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smoke from fuse box - fuse didnt trip

Discussion in 'Electrics UK' started by andrew55, 20 Nov 2005.

This topic originated from the How to page called Fitting an electric shower.

  1. ban-all-sheds


    27 Aug 2003
    Thanks Received:
    United Kingdom
    That doesn't need a 45A supply...

    How did you resolve that?

    4mm² is rated at 37A if it's just clipped to the surface of a wall, but introduce any thermal insulation, or bunching with other cables, into the mix and the capacity drops off, quite conceivably to well under 30A.

    Also bear in mind that all cable ratings assume that the temperature of the copper core will rise to 70°C, which is pretty hot - well over the pain threshold if you should touch it, so it only needs a bit of extra heat from a poor connection, and hey presto - damaged insulation.

    It is more likely to be threaded through holes in the noggins?

    However it is installed, it won't be that easy to replace, I imagine, so if you're going to do it, you might as well put 10mm² in, then you have the option of a more powerful shower when the current one packs up...
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  3. andrew55


    20 Nov 2005
    Thanks Received:
    United Kingdom
  4. mapj1


    24 Nov 2004
    Thanks Received:
    United Kingdom
    Unless you are sure the holes they drilled originally are oversize, pulling in a new cable with the old carries the risk of a jam..
    You might do better running the new cable over the surface and rising in an understairs cupboard or similar (not seen your house, may not work).

    However, the saving grace, and the only reason it has been more or less OK so far, is that the cable ratings are for continuous use, at a 30 degree ambient temperature. (i.e. nominal load current gives a 40 degree rise to an operating temp of 70C)
    I suspect the load is on for 15-20 mins at a time tops, and usually the ambient temperature is rather lower (this being the UK and all that.)
    If the cable is run hot, it doesn't suddenly catch fire at one amp over its normal rating, rather the PVC starts to oxidise where it is exposed to the air, giving it a brown and crackled look. Grosser overloads that raise the temp to 120 degrees plus cause the copper cores to migrate like a hot wire cutter (which they are :LOL: ) through the plastic, until something goes bang.

    See here http://www.iee.org/Publish/WireRegs/Commentary-UpdateApr04.pdf for some tables of PVC cable lifespan under under/overload conditions.
    For modest overloads (perhaps up to ~200% of normal power dissipation -i.e an 80 degree rise to a final temp of 110C instead of a 40 deg rise to 70C) the expected life of cable is divided by 3 for every 10 degrees increase.
    In practice, this should not happen very often, even in situations like ring main with a break, and for an overload that is only on for some minutes every day the effect may well not be noticiable for years. This doesn't mean it is a good idea though, and while just repairing the damaged end will get you on the road for the winter, a new, larger cable is certainly the way to go.

    PS note, a 200% cable overheat occurs at a 141% current overload - as the voltage drop also rises as well as the current, dissipation is current squared.. So, no more than 30A through 20A cable please, and do realise that will shorten the cable life to less than a year of overloaded operation !!
    (It is with this in mind that thermal circuit breakers are set to operate before or around 1.4 times their nominal continuous rating)
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