Maximum Wattage for Shower

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Hi

My shower has broken (keeps on cutting out 5 mins into a shower) and i need to replace it.

I've got a 40amp fuse and a 10mm cable with an 80A30mA RCD. I want as powerful a shower as possible and my electrician that has just done a lot of work on the house (but has now finished) said i could get away with anything up to a 10.5kw shower. Having researched on line though Ive seen a lot of people say that the max that should be fitted is 8.5kw with this set up. Can anyone help? I trust my electrician to an extent but he's also advising from memory and I want to research what shower to get before getting it fitted.

Thanks
 
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40 x 230 = 9200 so 9.2 kW is the largest unit that can work continuous, however time in a shower is limited, so as a result likely 10.5 kW will not trip within the time it is used for, so your electrician is correct.
 
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40A @ 240V is 9.6kW - the maximum you can have as it stands.

Depending on the cable installation method and if it really is 10 square millimeters then you may be able to increase the MCB to 45A/50A and have up to 10.8kW/12kW respectively @ 240V.
 
The maximum current a 10mm2 flat grey t&e, could carry, would be 64A (that could depend on manufacturer). 64A @240V would calculate out at 15.36kW
But there are number of things to get round first.
The size of breakers available for your consumer unit, which rarely are above 45A, occasionally 50A MCBs are available.
Then there is also any derating that must be factored in, with regards to how the cable is routed and whether it is contained or within thermal insulation.
As above the calculation is normally done using a 45A breaker giving 10.8kW @240V. But if a 50A MCB is manufactured for you CU then that could be increased to 12.0kW. Derating allowing.
 
40 x 230 = 9200 so 9.2 kW is the largest unit that can work continuous, however time in a shower is limited, so as a result likely 10.5 kW will not trip within the time it is used for, so your electrician is correct.
I disagree.

One cannot rely on a limited duration of overload to say that it is acceptable. Basically you're assuming diversity for a water heater - never seen that as being OK.
 
One cannot rely on a limited duration of overload to say that it is acceptable. Basically you're assuming diversity for a water heater - never seen that as being OK.

And in many circumstances (teenage daughters, I am told) it is not uncommon for multiple 15-minute showers in quick succession.
 
Knowing nothing about this I can't believe there are so many varying views. I would have thought this was pretty standard. Anyhow maybe I'll go with a 9kw Mira sport airboost in the hope the air boost will give me the power I'm after? Any thoughts? Thanks!
 
As said 9.2 kW is the maximum which should be fitted, however I can also see why an electrician would quote a higher figure, it is like saying maximum speed on the motorway is 70 MPH but your unlikely to have a problem if you don't exceed 80 MPH, there will be the odd Policeman you will send in a report when you exceed 70 MPH by just 4 MPH but in the main they will not, and in the same way most of the time you can exceed the 9.2 kW, but in the odd case where it is being used as a shower at side of swimming pool running most of the day, when it would cause problems.

However like with the speed limit, you may still get away with it under 100 MPH but the chances are reducing, as you go higher and higher above the 9.2 kW so the chances of it going wrong increases, and there is not a fixed figure over which it is really a problem, it is all on a sliding scale. I would not want to say your electrician is wrong stating 10.5 kW.
 
As said 9.2 kW is the maximum which should be fitted,
No, it isn't.

Showers are primarily quoted at 240V, e.g. 9.5kW @ 240V = 39.6A.
This is not a coincidence. It is so they may be on a 40A MCB.

The quote may also mention that 9.5kW @ 240V = 8.7kW @ 230V. This latter figure is only 37.8A.
Therefore you may have a slightly higher wattage if you are designing at 230V, although this may not be wise, as (as Winston is always telling us) the actual voltage is likely to be around 240V.
 
Presumably the 240v>230v factor is already taken into account with the cable sizing rules, otherwise it's a pointless extra calculation to be made. Although the 0.95 factor has to be taken into account for fault disconnection, so my logic doesn't always apply!
 
Presumably the 240v>230v factor is already taken into account with the cable sizing rules,
Yes, although they weren't altered when the 240 to 230 change was made.
I assume there is enough tolerance already.

However, I don't think it would be advisable as far as the MCBs are concerned.

There could be a Cmax for such things which would really mess it up.
 
Yes, although they weren't altered when the 240 to 230 change was made.
I assume there is enough tolerance already.

However, I don't think it would be advisable as far as the MCBs are concerned.

There could be a Cmax for such things which would really mess it up.
Well it's just maths, and the mcb spec states they must trip between 3 to 5 times, so they could just mandate they have to trip slightly reduced range (3.1 to 4.9 or something) to cover the possibility. But anyway it is what it is!
 

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