Electrical Question

What size mcb do i need if maximum current is 13a?

I want to install a supply to a single socket in a log cabin 200metres away from the house. I have calculated that to have a maximum load of 13A, I must use a 10mm sq armoured cable. What size mcb do I need to fit in the CU taking into account that 13A will be the maximum current taken?

I have a 10mm 3 core cable, using the spare core as a earth connection from the CU, also the swa is bonded to earth at both ends will this be sufficient, or will I still need to use an electrode? I have a 2 way CU in the cabin fitted with 30ma RCD, and have a 10A mcb feeding the socket. Is this acceptable?

There is no running water in the cabin, just clarifying is discrimination from the house CU to cabin CU ok if I fit 16A mcb in house , and ok with 10A mcb RCD protected in cabin to limit max current.

2 Answers

Best Answer

Hi Bernie, it does look like you are trying to do this yourself and without trying to sound too negative, you have some knowkledge but maybe not enough to do this safely, so we can not really offer advice as to the safest method without knowing the application much better than anyone can explain in a quick note.

Regulation 708.411.4 permits the use of a PME earthing facility for use within permanent buildings and the ZS with a line and earth of 10mm over this distance will meet disconnection times if you are set up correctly. If you do not understand what this means, then please ask someone to come and do the work for you.

As for a TT system, the 100ohms as noted in an earlier response is only typical to a 500mA RCD, Assuming you are going with a 30mA as you would need to, maximum Zs, if RA is not known needs to comply with this calculation RA * Idn <= 50V, hence on a 30mA trip, it is 1,667 Ohms, 411.5.3, though anything over 200 Ohms in reality is not really stable.

Again, if you do not understand these terms and what they mean on reading them (as opposed to using google to try and understand them) then get a qualified Part P registered (this means that they have had to prove their competence annually) electrician to do the work for you.

If you think that you can save money by buying the materials, most of us will allow that, but this really is false economy.

Answered 4th Oct 2013


Member since 4 Oct 2013

It must be stressed that you have a LEGAL obligation to Design, Install and Test this electrical installation to the required standards set out within BS7671:2008. I feel that its likely that you do not have a sufficient knowledge of BS7671 to be able to do the cable calculations and electrical design to justify this requirement?

Just asking a few questions on this website is a far cry from doing a suitable and sufficient design. For your own safety and to prevent a costly future prosecution by LABC, I would get a Part P registered electrician to design and install this for you and abandon trying to do it yourself.

At 200m irrespective of the use of a separate core of the SWA, it is not possible to export the main earth from the house out to the outbuilding. To do so would be dangerous and may well not be permitted by your local DNO, depending on what your incoming service arrangement is. The main reason is that the disconnection time for the circuit protective device is unlikely to be able to be met due to the high impedance at the remote end. Therefore a TT installation with an Earth Electrode and 30mA RCD must be used. The earth electrode must be installed and have a RA reading of less than 100 ohms (measured in dry weather).

In fact it could be argued that depending on whether there are extraneous-conductive-parts inside the cabin?, that by exporting the earth, you are actually introducing an earth potential difference. Thus it could be that the local earth at the outbuilding has a different potential to that of the main house, so again another good reason to TT the remote end and treat its as completely separate installation.

Answered 4th Oct 2013

Electrical Safety Services

Member since 17 Oct 2011

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