]

Yes it can, but there used to be a restriction on 4mm radials on a 30amp device. You could not use a rewireable with those.