433.2.1 p74 bs7671 question for anyone?

anyone fancy answering a regs question?


We are pricing up 8 separate 110 volt bus bar systems powered by 5kva transformers which will have c40 double pole mcb as transformer secondary overload.

I'm aware bs7671 diversity is 100% first socket and 70% the rest,but with customer agreement we are planning up to six c16 mcb/rcd sockets per transformer with the loading, diversity and usage it should be adequate.( the site has an existing 110v system and there are feed issues with going bigger)

Each run will be maximum of 40 meters 160A bus bar trunking(to get earth loop low enough) and a 10mm swa feed to socket from bus bar plug to 16A mcb/rcd protected socket.

The supplier is quoting 433.2.1 overload within 3 meters of csa reduction.
I disagree as 433.3.1 my lowest size (10mm)conductor is rated in excess of primary supply overload. Then the 110v 16 amp socket has c16 mcb which is protecting any lead extension fitted.



Hope it makes sense as it's been a long day and drinking a couple of beers

ang on while i dig my regs book out... don't use it that often!

If i understand your post correctly, you are protecting bus trunking with a device which is below the maximum current capacity of the 10mm SWA...

In which case I would say no further protection is required

try 433.2.2 (ii) distance between change in CSA and protective device <3m...

Thats how it reads to me and i agree wi ya, buts its been a while since i've needed the knowledge.
Cant find me regs book either. Saw me on site guide the other day but cant remember where:doh:

lerk said:

ang on while i dig my regs book out... don't use it that often!

If i understand your post correctly, you are protecting bus trunking with a device which is below the maximum current capacity of the 10mm SWA...

In which case I would say no further protection is required

Click to expand...

correct the bus bar is rated to 160A only due to the need to meet loop impedance. The 10mm is rated in excess of c40 amp which is feeding the bus bar.

433.2.2 is superseded by 433.3.1 as I understand it as my bus bar is only on a c40 and the ten mm will take it. the socket is then protected by c16= any lead plugged in is on 16a breaker an not c40

433.2.2 as I understand it would be a 200 amp fused or mccb protected bus bar, which you would then have protective device eg 100 A mccb feeding 25mm tails to db

Well I know what i'd do!

Sometimes you have to wonder if the IEE actually has any members that do any sort of electrical work any more...