New house and the garage is "built-in" under our bedroom on the corner of the house. Previous owners have taken a feed from an adjacent room into the garage with a switched fused spur, and then put a couple of sockets on one wall. All in surface-mounted plastic conduit, RCB protected at the CU. I need to add an extra 4 sockets for convenience around the garage. Don't expect them all in use concurrently, so no issue with the 13A fuse on the spur. But I also have the option of bringing the ring down from above, so all the sockets are on the ring rather than a single fused spur. Given I don't need more than 13A, is there any advantage in the extra work of extending the ring? Feels odd having so many sockets on a single spur, even though I know it's safe. One advantage of extending the spur around the garage is I have a single kill switch for all sockets in there, should there ever be a reason for such isolation. Probably overthinking this, what would you do?
I would not run of house fused spur as you may more than likely increase what you do in garage ,ie whack a welder or compressor on it
A 13 amp supply will allow using 2.5 mm² direct from consumer unit around 54 meters to be within volt drop, clearly not that long as coming from ring, but one is unlikely to exceed the limits, a ring final can have 106 meters to comply with volt drop, so step one is measure the loop impedance or prospective short circuit current at consumer unit and the doner FCU and calculate volt drop from the loop impedance or PSCC to find how close one is sailing to the wind before extending the ring. As to using R1 + R1 using a low ohm meter or loop impedance using a loop impedance meter or the figures from the installation certificate is up to you. However if lacking test equipment then you should of course not do the work, but assuming you will DIY your less likely to exceed the limits with a fused spur than extending the ring final, so I would when using guess work rather than meters go for the FCU method.
R1+R2 has little bearing on earth fault disconnection if an RCD is fitted, we do need to know it as part of the ring final circuit continuity test procedure. In a house with a ring final circuit serving a floor area of 100mt 2 or less, it is unlikely that the R1+R2 value for earth fault disconnection time without a RDC will be exceeded as the ring final circuit is over engineered as far as cable CSA is concerned. It is correct that the FCU method is the simple method more suited to the DIY person.
Thanks for the replies. The ring I'd be extending is pretty small, serves a bedroom above and utility to the side of the garage. Even if the installer wanted to waste cable they'd probably struggle to use up 30m so voltage drop over the length won't be an issue. As a DIYer I obviously don't have £700+ test equipment. I could take the readings from the last EIC and do the math from there to calculate a revised loop impedance but the one I got when buying the house is around 15 years old so I can't be sure my starting values are accurate. To that end, keeping it all on the FCU and not extending the ring seems like the safer option.
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