ring final spur

Hi all,

can anyone comment on the following with respect for compliance with BS7671?

2.5mm spur off a ring final circuit suppling a 100amp main switch 2way consumer unit suppling 6amp mcb for two lights and a 16amp mcb for one double socket.

any responses appreciated.

RMS

The 2.5mm spur ought to be fused. An FCU with a 13Amp cartridge is the easiest way.

You do not say if the subcircuit is out of doors or in a different building like a shed.

As there a separate consumer unit it would be better to treat it as a sub-main from the main consumer unit, rather than as a spur.

Hi johnd,

This is just an observation i have made on a job. The board is in a out building that is intergrated with the house. The socket circuit is supplied via an rcd at the main consumer unit.

The rcd was tripping and i was called out today after an electrician had renewed the main consumer unit and fitted the submain and left the job in this condition. The rcd still tripped when all circuits were off so i dropped the neutrals out one by one and the downstairs socket circuit cleared the fault when i dropped the neutrals out. I later found that this submain was fed from the ring circuirt and isolated the submain which cleared the fault. Have not looked into what on the circuit is causing the fault but noticed the way it was supplied from the spur.

RMS

The cable should not be spurred direct off the ring as it is quite possible to have 32A running on the 2.5mm² without an OCPD operating. As the cable is feeding a fuse board a 32A load is not infeasable.

The rcd was tripping and i was called out today after an electrician had renewed the main consumer unit and fitted the submain and left the job in this condition

Click to expand...

mmmm, pretty poor if you ask me! with regards to BS7671 its not right. potentially the sub main could be altered by some unsuspecting/ unknowledged person who could run more circuits of it, therefore more current than the spur cable can take.

the fcu would offer the protection but its poorly done by the sounds of it and id be disconnecting it if i was you and doing it right.

sounds like a bit of a lash up to me