Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about circuit protective conductor (CPC) sizing for a ring-main circuit serving 13A socket outlets, is answered by the NICEIC:
Therefore, high integrity earthing will be required in accordance with Section 543 of BS7671:2008.
Where RCD protection is not provided - as it could cause nuisance tripping - is a 2.5mm twin & earth cable, incorporating a 1.5mm CPC installed as a ring circuit acceptable?
By this I mean CPCs terminating in separate earth terminals at the back of each socket.
There is a clause stating that the CPC should be no less than 4mm if copper; therefore some engineers are of the opinion that where a 2.5mm live conductors are used, the CPC should be 4mm. Is this so?
Answer: A ring final circuit, by its particular configuration, provides duplication of the protective conductor and, provided the requirements of Section 543 of BS 7671 are met, an acceptable high integrity protective conductor arrangement (Regulation 543.7.2.1 (i) refers). However, for the requirements of Regulation Group 543.7 to be met, the ends of the conductors must be terminated independently of each other at all connection points in the circuit. This requires distribution boards, and accessories (such as socket-outlets, junction boxes, and connection units) to be provided with two separate earth terminals (Regulation 543.7.1.4 refers), one terminal to be used for each protective conductor, of a minimum conductor size of 1.5mm.
Therefore, 2.5mm twin & earth cables, incorporating 1.5mm CPCs and installed as a ring final circuit meeting with the all the relevant requirements of BS 7671:2008, would be an acceptable arrangement (Guidance Note 5 - 12.4.5 refers).
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