30 amp wylex rewireable fuse.

[entech]

hi all, quick question ,... i have noticed that my ring circuits are wired in 2.5/1.5mm pvc/pvc and are protected by 30 amp re wireable fuses .. is this correct ???? as the max 2.5mm pvc/pvc can carry is 27 amps depending on install reference.

Cheers

 

[IanG1]

The clue is in the word "ring". Circuit is fed from both ends hence the 30A rating of the device exceeding the cable if that makes sense?

 

[entech]

thanks for the reply Ian. whats confusing me thou is that if you work out the design current and then decide on your fuse, then according to the osg you then select a cable which is equal to or exceeds the fuse selected. it doesnt differentiate for rings or radials does it??.

with your theory are you sort of saying that a ring wired in 2.5mm pvc/pvc could carry up to a max of 54 amps (2 x 27 amps)???

 

[DurhamSparky]

Ring circuit - Wikipedia, the free encyclopedia


a good read!!

 

[brucelee]

Hi mate because its a ring half of the current will flow down one leg and half down the other so the cable will not be overloaded so 2.5mm is ok with a ring circuit upto 32A

 

[entech]

Ring circuit - Wikipedia, the free encyclopedia


a good read!!

cheers mate. still not sure thou as i mentioned earlier that according to osg and cable selection formula you must be equal to or higher than the protective device.

in the wiki explanation it says that the cable current carrying capacity must not be less than 2/3rds of fuse rating (i def cant find that info in bs7671).

 

[POWERFULL]

They say that we needed to reduce the apprenticeship to 3 / 4 yrs.
By the general questions I see posted on this site, we need to go back to the 6 yrs apprenticeships as the underpinning knowledge is sadly missing.
More time is need within the theory side of the qualification.
Anyone can run cable in and cut boxes into a wall. It ends there.
That when the knowledge comes into the great scheme of things

 

[entech]

They say that we needed to reduce the apprenticeship to 3 / 4 yrs.
By the general questions I see posted on this site, we need to go back to the 6 yrs apprenticeships as the underpinning knowledge is sadly missing.
More time is need within the theory side of the qualification.
Anyone can run cable in and cut boxes into a wall. It ends there.
That when the knowledge comes into the great scheme of things

i was hoping that perhaps someone could point out exactly where in bs7671 it shows how to calculate cable sizes for ring final circuits.

 

[cornburn]

i was hoping that perhaps someone could point out exactly where in bs7671 it shows how to calculate cable sizes for ring final circuits.

If you flip to the back of the big red book there are some useful diagrams that show both rings and radials, including radials and spurs off ring circuits. If you get on better with diagrams than descriptions these will be really useful. If you ran 2 lots of 2.5mm in parallel you would have 5mm total, giving you twice as much current carrying capacity. In basic form a ring is no more complicated than this. Your OSG also shows standard circuit layouts for domestic circuits. Don't know where exactly is mentions calculating rings specifically in the BRB but I was up at 5 this morning so going to bed - you'll have to trawl it yourself I'm afraid

 

[entech]

If you flip to the back of the big red book there are some useful diagrams that show both rings and radials, including radials and spurs off ring circuits. If you get on better with diagrams than descriptions these will be really useful. If you ran 2 lots of 2.5mm in parallel you would have 5mm total, giving you twice as much current carrying capacity. In basic form a ring is no more complicated than this. Your OSG also shows standard circuit layouts for domestic circuits. Don't know where exactly is mentions calculating rings specifically in the BRB but I was up at 5 this morning so going to bed - you'll have to trawl it yourself I'm afraid

thanks cornburn. i had a look at the diagram in brb and it comes across well in diagram form.shows the spur fcu and non fused spur well also. will have a look for the ring calcs another day.

cheers again.

 

[Sintra]

Reg 433.1.5 is specific to ring final circuits.

 

[Phil]

some good guides on here

 

[malcolmsanford]

some good guides on here

I wonder if they are ever going to update that for the 17th. John Whitfield as wrote a 17th, wonder if him and TLC have fallen out