Q1. How can I rearrange the CU MCBs to allow a garage feed (assuming I should not use the RCB side)?
Although convention has the largest MCB’s closest to the feed, with modern consumer units the buss bars are big enough to allow one to change the order without any real problems. The buss bars in some consumer units are supplied uncut and the installer can select the number of MCB’s to connect to each buss bar. If the consumer unit is current then likely new buss bars could be obtained and cut to new lengths.
Q2. Can I use the spare 16A MCB unused for the garage?
The problem is 314.1 Every installation shall be divided into circuits, as necessary, to:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.
The division into circuits was assumed by many to refer to just the MCB but with the 17th Edition it was made plain this also included the RCD protection. To answer the question needs a risk assessment. If the garage was to trip the house supply or the house was to trip the garage supply would it cause a danger. Using emergency lighting could remove the dangers. But without viewing the installation no one can answer these questions for you.
Q3. As I am doing Kitchen work will I need to split its circuit away from the downstairs ring and provide a new one and use the other spare 16A?
Without viewing the installation no one can answer these questions for you.
Q4. What can I use to join the armoured cable near the CU?
There are loads of boxes that can do this job. From a standard socket back box with a blanking plate to an adaptable box complete with DIN rail and connectors.
The problem is changing MCB’s and installing circuits is not just a case of finding what will fit and work but includes testing and inspecting so that volt drop and the loop impedances will fall within the safe values. You will need to measure all these values and present them on the installation certificate to the local authority building control and even to hire the test equipment will cost around £75.
Add to this the LABC charges for any work with consumer units or kitchens under £500 it is just not worth doing DIY likely it will cost more than getting an electrician who is a member of a scheme and can self certify the work to do it all for you.
Although with the larger jobs because the LABC have a minimum charge of £100 plus vat for work up to a value of £2000 the DIY route may seem more appealing values like earth loop impedance don’t have a fixed pass value but vary according to types of MCB used and other protection methods. You will note the MCB has a marking like B16 or C16 and the letter is important when working out cable sizes and lengths.
Added to this in 2008 the regulations changed from BS7671:2001 to BS7671:2008 and with it the requirements for RCD protection was increased to nearly every circuit. Although there are exceptions and methods to avoid changing a consumer unit likely the easiest method would be a consumer unit change. Although in some cases this may be without any real problems, in many cases it high lights existing problems and can lead to a lot more work than just changing the box.
To ask the questions you have asked I would think for you to try to DIY this work would likely cause you so many headaches trying to jump through all the hoops presented by the LABC you would wish you had never started.
I am not saying don’t DIY but unless you understand things like earth loop impedance and TN-C-S or TT supplies then I would recommend you get an electrician to do the work for you.
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