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?
Ok, we have interesting answers. The first is Andy which is "a floor void is ref method B". I suppose you have looked at number 40, or 47 of the diagrams, but you have not followed the formula provided. a 6mm cable in a ceiling with 8" joists is outside the range, and is clipped to the wood also, in my opinion, is still C. I think the rating of a 6mm cable is perfectly fine as described above because you have two more points:
1. The ambient is unlikely to ever be 30C in a bathroom or under a ceiling. In the middle of summer it may approach this, but then you will not want a very hot shower on top, at least I only want about the body, temperature maximum.
2. It will take at least an hour to heat this 6mm cable to the maximum temperature, at maximum current, etc, starting at 30C. This is very unlikely to happen, and even if it does it will take another hour to heat it much higher. 2 hours of showers doesn't even happen with several daughters (experience). It might in a sports centre, but not in a domestic.
The point is that both plumbers thought the cable undersized, and would have insisted on changing it.
It may well suit plumbers to use 10mm cable, but when changing a shower this could be quite difficult and needless cost to the consumer.
Interestingly no one noticed the 40A MCB fitted above. Of course, it is not a problem, as even at maximum current would take a long time to trip, if ever (see characteristics of 40A type B.). The joint box point is a red herring, joint boxes have overlapping conductors, the rating is based on terminal size, not actual current capacity. I am less happy about Wagos.
Essentially there are two things to consider here. The Manufacturer's instructions are not really adequate, they would say the same size cable (which is not their problem) if it is clipped direct or run in very good insulation. In good insulation 10mm may be insufficient. Why do the instructions not just refer to BS7671, perhaps they don't know what is in it, which is serious. They may assume that the installer is unaware, in which case they should refer to a suitably qualified electrician and BS7671. Now I ask the harder question (generally passed by the answers above) how many electricians even understand the dialogue above, other than our own interested parties? How many of them could EICR this installation correctly? Comments please!
Ok, we have interesting answers. The first is Andy which is "a floor void is ref method B". I suppose you have looked at number 40, or 47 of the diagrams, but you have not followed the formula provided. a 6mm cable in a ceiling with 8" joists is outside the range, and is clipped to the wood also, in my opinion, is still C. I think the rating of a 6mm cable is perfectly fine as described above because you have two more points:
1. The ambient is unlikely to ever be 30C in a bathroom or under a ceiling. In the middle of summer it may approach this, but then you will not want a very hot shower on top, at least I only want about the body, temperature maximum.
2. It will take at least an hour to heat this 6mm cable to the maximum temperature, at maximum current, etc, starting at 30C. This is very unlikely to happen, and even if it does it will take another hour to heat it much higher. 2 hours of showers doesn't even happen with several daughters (experience). It might in a sports centre, but not in a domestic.
The point is that both plumbers thought the cable undersized, and would have insisted on changing it.
It may well suit plumbers to use 10mm cable, but when changing a shower this could be quite difficult and needless cost to the consumer.
Interestingly no one noticed the 40A MCB fitted above. Of course, it is not a problem, as even at maximum current would take a long time to trip, if ever (see characteristics of 40A type B.). The joint box point is a red herring, joint boxes have overlapping conductors, the rating is based on terminal size, not actual current capacity. I am less happy about Wagos.
Essentially there are two things to consider here. The Manufacturer's instructions are not really adequate, they would say the same size cable (which is not their problem) if it is clipped direct or run in very good insulation. In good insulation 10mm may be insufficient. Why do the instructions not just refer to BS7671, perhaps they don't know what is in it, which is serious. They may assume that the installer is unaware, in which case they should refer to a suitably qualified electrician and BS7671. Now I ask the harder question (generally passed by the answers above) how many electricians even understand the dialogue above, other than our own interested parties? How many of them could EICR this installation correctly? Comments please!
