permitted cable runs..help me if you can

[linnsound]

hi there, just stumbles upon this forum and hope someone can furnish me with an answer. Studying at the moment for C&G 2381 and 2531 and have ?'s relating to the regs as listed in the Green Book ( Electricians Guide to the Building Regs). It states that a ring, prtected by TypeB 32A circuit breaker, with 2.5/1.5 twin and earth has a permissible cable run of 90m. Now in my house the ring clearly exceeds that run when cable drops to sockets etc are taken into consideration. Similarly the floor area covered is more than 100msq...it passed all tests on change of occupancy. The same deal really with fixed lighting is stated ie 10a prtecting 1.5/1.0 has max run of 100m including switch drops. doesn't seem enough.
So what are your thoughts?
And what happens when this limit is reached.? Does one install 2 rings on the same floor if the floor of that property exceeds 100msq
Or is that in the case of the ring, 90m max to first socket.?
..i know i should know the answer to this but cant find it in my endless mound of books. I mean my place is an upper floor maisonette. 3 bedrooms, living room, bathroom, kitchen. The total floor area is well over 100msq, although the rooms individually are not huge a long hallway separating the front and rear of the flat dictates a large floor area. By the same token the consumer unit (located at foot of stairs) to the loft (where fixed lighting cables are run) is in itself a significant run and consequently running to all rooms and including drops to switches plus a two way switch at top and bottom of stairs ( all three plate).......well i don't understand the clause 100m cable including switch drops, 35m loop on length
Any advice or clarification would be greatly appreciated
Also what are your thoughts on a multi fuction tester - Megger, Fluke or AN other in terms of an MFT......many thanks
Peter

Should I have become a plumber instead? dont like poo though

 

[Steve__M]

I'm currently rewiring a house. The electrician who is going to do the Part P inspection repeated the 100 square metre rule,

But when I asked him about the cable length rule he looked at me as though I was an idiot.

I found my info on

www.diydoctor.org.uk/projects/electrical_safety.htm

where it says:

2.5mm (as above but larger) is usually used for the installation of power sockets on a ring main. The circuit uses a 30 amp fuse at the consumer unit (32 with a MCB). Cable length must not exceed 60m or 50m with an MCB.

I'm only wiring a medium size 3-bed mid-terrace, but I'm on my third 50m roll already (my friend likes lots of socket outlets).

 

[Adam_151]

If you go over the lengths in the OSG then its no longer a standard circuit and its upto the designer to calculate that it complies

As to floor area, I can't really imagine you'd want a ring of over 100m² unless you are sure you are always going to have a load 'load density'

As far as the lights go, it doesn't make sense to add all the switch drops to the length, for a fag packet idea add the longest switch drop (including any 2 way arrangements) to the length of the main radial, its still not strictly correct but its a quick thing that gives worst case.

(Of course if you had nothing better to do you could work out the volts dropped at each point and the R1+R2 at whatever the most distant point turns out to be, but its usually more hassle than its worth if the only difference is you might be able to install a 1mm² cable instead of a 1.5mm² one)