Shower wattage?

Postby bluenun » Sun Dec 30, 2007 7:42 pm

I want to get an electric shower installed.

Can anyone tell me whether it is best to buy a 8.5W, 9.5W or a 10.5W shower?
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Postby marrtin » Sun Dec 30, 2007 10:51 pm

Generally, the higher the wattage, the better the shower, but of course the more the cost. The most common shower is either a 8.5kw for basic installations, or a 9.5kw for a reasonable shower.

10.8kw showers are better but you may have problems with power supply.
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Postby bluenun » Mon Dec 31, 2007 10:44 am

Thanks for the reply.

What do you mean "may have a problem with the power supply"?

There is a very old shower here already that does not heat the water up.
The property was built in 1985 and I expect the shower is about that old.
It is a Heatrae sadia carousel slimline.

There is a pullcord in the bathroom (the little red light comes on) and the shower has it's own fuse in the fuse board in the hallway.
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Postby kbrownie » Mon Dec 31, 2007 3:41 pm

Hi bluenun,
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.
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Postby stevenc1603 » Mon Dec 31, 2007 3:52 pm

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.
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Postby marrtin » Mon Dec 31, 2007 5:26 pm

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.
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Postby bluenun » Tue Jan 01, 2008 4:03 pm

Thanks for the replies.

I will either go for an 8.5 or 9.5KW shower.

I went into the loft this morning and found the cable that goes from the hall to the bathroom (its a bungalow).
It measured 13mm across.

Excuse my ignorance but is that how you measure a cable, from side to side including the outer insulation?
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Postby marrtin » Tue Jan 01, 2008 4:41 pm

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.
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Postby bluenun » Wed Jan 02, 2008 2:56 pm

It is a 30 amp fuse, so it looks like I will need to get an 8.5kw shower.
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Postby bluenun » Fri Jan 04, 2008 8:45 pm

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 £49.99" or a Caselona 2 9.5kW Electric Shower for £89.99.

Thanks for any help.
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Postby marrtin » Sat Jan 05, 2008 12:07 am

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.
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Postby bluenun » Sun Jan 06, 2008 10:12 am

Thanks for explaining the benefits of a 9.5kw over an 8.5kw shower.

Yes I will post over in the electrics forum.
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Postby ericmark » Sun Feb 03, 2008 1:47 pm

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 70°C or 90°C unless we find marking on the cable we assume the 70°C to be on safe side. We would normally consider 4mm² as 27 amp, 6mm² as 34 amp, 10mm² as 45 amp and 16mm² as 57 amp (Table 4D5 BS7671:2008) but these can change if the route goes through insulation etc. Also can be higher if 90°C cable. The showers are sold in Watts so to convert we multiply by 230 the volts so 4mm² = 6.21Kw, 6mm² = 7.82Kw, 10mm² = 10.35Kw and 16mm² = 13.11Kw most miniature circuit breakers (MCB) will allow an over load before tripping out for some time and since we don’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 6mm² meter tails and 60 amp incoming fuses which would stop any idea of upgrade until the supply to house is changed. The MCB’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 “RCDâ€

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