Thoughts please

[D Skelton]

Each 20A load would need a 20A fuse to protect the 2.5, surely without this then is one of the loads increased through whatever reason, the 2.5 would melt well before the 100A tripped.
I'm with shanky887614 on this one.

2.5mm 100a mcb over-rated

I'm really surprised at the fundamental lack of understanding regarding section 434 a large number of sparks seem to show!

 

[NickD]

I admire their natural suspicion of cable protection through a downstream protective device though. It's difficult to feel good about, regs or no regs. One might argue that a good design goal is to not have anything unexpected in it? I'm trying to think of something bad that might happen if someone working at one of your 20A loads blithely assumed there must be adequate upstream protection for the cable. Idiot proof designs are to be preferred.

 

[ackbarthestar]

The basic methods of overcurrent protection at NVQ level 3 is not to teach S/C protection up stream and O/L downstream. This is probably why some people have problems getting their heads around a non fused 2.5mm2 spur at the origin of an RFC.

However, your design does not take into account section 314 unless this is a German lash-up on one circuit.....

 

[Sparkytlec]

It's the same sort if thing as a 10A lighting circuit wired in 1.5mm to klick multiple point lighting box then having 0.75mm flex from the klick plug to the fittings.

As its affixed load at the end of the 0.75 flex you can never overload the flex. But you need 1.5 and a 10A o/c to have all the fittings on one circuit.

 

[ackbarthestar]

Just like to add that this is a good debate, except for the 300A deviation in the middle. Better to have stuck that one on a different thread.

 

[Engineer54]

As far as the original layout is concerned, (showing an over current device instead of just loads) this would be considered as an unconventional circuit as far as BS7671 is concerned. Another example of an unconventional circuit would be the ''Lollipop Ring Circuit, again perfectly acceptable providing it meets certain criteria. So providing due care and attention has been applied in the OPs design and installation it's perfectly fine if maybe not ideal!! Unconventional circuits are permissible under BS7671, if the designer can substantiate by calculation and method, etc. Our American (and even some of our European) friends quite often apply this type of distribution circuit.

The problem being with all the critics here is, that many here need an actual Reg No, to put against any specific, or they become flummoxed, or totally flummoxed. The Reg's are not a step by step guide, or a ''How To-Do book''!! That's not what the Reg's were ever intended to be, they actually require a reader to hold a good basic knowledge of electrical work and theory!!!

 

[durhamspark]

Just like to add that this is a good debate, except for the 300A deviation in the middle. Better to have stuck that one on a different thread.

Sorry about that , just posted in here cos it's an actual situation using the same theories discussed here.

 

[D Skelton]

However, your design does not take into account section 314 unless this is a German lash-up on one circuit.....

How so? It is designed as one circuit. Think of it like a radial but instead of taking the 25mm to each point, it has a 2.5mm branch spurred off to each point.

 

[Rockingit]

We get this argument in rock and roll whereby you arrive at a venue to find the house supply/outlets are physically and electrically larger than the inputs to the touring distribution that you're using. Typically a 125/3 > 63/3 or 400/3 > 300/3 scenario - so we carry jumper adaptors to suit. Some of the venues go ape and just can't get their heads around the fact that if I have d/s OL protection at 'source' then it doesn't matter that the feed cable is down-sized from the source S/C protection.