I had a phone call from my son yesterday, the shower in his house had broken and not being a plumber/electrician got a couple of quotes. The old one was 6.5 kW with 6mm cable. The plumbers all seemed to think that the cable and MCB should be replaced as the current rating was inadequate for a replacement, the MCB is 40A. I had a quick check what could be fitted as there are many more powerful ones available and this weather the water is not wonderfully warm! I have not really thought about shower cables for a long time as they are almost always in good condition. Table 4D2 6mm clipped direct says 46A, continuous rating. A 10.5kW shower takes 45.65A on 230V so is quite satisfactory on the 6mm cable. In fact, the model I was looking at (Mira fancy thermostatic, expensive) would only take this power at full flow and temperature rise as it has electronic thermostatic proportional control and normal running current would be modulated and somewhat less. I consider this to be perfectly satisfactory, but the plumbers obviously haven't read the regulations. I often find the same with cookers, various sellers suggest 10mm cable, which may well not be the existing size. Should any change be made, and why do would you do this?
I had a phone call from my son yesterday, the shower in his house had broken and not being a plumber/electrician got a couple of quotes. The old one was 6.5 kW with 6mm cable. The plumbers all seemed to think that the cable and MCB should be replaced as the current rating was inadequate for a replacement, the MCB is 40A. I had a quick check what could be fitted as there are many more powerful ones available and this weather the water is not wonderfully warm! I have not really thought about shower cables for a long time as they are almost always in good condition. Table 4D2 6mm clipped direct says 46A, continuous rating. A 10.5kW shower takes 45.65A on 230V so is quite satisfactory on the 6mm cable. In fact, the model I was looking at (Mira fancy thermostatic, expensive) would only take this power at full flow and temperature rise as it has electronic thermostatic proportional control and normal running current would be modulated and somewhat less. I consider this to be perfectly satisfactory, but the plumbers obviously haven't read the regulations. I often find the same with cookers, various sellers suggest 10mm cable, which may well not be the existing size. Should any change be made, and why do would you do this?
I normally go up to a 8.5kW shower on a 6.0mm2 T&E with no problems if the run is relatively short. Some people that have new 10kW plus showers installed only ever use them on half heat setting as full heat is just too hot. Of course if the cable is covered in loft insulation or is run in hot conditions it must be down rated. I have seen shower cables covered in thick loft insulation which has been installed years after the cable. There again unless the shower is used by teenage girls it is only in use for a few minutes.
The plumbers probably have not read the wiring Regs. but have read the maker's instructions which often specify larger cables than are really necessary.
You can have an extra amp in table 4D5 for T&E - installation methods 57 or 58.
Problem is that even if those plumbers have heard of diversity, they probably think that it means that you have to genuflect at the start of a game or race. ?
in reality, if it is clipped direct, you could probably run the shower on a length of 4mm, as the shower will be time limited (unless I suppose you have several teenage daughters). It tends to be the switches that fail, rather than the middles of cables. However, the makers of the shower tend to assume the worst case cable routing and grouping factors to avoid trouble, and to copy the same instruction sheet for all models, regardless of wattage.
However, when I took out the cable for the one here, which was 6mm, and clipped direct, really on the surface, up the side of the stairs, so at about 45 degrees or so, I too fitted 10mm, as the price diff was not great and it is future proof against anything. And Mrs mapj1 did not like it at ankle height up the stairs so it had to be converted to a more vertical run and chased under the plaster..
To a plumber I can see the attraction of simply always using a one size fits all reel of 'shower cable' rather like kitchen fitters and 'cooker cable'
If there’s no insulation in contact the cable installation method B usually fits the bill with part of the cable running in building voids and other parts in mini trunking.
