Calling All Sparks!

[securespark]

Hi guys.

I can't remember which one of you it was, but I remember a discussion about cable sizes on cooker circuits. I was arguing that the csa must remain at least the same as the rest of the circuit. one of you argued that for a short length, the csa can be reduced, as long as it is suitable for the load.

Who was it? C'mon, own up!! And while you're at it, can you reference it to the regs, please?

I have been asked to find the reg. I can't! All I can find re 2.5 for radial circuits is in Appx 15 where it says you can use 2.5 on a 32A radial to feed one outlet.

 

[ColJack]

that would have been me.. or at least I said it to someone..

I believe we were discussing the use of 2.5mm² to connect a single socket to a cooker outlet for the plugging in of either a gas hob igniter or a single oven..

reg is ...

434.2.1..
it says that

Except where Regulation 432.2.2 or 434.3 applies, a device for protection against fault current may be installed other than as specified in Regulation 434.2, under the following conditions:

The part of the conductor between the point of reduction in cross-sectional area and the position of the protective device shall:
(i) not exceed 3 m in length, and
(ii) be installed in such a manner as to reduce the risk of a fault to a minimum, and
(iii) be installed in such a manner as to reduce to a minimum the risk of fire or danger to a persons.

which we were told at college ment that you could use a smaller cable for a for a short length as long as the cable was protected downstream by means of limiting the load..

this is the reason you can have a 2.5mm² spur ( 27A max, clipped direct )from a 32A ringmain as long as it only feeds one single or double socket, or a fused spur, as the load on the cable is limited to 13A for a single or for a fused spur, or 26A for a double, which is within the cables limits ( not allowing for correction factors )

and just in case you were interested, 434.2 says ( paraphrasing ) that the protective device ( fuse / breaker ) shall be placed at the point where the reduction of CSA of the conductor occurs..

 

[Adam_151]

433.3.1 (ii)

Of course, you do need to do the calculation to prove its protected against fault currents... (its not a standard circuit like a spur off a rfc, so it needs designing rather than just chucking in )

 

[ColJack]

it's quite a few on those pages..
433.2 also says it, and then points you at 434.2.1

433.2 is regarding the position of the protective device for overload protection, 434.2 is regarding the position of the protective device for protection against fault current.

 

[securespark]

Thanks, Col, you're a star.

EDIT: I've now got my teeth into these regs and can't quite see what you have, Col.

The bits that I don't follow are:


Quote:

The part of the conductor between the point of reduction in cross-sectional area and the position of the protective device

and

Adams quoted reg: provided that the conductor is protected against FC.

 

[ColJack]

what? no thanks for adam too?
if he hadn't posted I wouldn't have realised about 433.2 as well.. I automatically skip to the bit they told us to highlight..

 

[securespark]

Adam, I'm really sorry. No offence intended. I rushed through the thread and completely missed what you said.

I edited my post after to query the reg you quoted.

 

[securespark]

and just in case you were interested, 434.2 says ( paraphrasing ) that the protective device ( fuse / breaker ) shall be placed at the point where the reduction of CSA of the conductor occurs..

But there is none in our scenario, protective device, that is.

which we were told at college ment that you could use a smaller cable for a for a short length as long as the cable was protected downstream by means of limiting the load..

But 433.3.1 (i) says no need as long as conductor is effectively protected against overload on the supply side.

Which 2,5 on a 32A circuit would not be surely?

Please don't get me wrong, Col. I'm not having a go at you, just trying to get to grips with the wording of these regs & trying to make sense of it all!

 

[333rocky333]

These regs arent they more to do with supplys from large bus bars or henley blocks to things like 30 amp switchfuses etc, to allow smaller cables like 6 and 10mm D/Insulated to be used.

 

[securespark]

Possibly. I'm just trying to firm-up Col's suggestion that you can use (for example,) 2.5 on a 32A cooker circuit without fusing down.

A bit confused now... I'm off to bed!

 

[Spark123]

Depends what type of smaller sized conductor and how it can dissipate heat.
Afaik you are allowed to use a smaller sized conductor than the current setting In of the upstream circuit protective device permits as long as things like the current using equipment load is fixed or there is a protective device is downstream which I think is what the others were getting on about in this thread.
I'd question a cooker flex in 2.5mm tho.

 

[securespark]

I can see that if you have a 6milli circuit on a B32 feeding a CCU, with 6milli coming out to a hob & then another 2,5 to an oven, providing that oven load is not increased and is less than the capacity of 2.5, there is not going to be an overload scenario.

But I cannot glean that infomation straight-forwardly from the regs.

Really am off to bed now!

 

[ColJack]

aww man... you want me to decode them for you too... this is going to be a late night again..

ok, paraphrasing..

433.2 says that an OCPD has to go in the circuit where the current carying capacity of the circuit ( CCC from now on ) changes to lower than the OCPD protecting it, be this through the cable size changing, the cable being burried in insulation etc..

now 433.2.2 says that you can put the new OCPD in anywhere along that length of cable after it's CCC chages as long as there are no sockets on it or branches to other things, as long as it's less than 3m long, won't electocute anyone or set anything on fire..
this allows for wiring in 2.5mm² to a fused spur. the 13A fuse in the spur protects the cable from overload by cutting off the load.

433.3..1 ( ii ) says that you can miss out the OCPD entirely if the characteristics of the load means it's unlikely to overload the cable.

this allows for a 2.5mm² unfused spur to one single or double socket, because the fuse in the plug will prevent the cable to the socket from being overloaded..

434.2 is about the positions of devices for protection against fault current..

 

[securespark]

And can you apply the above to your scenario of 2.5 on a 32A cooker circuit which is not fused down?

PS, I am posting this in my sleep, honest.

 

[ColJack]

These regs arent they more to do with supplys from large bus bars or henley blocks to things like 30 amp switchfuses etc, to allow smaller cables like 6 and 10mm D/Insulated to be used.

it's the rules for the entire distribution in your house..

you have 25mm coming in, fused at 100A.. then you fuse down to 20A for a 2.5 socket radial, then fuse down further with a 3A fuse in the plug, to protect the 0.5mm cable to your clock radio..
at each point the conductor size ( and therefore it's current carrying capacity ) is reduced, an OCPD is inserted into the conductor path to protect the smaller cable downstream...

the are exceptions but they have to meet a certain criteria, such as on a 32A ring, you can have an unfused 2.5mm spur as long as it only feeds one single or double socket, or a FCU.. the 13A fuses in the plugs or FCU protect the cable upstream by limiting the load on the cable.