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Extra socket question

Discussion in 'Electrics UK' started by Andy Byford, 27 May 2021.

  1. Andy Byford

    Andy Byford

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    Hi all,

    Thought I'd pop a question on here if anyone doesn't mind sharing their expertise it would be greatly appreciated :D I'm fairly proficient at DIY but regs can get complicated with electrics, so wanted to ask first.

    I understand safe zones for electric cables, I was just wondering if it would be possible to spur 4 sockets (3 horizontal, 1 vertically from 2nd spur) from an existing socket if using a 13a FCU between original socket and first spurred socket. Im presuming the original socket isn't a spur as its the only one on that whole wall and no work has been done to the house since being built, but have not checked yet (I will check before doing any work)

    Is there a way to tell if the total length would be under 100m? I'm looking at adding approx 5m extra cable in total as all these sockets will be fairly close together. (Installing new desk setup for multiple monitors and devices such as audio amplifier, consoles and PCs)

    Was going to channel these into the wall inside some oval conduit and get a spark to come sign off on it before plastering the cables in.

    Thanks in advance!!
     
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  3. EFLImpudence

    EFLImpudence

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    You seem to have been overlooked.
    Yes, it is possible.

    Right.

    You could measure the resistance of the wires at the existing socket between the two Ls, two Ns and two Es.
    100m. of 2.5mm² T&E should be 0.73Ω for the L & N and 1.22Ω for the E.

    The 100m. is only a guide working on standard values. Is it a big house?


    Oval conduit is optional; you can just bury the cables if you want.

    You can ask en electrician to test your work and record the measurements but there is nothing to officially "sign off".
     
  4. JohnW2

    JohnW2

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    Not only that but, also, the OP is talking about a fused spur, not an extension to the circuit.

    I would guess that the "100m" figure is approximately that (often cited by eric) total length of a ring circuit that, under worst-case conditions would keep voltage drop below the guideline maximum figure.

    If so, then adding a spur would not add to the ring's total length (whatever that may be), hence VD anywhere on the ring. Furthermore, even if the ring were already 100m (as you imply, unlikely to even be approached other than in a very large house), then a spur attached to the ring 'not too close' to the middle of the ring would not result in worst-case voltage drops, anywhere, exceeding the guideline maximum.

    We appear to be talking about ~5m total length of a 13A fused spur. In the worst-case (a 13A load at the end of that spur), the VD at the end of the spur would therefore only be about 1.17V (about 0.5% of 230V) more that the maximum possible VD at the point the spur joined the ring (which itself would inevitably be appreciably less than the guideline minimum, if a significant distance from the middle of the ring).

    The bottom line of this rambling is that I don't think the OP should worry at all about cable length, and I imagine that you would probably agree.

    Kind Regards, John
     
  5. Andy Byford

    Andy Byford

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    Thanks for the replies.

    The house is a 5 bed over 3 stories, the socket I'm extending is downstairs. It's not overly big though as it was built as a 3 bedroom originally before we had a loft conversion.

    It's good to know what you've both said, I know regs can have things written into them that sometimes I'd not think to take into consideration. Didn't want to start channeling walls out until I had a better idea, the misses would not be happy haha!

    I'll proceed with fitting it all and for my own peace of mind, I'll grab a spark to test it before it's covered over.


    Thanks again
     
  6. ericmark

    ericmark

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    As said the 100 m² is a guide to help not have too much volt drop or loop impedance the latter is to ensure the magnetic part of the MCB will work, or a fuse will rupture in time with a direct short circuit.

    I a long time ago wrote myself a java script program so I could enter the readings to ensure I had not exceeded the limits. upload_2021-5-28_7-55-35.png
    However when I came to use the program I found the accuracy of the loop impedance meter and the fact that the design current is not really fixed, means that one would have to be well over the limits before one could really take an electrician to task for going over the limits. Readings shown is what is claimed it the maximum length of a ring final. They assume 20 amp at centre and 12 amp even spread, so average of 26 amp. I am sure you can see flaw in that.

    About the only item today which may have problems with volt drop is refrigeration, the motor needs to reach speed before the pressure raises too much or it will stall, and the overload will trip and once cool it will try again, if the volt drop is too much then the overload with repeated use will fail, I did see a few AC units fail this way, but the voltage was way under the design voltage, looking at less than 200 volt for a 220 volt unit. The other is fluorescent lights not striking, but most other stuff used today will take quite a volt drop, so no longer important.

    The other is the tripping of the MCB or rupturing of a fuse with direct short, in the main worried about short to earth, but with RCD protection the RCD will open even if the earth loop impedance is well over the permitted level for the MCB/fuse, so although we should work out the earth loop impedance in real terms it is unlikely to cause a problem if high.

    Note DC it is called resistance and AC called impedance, both measured in ohms, so although not strictly true you can regard them as the same. So just do it.
     
  7. Andy Byford

    Andy Byford

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    Thanks
     
  8. Andy Byford

    Andy Byford

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    Whilst I have some knowledgeable folk here, could I ask another question:

    I want to spur one socket onto my chimney breast, the depth of this is around 250mm, could I run a socket from a wall next to this to the front of the chimney breast? Technically the sides are slim enough so that the safe zones cross over and the front would be covered with the new socket showing cables could be either side..

    I've attached a photo to show where I mean.

    Thanks again
     

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  9. JohnW2

    JohnW2

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    I would say that your interpretation of the rules is probably strictly correct.

    Kind Regards, John
     
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  11. Andy Byford

    Andy Byford

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    Thanks for the quick reply, technically it's all still following safe zones but just wondered if I missed something.

    Cheers John
     
  12. JohnW2

    JohnW2

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    I think the only issue is probably that, as I said, it is 'not ideal' - since, although it is 'technically correct' (and hence 'compliant with regs), I would not like to guarantee that others (with drills in their hands!) would necessarily 'twig' that the two overlapping 'safe zones' (I tend to call them 'dangerous zones' :) ) rendered the entire side of the chimney breast a 'safe zone'!

    The new cable would, of course, need to be RCD-protected in order to be compliant.

    Kind Regards, John
     
  13. Andy Byford

    Andy Byford

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    Hi John,

    My consumer unit has RCD protection built in, does this cover the new socket?

    I don't plan on moving any time soon but I can label up the back box to ensure any future occupiers can find where it is cabled to, I measured just now and it's actually only 143mm deep (Not sure why I had 250 in my head!) So the whole wall is classed as a safe zone anyway
     
  14. JohnW2

    JohnW2

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    Yep.
    Fair enough.

    Kind Regards, John
     
  15. Andy Byford

    Andy Byford

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    Thanks again :D
     
  16. JohnW2

    JohnW2

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    you're welcome.

    Kind Regards, John
     
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