when electric shower are being installed. Cable calculation have to be taken in to consideration and this done the power of your Shower, fuse rating, length of cable run, route of cable run and design current.
It's not as complicated as it sounds, but neitherless should be worked out for your appliance to work safely and correctly.
If I remember correctly anything over 9.5kw would need 10mm2 cable to supply it. You would need to check the size of the cable feeding your shower to see what you can use. 6mm2 is OK for 9.5kw and below from what I have been told.
10.8kw showers draw so much juice, in some older installations the incoming supply is barely able to cope and needs to be verified suitable before purchasing a shower.
I do believe under the current regs any shower above 9.0kw will need a 10.0mm cable. Up until a few years ago, nearly all showers were supplied in 6.0mm although I have seen even 4.0mm used on old low wattage appliances.
Unfortunately it is not as simple as that. The size of cable insulation is irrelevant as measurements are expressed in the overall copper conductor size in millimetres squared, ie, 6.0mm2 or 10.0mm2 (where the 2 represents "to power of 2" but you can't do superscripts on this website).
Sometimes the size of conductor is embossed on the side of insulation along with the manufacturers name but usually it can only be recognised by a trained eye.
Have a look at the trip or fuse in the fusebox. If it is 30 or 32 amp only a shower up to a 9.0kw can be fitted.
I had an electrician round today to quote me for fitting an electric shower, he went in the loft to see the size of the cable and looked at the fuse box and said I was able to have any wattage shower I wanted.
So my question is, what is the advantage of a 9.5kw shower over an 8.5kw?
I ask because I have seen a "Creda 8.5Kw Electronic Shower for Â£49.99" or a Caselona 2 9.5kW Electric Shower for Â£89.99.
Obviously the higher wattage shower will produce a better shower by an increased flow rate particularly during the winter months when the incoming water is colder.
It is always best to fit the highest wattage shower that is within your budget and your power supply can handle.
I am concerned that your electrician says any shower will fit, as a 9.5kw shower draws a constant resistive loading of about 41 amps which is well in excess of the 30 amp fuse you have installed. If you want further comments, post this question to the "electrics" forum and see what the Sparky's there have to say.
I expect it's all complete now but cable current is not just look up in a table and what it runs through and the insulation on the cable also have an effect. Two types of cable normally used can run at 70Â°C or 90Â°C unless we find marking on the cable we assume the 70Â°C to be on safe side. We would normally consider 4mmÂ² as 27 amp, 6mmÂ² as 34 amp, 10mmÂ² as 45 amp and 16mmÂ² as 57 amp (Table 4D5 BS7671:2008) but these can change if the route goes through insulation etc. Also can be higher if 90Â°C cable. The showers are sold in Watts so to convert we multiply by 230 the volts so 4mmÂ² = 6.21Kw, 6mmÂ² = 7.82Kw, 10mmÂ² = 10.35Kw and 16mmÂ² = 13.11Kw most miniature circuit breakers (MCB) will allow an over load before tripping out for some time and since we donâ€™t shower for too long running a 8.5Kw shower on a 32 amp will most likely work OK but at 9.5Kw it will trip after a few minutes and will stress the MCB and get worse and worse as time goes on. Up grading the MCB would depend on cable fitted and supply into the house. I have seen 6mmÂ² meter tails and 60 amp incoming fuses which would stop any idea of upgrade until the supply to house is changed. The MCBâ€™s jump up in steps so next is 40 amp then 45 amp after that it depends on the make of Consumer unit as to if a 50 amp is available. Once you change things then you have to follow latest regulations so if not already protected with an Earth leakage device â€œRCDâ€
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Last edited by TheDoctor5 on Wed Jan 21, 2009 12:07 pm, edited 1 time in total.