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10mm t&e with 4mm cpc

why does 10mm T&E have a 4mm CPC, when it should be at least half conductor cross sectional area. The cable I refer ton is branded "basec", from a reputable supplier.

1 Answer from a MyBuilder Electrician

Best Answer

I think you are getting your regulations mixed up. Its only the main incoming supply meter tails where the CPC is recommended to be 50% of the size of the Line and Neutral conductors.

BS7671:2008 allows for CPC's to be either;

(i) calculated in accordance with Regulation 543.1.3, or
(ii) selected in accordance with Regulation 543.1.4.

Where calculated, the csa of the conductor to be used as a circuit protective conductor has to be not less than the required value (S). The normal method of calculation is by means of the adiabatic equation, given in Regulation 543.1.3

6424Y Twin & Earth PVC Cables to BS6004 are permitted to have a smaller size cpc at 6.0mm, 10.0mm and 16.0mm, and under most circumstances will comply with 543.1.3. Therefore provided you design the circuit corectly and select an appropriate circuit protective device you should not have any problems with earth fault loop impedance or need to run a separate cpc.

If the CPC size needs to be larger or you are expecting high earth fault current or earth leakage current, then it would be better to select a different type of cable and installation method, e.g. 10mm singles in conduit or a 10mm 3 core Steel wire armoured cable.

Hope this helps, please click on "like" if does.


Answered 20th Jan 2013

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