I intend to replace my existing 8.5kW shower for a 9.5kW one. The present one is wired in 6mm through a 40A mcb in the CU with a run of about 7 metres with a double-pole isolator in the airing cupboard.
My intention is to replace the cable with 10mm through a 50A mcb, and to replace the isolator with a separate shower CU (of this type, Wylex NH204/63), again with a 50A mcb. Any thoughts/recommendations?
Joe Bloggs presented the following explanation :
Assuming 240v - Your 40a and 6mm is just on the borderline for 9.5kW, ignoring any volts drop.
Our 9.5Kw was originally an 8Kw shower wired in 6mm on a 40a MCB. It has been fine for the past 10 years. Modern showers can modulate their current needs as well as their flow.
Modern showers can modulate their
But I bet most users need them on full all the time.
That's why I'm upgrading the supply cable and mcb.
What I was really unsure about was the necessity of the separate shower CU. I intend to fit one, even though the shower would be fed from the main CU which is RCD protected. But am I correct in thinking that the shower needs a separate RCD?
The shower does not need two lots of RCD protection.
I would not call it borderline.
Borderline, as in just adequate. Were it already installed I would leave it as is, but would increase the capacity for a new install.
No point in adding a second RCD of the same trip rating, there would be no discrimination - so either or both could trip. They don't need a separate CU, just a local means of isolation. Either a pull switch or an isolator outside the door, with an indicator.
ARW expressed precisely :
Just comfortably adequate then?
The OP seems to have his mind set on replacing it with a higher rated cable.
How many sq mm do you get in a pair of 22mm water pipes?
NT
Go for a 10.5 kW one. It's the difference between an 11% and a 23% better shower.
You're upgrading the cable etc anyway.
Owain
Harry Bloomfield explained on 22/01/2017 :
47amps if installed clipped direct to a wall, or embedded in none insulating materials. 34amps if installed in insulation materials. Likely the OP had in mind some installation method in between? 240v 9.5Kw = 39.58 amps.
You have not mentioned a RCD... is there one on the current circuit?
(if just changing the shower and leaving the rest alone, and the existing installation is meets the required disconnect times - then you could argue there is no requirement to do a full 17th edition style upgrade)
The "clipped direct" installed capacity of 6mm^2 is 47A. Your load of
9k5W will probably be specified at 240V, so you nominal current will be 39A. So in loading terms you would be fine and nothing is going to overheat etc.Voltage drop is fine at under 2V
Chances are the loop impedance that that length is in spec (even with the new 17th edition amendment 3 reductions is max Zs)
The adiabatic check suggests a minimum protective conductor size on 40A MCB of s = sqrt( 400^2 x 0.1 ) / 115 = ~1 mm^2 which is also fine.
If you don't already have a RCD on the shower, then you could use an enclosure like that to add one. However you may just be able to use B40 RCBO in the current CU (and keep the existing isolator).
No.
New 17th edition installs would need RCD protection at a maximum trip threshold of 30mA. There is no requirement for that to be an additional or dedicated RCD.
If the install is as the OP describes, then its adequate - and that is all that counts really - since it will not exceed any technical parameter.
With that I would concur... No point going to all that hassle and fitting a 9.5kW shower. If doing a new install it would obviously be worth going for a higher spec to allow for a more powerful shower.
If the OP is set on cable upgrades etc, then he may as well go for a better shower as well.
You mean in terms of CSA of copper? Over 27mm^2 per pipe if my sums are right... (assuming a 0.8mm wall thickness)
Yup, if going to the hassle of putting in a bigger cable, you may as well get some benefit from it!
Thanks for all the, very helpful, replies. To clarify things I'll do a block response to some of the queries raised.
I have already run a length of 10mm from the CU to the airing cupboard (not yet connected) so there's no question of sticking with the existing cable. There is a 30mA RCD in the existing CU, so it sounds like that's all that's required. As stated, the cable run is about 7 metres and runs from the CU up into the loft (this is a bungalow) and across and down to the airing cupboard. I will run it under the loft insulation (not clipped to anything) - or over it if this is likely to be a problem.
I think I'll just upgrade the existing double pole switch in the airing cupboard - the current one doesn't have a neon indicator - and dispense with the idea of a separate shower CU. The existing shower is fed through a 32A mcb, which I will upgrade to 50A. The suggestion of a
10.5kW shower seems a good one, which I will look into - although I was under the impression that this might run close to a requirement for 16mm cable.I should add that the property has TT earthing.
Any additional advice/comments would be very welcome.
Dependent upon the installation conditions of your present 6mm T&E, it might be able to support even the 10.5kW shower at 43.75amps full load(10500w/240v = 43.75amps). Your 10mm is definitely more than adequate for the purpose, absolutely no need for 16mm and you might find it a struggle/impossible to get into the terminals.
6mm unless covered by insulation is rated at 47amps.Avoiding running cables under or through insulation is always a good idea, especially those which carry heavier currents.
Your 30mA RCD covers what you need, but if it provides cover for ALL of the circuits, you might find it better to replace that with a 100mA and fit a separate 30mA RCDBO for just the shower circuit - it avoids all your circuits tripping out, due to a fault on one circuit.
Just run the 6mm t&E next to the showers water pipe for extra performance.
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