4mm radial circuit

[Brightspark2]

not if i have to use lube to get the buggers in.

ooo er...

 

[MDJ]

I've just been flicking through the regs to do some cable calcs for a job and noticed that a 4mm 2 core cable installed using method B (under floors etc) has a capacity of 30A. However it is ok to connect to a 32A MCB for a radial socket circuit. Don't know if its just me missing something (which I'm guessing it probably is!) but that doesn't sound right. I'm looking at table 4D2A. I just can't figure it out!

What you planning on connecting to the cable? not a block of flats I hope haha, JK, How many sockets out of interest only?

 

[marshr02]

Biff. I think I agree with you BUT strictly speaking the O/C device is rated at 32A, yet the cable is only QUOTED as handling 30A. So I guess the debate is how accurate the Ref Method B for Multicore is VERSUS T&E, in conduit IN the wall. So am I right in assuming you feel this cable method has a CCC of 32A. Cheers...

 

[storyelectrical]

What you planning on connecting to the cable? not a block of flats I hope haha, JK, How many sockets out of interest only?

I'm was actually working out for installing a cooker circuit when I noticed this in the regs. Its just something that I noticed and it triggered me thinking. The regs state that cables run under floors should be classed as method B. But then also show drawings of 4mm radial socket circuits. If I was installing a 32A cooker then 4mm wouldn't be acceptable but for a socket circuit connecting into the same MCB is fine?

 

[Heisenberg]

read posts #7 , 9 & 11.
and cables under floors are nothing like as derated as conduit values.
why is it not fine ?

Yeah, why didn't I think of that, read replies that had not yet been written :dizzy2:

You should be a politician, answering one question with one of your own. LOL

 

[marshr02]

Yeah, why didn't I think of that, read replies that had not yet been written :dizzy2:

You should be a politician, answering one question with one of your own. LOL

I really want to believe that there is no method B for T/E - hence if not insulation then it's method C. BUT the OSG references it - page 52 Table 7.2.

 

[Richard Burns]

The standard circuit A2 in the OSG does mention that the csa of the cables has to be assessed for CCC based on the installation method and the "informative" radial final circuit in the appendix of BS7671 also says that over current protection is not considered.

Because this is such a common circuit, it may well be installed incorrectly fairly often, but one can consider that domestic circuits almost never trip on overload and the likely current being carried will be far less than 30A so there is no significant danger, although as you say it will not meet the requirements of the regulations in many cases.

(copied my reply from the reference method explained thread)

 

[marshr02]

The standard circuit A2 in the OSG does mention that the csa of the cables has to be assessed for CCC based on the installation method and the "informative" radial final circuit in the appendix of BS7671 also says that over current protection is not considered.

Because this is such a common circuit, it may well be installed incorrectly fairly often, but one can consider that domestic circuits almost never trip on overload and the likely current being carried will be far less than 30A so there is no significant danger, although as you say it will not meet the requirements of the regulations in many cases.

(copied my reply from the reference method explained thread)

Thanks for the reply. Yes, your info is what I've read - I guess I really want the BS7671 & OSG to 'make sense'. In this case there seems to be a hole. Ref B, I understand includes cables under the floor (in a building void, BS7671). So even if you remove ALL conduit, almost ALL domestic installations will involve cables under the floor - ref B. Hence the OSG A2 cct recommended for domestic installations must implicitly assume ref B (@30A) is JUST okay. The frustrating thing is the regs don't come out and say this - which is the sort of leadership I hoped the IET would provide.

 

[Richard Burns]

The frustrating thing is the regs don't come out and say this - which is the sort of leadership I hoped the IET would provide.

This is a vain hope indeed!!!:nonod:
With all the verbiage coming from the IET you would expect clarity but it is never there, always grey.

 

[Tuttle]

Why don't you just use 6mm if you are concerned about it (that's what I'd do)?

 

[marshr02]

Why don't you just use 6mm if you are concerned about it (that's what I'd do)?

Because I'm not discussing what to do about a ONE off job. I'm discussing a flaw with a prescribed cct that is being installed by thousands of electricians across the country for years

 

[marshr02]

I'm coming to the conclusion that if the layout of a house suits radial ccts then 20A, with 2.5mm is more flexible, just likely to be running one more cct. With 32A radial used sparingly, say for the kitchen. (Obviously if the layout suits ring then I would use ring)

 

[kingeri]

I'm coming to the conclusion that if the layout of a house suits radial ccts then 20A, with 2.5mm is more flexible, just likely to be running one more cct. With 32A radial used sparingly, say for the kitchen. (Obviously if the layout suits ring then I would use ring)

Which is exactly what many sparks do these days. 20A radials for bedrooms, living room etc. and (personally) a 32A 2.5mm ring for kitchens, usually installed 'staggered'. I'd much rather work with 2.5mm anyway, much easier to terminate into socket outlets. 32A radials pretty easily fall below 32A CCC depending on installation method, and also because 4mm only has a 1.5mm CPC the circuit length is quite limited by Zs.

 

[marshr02]

Which is exactly what many sparks do these days. 20A radials for bedrooms, living room etc. and (personally) a 32A ring for kitchens, usually installed 'staggered'. I'd much rather work with 2.5mm anyway, much easier to terminate. 32A radials pretty easily fall below 32A CCC depending on installation method, and also because 4mm only has a 1.5mm CPC the circuit length is quite limited by Zs.

Thanks for reply. Just out of interest, would you be happy, putting 4mm in oval conduit into a plastered wall, and clipping to joists, and then running on a 32A CB?

 

[Tuttle]

I'm coming to the conclusion that if the layout of a house suits radial ccts then 20A, with 2.5mm is more flexible, just likely to be running one more cct. With 32A radial used sparingly, say for the kitchen. (Obviously if the layout suits ring then I would use ring)

I thought about using a 4mm 32A radial once and came to the same conclusion as you. It only seems to be good for ref method C and I think there could be a problem fitting three 4mm conductors into accessories if you need to branch off at any point or modify the circuit at a later date, so I used a 2.5mm ring instead.