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CU 32->40a fuse upgrade

Discussion in 'Electrics UK' started by nice-but-dim, 1 Aug 2020.

  1. nice-but-dim

    nice-but-dim

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    Hi and thanks in advance to all the givers of advice here.

    I want to upgrade a Triton electric shower from 8.5kW to 9.5kW but would need to swap the existing 32a CU fuse for a 40a fuse. Is there a way for an idiot to safely tell what size the wiring is on that circuit as I gather I need 8mm for the bigger fuse and have no idea what's installed?

    Also, I fear that my CU is at or near max capacity so need to check that the fuse upgrade won't exceed it. How do I find out? The board is a Wylex 1206 with a max capacity of 100amps but the aggregate rating of the fuses is over 200amps. Does that mean I'm chancing my arm if ovens, kettles, heaters, driers, laundry machines etc are all running at the same time?

    (Ideally, I should probably upgrade my CU but it's in a very inconvenient place (in an internal living area from a time before house extensions were added) and would be a major engineering job to run any more wires/cables to it so I will only upgrade as a last resort.)

    Thanks for any help - and apologies if my questions prove my stupidity.
     
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  3. terryplumb

    terryplumb

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    Do you have fuses ,or circuit breakers ?
    Cable size for 9.5 kw shower would typically need to be 10mm² in most installations .
    Although all the circuits " fuse" ratings add upto 200 amps ,its unlikely that you will actually draw anywhere near that ,due to diversity.
    Cable size may be found embossed into the outer sheath.
     
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  4. nice-but-dim

    nice-but-dim

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    Thank you terryplumb. I have a master RCD attached to the consumer unit and I think I also have a fuse in the shower spur socket. But I can't easily see the cable to find the size. Would I have to take the shower unit off the wall to find it or is there an easier way?
     
  5. terryplumb

    terryplumb

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    Turn mains power off and open the shower front casing .The cable is visible there. Take some pics and post here. Also look at the showers isolation switch ( ceiling pull cord type or wall switch maybe ) the supply cable is visible there ,again take pics and post here.
     
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  6. AndyPRK

    AndyPRK

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    I don't think 8mm cables exists.

    It will be either 6mm (likely the same as your cooker circuit) or 10mm
     
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  7. nice-but-dim

    nice-but-dim

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    Based on the Triton manual I have just read, you're right - but their cable sizing guide looks a bit clunky. Hopefully I'll find it embossed on the sheath.
    Cable sizing.png
     
  8. terryplumb

    terryplumb

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    What do you mean by " fuse in the SHOWER SPUR SOCKET" ??
    Pics would be useful !!
     
  9. nice-but-dim

    nice-but-dim

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    I'll get some photos later this afternoon. From memory I thought the shower was on a fused spur socket with an integral fuse caddy insert but I'll check.
     
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  11. EFLImpudence

    EFLImpudence

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    Technically a 32A MCB is too small for a 8.5kW shower.
     
  12. flameport

    flameport

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    No.
    It's unlikely you could get anywhere near 100A even with everything on at once.
    Even if you did, the supplier fuse for the property won't be more than 100A, so that will fail and disconnect everything.
     
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  13. nice-but-dim

    nice-but-dim

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    Thanks - that's a relief …and so I won't fret about upgrading my shower fuse from 32 to 40a.

    But I'm getting confused about the different types - and prices up to £30+ - of Wylex B40 40 amp fuses I can buy. I've found this one at £5.40 but it sounds like it won't work in my old-school CU:
    https://amzn.to/3jZU5cs
    …and then some are C40 which means nothing to me except that I should presumably avoid.

    I do have one (old) Wylex B40 40amp fuse installed which has "3000" stamped on it and not much else. So any advice will be much appreciated.
    Consumer-Unit-MCB-fuses.jpg
     
  14. terryplumb

    terryplumb

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    You can't just put a higher amperage MCB ,on your shower circuit ,without establishing the cable size and it's suitability. You should consider engaging an electrician . By the way they are not fuses ,but MCB ''s.
    And where is the RCD ??
     
    Last edited: 1 Aug 2020
  15. nice-but-dim

    nice-but-dim

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    I wouldn't dream of it, terryplumb. I'm waiting to get into that bathroom to take some photos, as advised.

    Sorry for the wrong terminology: I'm conditioned over 60 years to calling the CU a fuse box and its MCBs fuses.
     
  16. terryplumb

    terryplumb

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    Glad to know that you wouldn't just swap the MCB .
    Still none the wiser about where the RCD is .
     
  17. flameport

    flameport

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    40A MCBs for that fusebox are obsolete. 6/10/16/20/32 are the only values available now, other than dubious used efforts from ebay and other vendors of tat.

    It might be possible to use the 40A 'hob' one for the shower and the 32A for the hob, however that also depends on whether that particular fusebox can support 40A devices at all - many were limited to 30A maximum.
    Those that can do 40A devices typically only allowed them to be installed in one specific location.

    If the shower cable is 6mm² is may be possible to use that with a 40A device, although how and where the cable it's installed will need to be established, as will the total length and various other factors.

    Although one has been mentioned, there is no RCD in that picture.

    Ultimately that fusebox is several decades past it's replacement date, your existing 8.5kW shower is already overloading that 32A device, and the noticeable difference between an 8.5kW shower and a 9.5kW shower is somewhere between insignificant and none at all.
     
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