Help a poor student whose hand in is tomorrow!!!

[lazy student]

Hi Folks

Sorry, cable rating question! No doubt this has been asked before and I apologize profusely for not trawling the boards to find it but time is short.

Part of my brief is to install two 13 amp single sockets with single core on a radial circuit, (its less than 30m length and in PVC conduit) my notes say it should be 2.5mm 2 core and earth, but the brief says single core. Can you get single core 2.5mm? should i just use 2 core?

Also its protected by a 32A MCB but i thought 2.5mm had a rating of about 22A? And it could potentially draw 26A! whats with that?

I think I've been looking at this too long and just got myself confused, can somebody dig me out of this.

Many thanks

Lazy Student

 

[telectrix]

in conduit, you use single cables. i.e. a brown, a blue and a gn/y. if 2.5mm cable on a 32A MCB, then make it a ring. sssimplesoops, just seen you stated a rfadial. use 4mm or 6mm singles then.

 

[lazy student]

Thanks Telectrix.

That was my first thought but the lecturer poo pooed it for some reason, cost maybe. Either way I'm pretty much past caring so that's what i will do.

Fancy explaining why the regs let you put a 32 A MCB on a 22 A cable?

Thanks again

Lazy

 

[kingeri]

You need to understand the concept of a ring final circuit. 2.5mm can carry up to 27amps depending on installation method. So, obviously you couldn't use a 32amp OCPD to protect it in a radial configuration. When wired as a ring, you have 2 x 2.5mm cables supplying the accessories. However, it's not quite so simple as if you had 2 x 2.5mm in parallel, as the amount of current each cable will be carrying will vary depending on the position of the load on the ring. This is why thought must be given to balancing the load on a ring, so as to avoid overloading one leg.

 

[telectrix]

so give the lecturer this little gem. shoots him out the water. Iz>In>Ib.

 

[Sintra]

With lecturers like that what hope is there for the future of this game.

 

[telectrix]

about the same hope that there is for that 2.5mm cable getting hot while the 32A breaker just sits there, happily shoving 40A down it.

 

[shanky887614]

put it on a 16amp type b mccb.

bsen60898.

or if you want to be clever put it using 2.5mm tri-rated pvc singles.

a 32 amp breaker is too high for a radial using 2.5mm

 

[lazy student]

Hi Gents

Kingeri, I think I was told that it was suitable for a radial, something to with sustained current, and he pointed me to the regs, sadly i don't have a copy of them now when i need them, but yeah it didn't seem right.

Telectrix, will do, but I'll wait till he has marked this,I think I will need him to be lenient.

Sintra, I think the mistake is more likely to lie with me than him.

Thanks for taking an interest lads

Lazy

 

[shanky887614]

ok ill explain why your getting confused with your notes.

depending on the installation method dictates which type of cable to use.

for example

cliped direct,conduit,trunking,ducting etc.

if your using clipped direct you normally use twin and earth (2c+earth)

in conduit,trunking etc you dont need the extra layer for mechanical protection so you just use singles.


now this is only applicable to this scenario and installation method

 

[lazy student]

Cheers Shanky, if carrying capacity is 13.5 A and i've got two 13A sockets and i run a ring final should i use a 25 A MCB?

 

[shanky887614]

no mate. by 13amp socket its the max it can pull.


let me respond to your question with a question.

if your house has 2 ring circuits on 32amp mcb's. a 60amp cooker mcb and a 32amp shower circuit.

why has it only got a 100amp incoming fuse?

its all to do with diversity mate

(type b mcb's come in 6,10,16,32, etc)


remember the mcb is to protect the cable not the device.

i recomend you buy the onsite guide and have a read through it

www.amazon.co.uk/gp/aw/d/1849192871/ref=mp_s_a_1_1?qid=1370374964&sr=8-1&pi=SL75

 

[lazy student]

Cause none of them are expected to pull their max. so i just need enough to protect the cables 16A MCB?

 

[telectrix]

CCC of 2.5mm in conduit , ref. method B , is 24A so a 20A MCB would be appropriate.

 

[lazy student]

brilliant thanks telectrix

 

[Outspoken]

As a reference for both you and your tutor I would refer you to Appendix 15 of BS7671:2008 Ring and Radial Circuit Arrangements : Regulation 433.1 on pp425.

Regulation 433.1 in the title of the Appendix directs the reader in pp81 and specifically 433.1.103, I strongly suggest you read it before proceeding and ensure you understand it.

 

[lazy student]

Ok having got a copy of the regs (don't ask how i did that at 11pm), I think the problem arose because I had forgot that the circuit is "unlikely to exceed for long periods of time the current carrying capacity of the cables" (433.1.5)

Cheers for all the help gents

Lazy