240V 9 kW Shower Fuse and Wire Question

Steve Mowbray 22 years ago

Hi - I am about to replace my old power shower and discovered the following:

9.0kW (240V) shower, 6mm² wiring (run length less than 10m), 30A rewirable fuse in consumer unit.

To my mind this should not be possible and should really require 10mm² wiring and a 45A fuse. Just to make sure that nothing was amiss I rewired the fuse and also tried a replacement 30A cartridge fuse - both worked fine.

Just to make things more difficult - my old style consumer unit does not support new style 45A fuse holders. So my simple (ish) shower replacement job has become a much bigger new consumer unit and rewire shower fitting job..... OR am I missing something??? The supposed 9.0kW shower has worked fine for at least 5 years (since I moved in) and probably a few more in use at least twice a day (at a hot setting).

I can possibly accept that a _shortish_ run of 6mm² wiring might survive working at 110% capacity for the infrequent shower use without overheating (much) or too high a voltage drop - but I cannot see how a 37.5A rated device does not appear to trouble the 30A rated fuse.

Can anybody help me understand.

Ta Steve

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Paul 22 years ago

A Semi Enclosed (rewirable fuse) to BS3036 will not interrupt the supply (for at least 10000secs) until the current reaches 55 amps. This however is not good practice, as the fuse will inevitably get warm and start smelling like a fish factory!!! The max current for a twin core (and earth) 6mm cable run is determined by how the cable is installed. In an insulated wall it is 32 amps or if clipped direct to a non metalic surface it is 46 amps. For 10mm cable it is 43A and 63A respectively. Without knowing how the cable is run, I cannot advise on which cable should be used. A (WYLEX) 45A fuse can only be installed next to the main incomer (switch) as it is keyed. Hope this helps, PF

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Steve Mowbray 22 years ago

I didnt realise the over-current tolerance was so high - I also tried the

30A cartridge fuse type in place of the rewireable fuse to see if this was a little more accurate.

Thats cleared up the spec for me thank. FYI cable run is 3m under floorboards and 3m buried in wall plaster under tiling.

My consumer unit is MEM type, pre-45A. Time to upgrade to a modern split supply unit with breakers I think.

Yes it helps a lot - thanks.

replacement

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