DIY Doctor

Electric Shower

Postby Gavin Thorn » Thu Jan 24, 2008 8:52 pm

Hi there,

I wonder if you can help me please. My current 7.5kW shower is only outputting very luke warm water (almost cold in fact) at the maximum temperature setting. I’m not sure what the problem is but I have for a long time wanted to be able to upgrade to a more powerful unit. Currently I have a 6mm2 Twin-core-and-earth cable installed with a 32amp fuse in the consumer unit.

I have spoken to several shower unit manufacturers regarding the maximum kW allowed for a new electric shower and each gave me slightly different answers ranging from 7.5kW to 8.5kW. I have a Reader’s Digest DIY handbook which states that with a 6mm2 Twin-core-and-earth cable, a 30amp circuit can be used for cookers up to 13kW and shower circuit from 13m to 20m long. To further confuse me I am not sure what ‘clipped direct’ or ‘enclosed in insulated wall’ means as shown on your website which states different values again. Can you please explain?

My main question is ‘What is the maximum kW electrical shower unit I would be able to use with a shower circuit up to 13m long with a 32amp fuse?’

Many thanks
Gavin Thorn
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Simply Build It

Postby ericmark » Thu Jan 24, 2008 10:54 pm

Quick answer 7.5Kw but below is why you find different ratings.
Although we call it 6mm² twin and earth we really should call it 6mm² 6242Y (PVC) this can run to 70° or 6mm² 6242B (XLPE) this can run to 90° and looking at method 1 installation it jumps from 47amp to 63amp. (Values taken from Batt Cables website) In the back of the 16th Edition it has the list of methods Method 1 is Sheathed cables clipped direct or lying on a non-metallic surface or embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials) but a thermal insulated wall is Method 4 and 6242B 6mm² cable drops from 63 amp to 42 amp and with 6242Y down to 32 amp this is when ambient temperature is no more than 30° and it is not laid with other cables. Put the cable next to a hot water pipe and the ratting goes well down. Since we don’t know which 6mm² you have and the 6mm² 6242Y (PVC) is the popular one and unless marked on sheath near impossible to tell which is which and it may be in insulating wall we tell people 32 amp that’s 7.63Kw and unless the electrician has either fitted the cable or the one who did has filled in the paperwork with all information he will play safe and work to 32 amp limit. The length affects the volt drop and earth loop impedance. With 6mm² loses are 7.9mV/A/m so at 13 meters 3.3 Volt drop and you’re allowed 9.2 Volt drop so you’re well within limits. Can’t work out earth loop impedance without further information but unlikely to fail. Most likely you could fit a 10Kw but unless I could see whole cable run I would not fit one only needs to go through some loft insulation and you could overheat cable and house fires are not nice.

Postby sparx » Fri Jan 25, 2008 12:03 am

Hi Gavin,
as usual with leckie stuff lots of issues to consider,
'clipped direct' means surface run cable all the way from consumer unit to shower held in place with cable clips - hence name,
enclosed in insulated wall means it is chased into wall and plastered over for some of the run, the reason being all cables have some resistance and so produce some heat, if clipped on surface airflow will allow it to dissipate whereas buried cable will get warmer so must be limited to lower current rating. ie 6mm 47A surface clipped or 35A buried.
Next, length of run from cons.unit to shower, 6mm drops 7.3mVolts per amp per metre, so if drawing 32A [which is approx what 7.5kW shower does] with a cable run of say 15Mtrs we get : 7.3 X 32 X 15 = 3504mV divide by 1000 gives 3.5 volts dropped which is well within limit of 9.2v allowed.
Assuming cable is not surface then using an 8.5kw unit draws 37A approx, which is too much for your 6mm cable but would be OK if surface clipped, using 40 MCB, if volt drop ok, ie 7.3 X 37 X 15Mtr = 4.0V Drop, still OK .
here endeth the surmon, wish you hadn't asked?
anyway as you have I assume you are not Part-P registered so gotta get a sparky to change it anyway.....
regards SPARX
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Postby Gavin Thorn » Fri Feb 01, 2008 1:50 am

Thanks guys. Unfortunately I can not see the markings on the wire to determine which type it is so I'll play safe and buy another 7.5kW unit.
Gavin Thorn
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Postby bertbasset » Mon Apr 21, 2008 8:30 pm

I understand where you are all coming from, but can i not put a diversity factor into the equation. You say about size of shower for cable, but you will never reach 9.5kw from a 9.5kw shower you would have major burns would you not? so why could i not have that size with a 6mm cable on a 40A 30mA rcd? Thanks for your input
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Postby ericmark » Mon Apr 21, 2008 9:00 pm

Where we have a number of items all connected to the same circuit we can use diversity but here we have ONE shower connected to ONE cable so where does diversity come from? And a 9.5Kw shower does use 9.5Kw in my house with a 27Kw gas fired shower maybe I am not using full 27Kw available but with 9.5Kw you need every last watt of heat it can produce.

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