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Electric Showers and plumbers

David Stone
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?
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    f37511582eaeb52453cad3660c78dac4-original-20201119_122032.jpg


    All you need to do is keep out of the insulation and you can then possibly claim the installation method is C.


    Just throwing the cable across the loft insulation does not do it.
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    Farmboy and Dave, you both mention RM A in your posts when i think you mean RM C. RM A refers to cables in conduit in an insulated walls (and architraves & windowframes). I agree it should be RM C in most ceiling voids.

    I did remedials on an EICR recently and the cable type on the schedule of test results was given for all circuits as "A", as was the reference method. I was searching everywhere for cables in an insulated wall, especially near the CU since all circuits were classed as RM A, could't find any insulation. It was the same situation as Dave has, 6mm T&E, 8.5KW shower on a 40A MCB. The inspector C2ed it and recommended putting a 32A mcb in instead, "over-sized over current protective device".

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    Thanks OM for pointing that out, Error on my part typing! I have corrected it.

    That C2 you mention is ridiculous, it doesn't matter, except it is likely to trip with long use. I hope you did nothing!
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    Nice picture Andy, and a right mess! Now is a cable above the insulation not method C? Why or why not? Not it is not clipped, it will not do!
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    davezawadi (David Stone):

    Thanks OM for pointing that out, Error on my part typing! I have corrected it.

    That C2 you mention is ridiculous, it doesn't matter, except it is likely to trip with long use. I hope you did nothing!


    Yep, I did nothing. But I think we have found a solution to "teenage girls" problem. ?


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    OlympusMons:

    Farmboy and Dave, you both mention RM A in your posts when i think you mean RM C. RM A refers to cables in conduit in an insulated walls (and architraves & windowframes). I agree it should be RM C in most ceiling voids.

    I did remedials on an EICR recently and the cable type on the schedule of test results was given for all circuits as "A", as was the reference method. I was searching everywhere for cables in an insulated wall, especially near the CU since all circuits were classed as RM A, could't find any insulation. It was the same situation as Dave has, 6mm T&E, 8.5KW shower on a 40A MCB. The inspector C2ed it and recommended putting a 32A mcb in instead, "over-sized over current protective device".

     




    I was referring to section 7.2 in regs appendix 4, page 384, which suggests RM A for 'cables installed in a ceiling', due to possible rise in temperature in JBs . So what I was trying to determine was should a cable in a ceiling void be considered as RM A even when no JBs are present, as the cables could still be subjected to the raised temperature?


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    It seems odd that the traditional combination of a floor of boards and a ceiling of lath and plaster do not get a mention. I go with RM C. Cable lying on the ceiling seems pretty similar to cable on a floor. The cable could, of course, be clipped to the joists. And what about cables passing through a row of joists - most of the time they are in free air, but should one apply a grouping factor?


    Why is the space above the ceiling a "ceiling void" and not a "floor void"? Cable in a ceiling means a suspended ceiling to me. Building voids as in IM 40 onwards suggest routes in a building which have been set aside for the various services.


    Then we have the modern requirement in Part E for sound insulation betwixt a ceiling and a floor. Granted it does not fill the void, but can you guarantee to keep the cables well above it (assuming that it rests on the ceiling). I use IM 100 for that.
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    Re Teenage Daughters i also like to take long leisurely showers  and I'm a bit passed my teenage years. My shower is 8.5 Kw and supplied by 10mm cable which seems to run cool just ever so slightly warm after a decent shower
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    wallywombat:

    In terms of power loss:


    If my back-of-the-envelope sums are right, a 15m run of cable for a 45A shower will give 1.7% power losses for 6mm2 and 1% for 10mm2. The larger cable will cost an extra £10 approx, so will pay for itself after about 70 kWh of reduced losses, or 10,000 kWh of shower usage, or about 1000 hours of use. Not sure what that is in units of Teenage Daughters.


    I get slightly more. The difference in resistance between the two cable sizes is 4.05 mΩ/m so 61 mΩ. Power = I²R = 123 W extra. For a 15 min shower, that is 1/8 x 1/4 = 1/32 kWh. At 16 p that is a ha'penny per shower, so pay back is after 2000 showers or about 2 years for a hygienic family of three.


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    Chris Pearson:
    wallywombat:

    In terms of power loss:


    If my back-of-the-envelope sums are right, a 15m run of cable for a 45A shower will give 1.7% power losses for 6mm2 and 1% for 10mm2. The larger cable will cost an extra £10 approx, so will pay for itself after about 70 kWh of reduced losses, or 10,000 kWh of shower usage, or about 1000 hours of use. Not sure what that is in units of Teenage Daughters.


    I get slightly more. The difference in resistance between the two cable sizes is 4.05 mΩ/m so 61 mΩ. Power = I²R = 123 W extra. For a 15 min shower, that is 1/8 x 1/4 = 1/32 kWh. At 16 p that is a ha'penny per shower, so pay back is after 2000 showers or about 2 years for a hygienic family of three.




    A nice warm cable near to water pipes in the loft will stop them freezing up. Like trace heating. It's an ill wind that blows nobody any good. Perhaps the shower cable should be tied to a cold water pipe for most of its run.


    Z.


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