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Sub-Board - Second Consumer Unit

Discussion in 'Electrics UK' started by FRAN1870, 7 Nov 2018.

  1. FRAN1870

    FRAN1870

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    I’m sure no honest electrician would sell anyone something they didn’t need, but I’d like to be in a position to qualify what I’m being told. Like remember the bloke who put the second electric shower with only 60amp total (claiming diversity)! That was HIS decision too, a different electrician but according to you I should just give license to ANYONE to choose cable and spec design completely unchallenged or understood!!

    That’s all I’m trying to do, there’s really no need to be rude or make a mountain out of a mole hill... it was a pretty simple question regarding cable spec.
     
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  2. mikeey84

    mikeey84

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    Your original postings certainly read differently from that, as you said you were going to do it, then get it checked.

    That is the part that has got some of the forum members, in your words, hostile, as it's a not the correct order to do things.
     
  3. FRAN1870

    FRAN1870

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    Running the cables .... ok maybe I did miss the bit about him hooking it up but I think I’ve made that clear throughout the thread in later posts.

    No need for the arsey replied imo
     
  4. mikeey84

    mikeey84

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    Its not just the hooking up, it's the fact that your electrician should be saying what cable size you need, not some randoms on the internet.

    As BAS says, whoever signs it off should have designed it, and that obviously includes cable routings, size and the myriad of other considerations an amateur sparkle may not consider, or even know about.

    That is what gets, especially BAS, riled up, and for a good reason
     
  5. FRAN1870

    FRAN1870

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    I think you’ve misunderstood me and for that I apologise if I’ve not made it clear.

    I’m not going to disregard what the electrician said I’m merely floating the question to see who agrees or disagrees with the spec..... it’s an open discussion about cable size so if some clever chap on here came back with a plausible calculation for using 10mm over 16 then I could take that observation back to the bloke who designed it and say “what about this” he then has the choice to discuss it based on what might be a more experienced sparkles view.

    Not everyone knows everything about everything and often design is about perspective and interpretation. I had the same with my loft conversion.

    One guy said the ONLY way was to sling two massive steels across... another guy came out and did it a completely different and way more cost effective way. The structural calculations were signed off as was building control so had I not questioned the first guy I’d of been 10k worse off!!

    Two people with very different views.... it’s nothing to do with Part P or who pulls the cable. It’s more about collectively finding a solution to a problem.
     
  6. mikeey84

    mikeey84

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    No need for an apology, it could be that I've read it wrong. I was just trying to explain one of the reasons why you might have had the reaction you got from certain forum members.
     
  7. EFLImpudence

    EFLImpudence

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    I haven't read the whole thread but -

    So why do you need to run new cables?
    Are there not cables supplying all the usual things?


    Also, although it depends on the installation methods,

    10mm² is often used to supply whole flats with a 60A fuse/63A MCB - not necessarily tiny ones.
    16mm² would be big enough to feed your whole house.
     
  8. ban-all-sheds

    ban-all-sheds

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    Just a thought - if you're supplying a CU with B32s in it, what upstream device do you need to get selectivity?
     
  9. JohnW2

    JohnW2

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    One might 'wonder', but, as a general comment, I don't think one could actually determine by calculation whether one was likely to get discrimination between a particular pair of MCBs unless (a) one had curves indicating actual (typical) disconnection times with currents considerably greater than (for a Type B) 5In (for the lower-rated MCB) and (b) a specification which indicated the duration of a current >5In (for Type B) below which it must not operate (for the higher-rated MCB). I don't think that the former are freely (if at all) available, and I don't think the latter is included in the spec of an MCB.
     
  10. SimonH2

    SimonH2

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    In the event of a "hard" fault then you'd get next to no discrimination between the B32 in the 2nd board and any practical size B curve MCB in the main board. The fault current would probably be significantly higher than the trip level of the upstream device as well as the B32 and unless one of them is slow to trip, then it's quite likely that both would - both having reached the point of "mechanical bits on the move" before either of them disconnects the fault current.
    Obviously the higher the fault impedance (ie lower the fault current) and the wider the difference between MCB ratings, the better the chance of the downstream one tripping on it's own. If the fault current was such that the B32 was only just tripping in the times required, and the upstream was (say) 63A, then there's a chance that the upstream MCB wouldn't trip before the downstream one had disconnected the fault. But I think that's more likely to be something of an outlier in terms of possible scenarios.

    For overloads, then it depends on other loads in the board. With large loads on separate circuits, it would be practical to reach load currents that don't trip any MCBs in the second board, but which can trip the MCB in the main board. But that's the compromise whenever you guestimate the diversity factor for the loads on the 2nd board - I say guestimate because while there are rules on how to do it, at the ned of the day even those are just guesstimating what sort of typical load patterns are likely to be found and won't be valid for all situations.
    So if, for example, you loaded up 3off B32 supplied circuits to (say) 30A then you'd trip an upstream B40, be close to tripping a B50, but probably wouldn't trip a B64 (nominally, it would depend on which end of the tolerance band the devices sat in).

    There is a reason why it's often recommended to use a cartridge fuse as the upstream OPD. There's a distinct difference in tripping curves which is likely to give discrimination.
     
  11. JohnW2

    JohnW2

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    That's all intuitively (and probably actually) true - but, as I just wrote, one could not attempt to 'determine' whether discrimination was likley without information which we generally don't have (and some of which may not even be available).
    Again, very true, but it's not conceptually different from the situation with the average domestic CU - if all the final circuits were loaded 'to their maximums/maxima', simultaneously, the current would probably be adequate to blow the service fuse pretty quickly.

    Kind Regards, John
     
  12. FRAN1870

    FRAN1870

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    It’s an old house and everything is on a downrated radial circuit... dogged off here there and everywhere including an outbuilding. Nothing has ever tripped but I did have a socket start to melt once, which I was told was just bad fitting

    (It was fitted by a part P electrician )

    Sparky advised kitchen on a ring main as well as obvious dedicated feeds for ovens etc.... it will also allow me to isolate the outbuilding.

    I might be able to use some of the cables already in the walls but I’m taking the opportunity to sort out that end of the building on its own board so I know exactly what runs from where. I think it’s the sensible thing to do rather than just adding to radials that have been added and added to?
     
  13. FRAN1870

    FRAN1870

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    I’m no sparky but I kind of follow what you’re talking about and I’m intrigued .... surely the concept of selectively applies to every plugged appliance with a fuse? 3amp or 13amp?
     
  14. JohnW2

    JohnW2

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    It does, and if there is a fault which results in a very high fault current (e.g. 'a short circuit'), it is essentially pot luck as to whether the fuse (whether 3A or 13A) will blow, the MCB will trip, or both (and if there's an RCD in the equation, in some situations that could trip first, without fuse blowing or MCB tripping - but, again, all three might 'do their thing').

    The point is that fault currents due to 'short circuits' are very high, typically hundreds of amps. Hence, not only is that many times what will blow a 13A fuse, it is also many times what will make a 32A MCB trip. What happens is therefore down to which manages to 'work' the quickest. If the 'other one' (fuse or MCB) is already 'past the point of no return' when the first one stops the current, it will also blow/trip.

    Kind Regards, John
     
  15. FRAN1870

    FRAN1870

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    Pot luck? Surely there’s a science to it?
    Although I’m not sure what that’s got to do with my question?

    I guess the basic requirement is that the cable and mcb can take the load applied to it under normal working circumstances.

    I’m not on a smart meter but is there a way to “measure” current usage in amps?
     
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