I'm looking to power my garage for a couple of sockets and lighting. Its about 10mtrs from the house and i aim to use SWA cable and a 2 way garage unit with RCD protection. Its quite an old house (1960s) and has no RCD protection at the cons unit (MEM) and all of the fuseways are already taken. I know that i require RCD protection for the garage, but was wondering which was the best way to wire this? Shall i take a SWA supply from the meter cupboard outside via a henley block and switchfuse unit and run it to the garage to connect to the 2way board with RCD protection? or Should the RCD be installed in the meter cupboard, via henley block before it goes to the garage supply?
Unless you live in Scotland the costs of DIY may exceed the cost of getting it done for you with LABC and test equipment hire charges to comply with BS7671 and Part P.
The problem with a henley block is unless the cable can take the whole of the supply (likely 100A) then some means to reduce to supply is required. Normally a mini-consumer unit. Since you need to fit this then I would be looking at a 32A (or less) RCBO to both add RCD and max amps protection in one device. Although really the RCD could be in the garage.
I have questioned many times fitting a consumer unit in a garage. (Unless using 16A sockets for welders) I would think in most cases a switched FCU with 3A fuse for the lights allowing the MCB/RCBO in the house to limit the output to sockets is ample.
Personally unless you intend to do welding which needs permission from DNO then a 13A supply to a garage is ample and normally a FCU from the final ring is all that is required.
So normally I would say a RCD FCU in house to supply garage and a further switched FCU in garage for lights and no henley block or consumer unit.
The big problem is earthing one does need to decide how it is to be earthed TT or TN before one goes any further. Loop impedances will need to be measured and consideration given to volt drop. Remember since also lights only 3% permitted.
I would get an estimate first and likely he will say how he would do the job. Then price up what you need. Add £350 for LABC and hire charges then decide is it really worth DIY?
Thanks for that Eric..On second view, it appears that i do indeed have a spare fuseway to connect into. I am going to try purchase an MEM Rcbo (prob 32A) to add to the board and then take a supply to the garage from this. I can drill through to the outside underneath the floorboards, to enable me to mount a metalclad 13A fcu on an outside wall. Then I will look to take 2.5mm SWA into the garage to connect to a metalclad socket or maybe a FCU. This will then allow me to add some light in the garage. Does this sound ok??
It will likely work although also likely will not comply. However it's not me who will be inspecting the work. What I would do is when you submit the plans to the LABC make sure they are detailed.
If one says I am going to wire up my garage here's my fee can I start. Then the LABC guy can latter inspect and say that's no good rip it out. However if you say I am going to use a B32 RCBO feeding 6mm three core SWA using a metal adaptable box etc. Then it would be very hard once permission was granted to say sorry rip it out I want 10mm cable. What you want is for any inspector so say before you ever buy the cable he wants you to use 10mm not after the job is complete. So submit detailed plans.
The reason I say likely will not comply is all cable has a variable rating according to how it is installed and the temperature the cable and items connecting to the cable can withstand.
2.5mm can vary from around 13.5A (thermoplastic Reference Method 103) to 33A (thermosetting Reference Method C) it is unlikely (although not impossible) that 2.5mm will be considered good enough for what you intend to do.
One also needs to consider the 3 meters rule when protecting against overload at the destination rather than the source.
The problem is over 2.5mm you may not be able to terminate in a socket or FCU where there are also other cables going into the same terminals. Although with MK one is suppose to be able to get 2 x 6mm in the holes.
There is also volt drop and loop impedance to consider. You have given no lengths or impedance figures at the consumer unit or what the earthing system is. So no one with information given can say yes or no.
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