Advice on fitting and electric shower

On Mon, 17 Mar 2008 15:32:48 +0000 someone who may be David Hansen

I forgot to add, for the avoidance of doubt. The section entitled, "Water heaters having immersed and uninsulated heating elements", contains the words, "This type of water heater or boiler is deemed not to be an electrode water heater or boiler."
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David Hansen, Edinburgh
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David Hansen wrote:

No - check the rating plate and/or instructions for the shower and you'll almost certainly find that the 10.8 kW rating applies at 240 V. For design purposes at 230 V the current will be close to 43 A and the output around 9.9 kW - so a 45 A fuse or breaker will be fine.

Certainly not - see http://www.wiki.diyfaq.org.uk/index.php?title rthing_Types

As already mentioned by others, this is not the case. The elements are insulated. RCD protection is now de rigueur and would be incompatible with an uninsulated element.

And a Part P application (building notice) is required of course...
--
Andy

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David Hansen wrote:

Nope. They're just glorified electric kettles.

It's not rocket science though, is it? If you don't know, why not just say so instead of being so fecking patronising? This is a DIY group unless you had forgotten. The *rules* may be a little complex (and I admit to not knowing them myself) but I'm sure anyone with a bit more eloquence and patience than you could explain them simply enough.
To the OP, this site may help.
http://www.diydoctor.org.uk/projects/bondingbathroom.htm
Tim
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On Mon, 17 Mar 2008 14:19:42 -0000 someone who may be "Tim Downie"

Incorrect. Such heaters merit a special section of the Wiring Regulations, at least they did in the 16th Edition. There are a number of requirements to install them safely. I have no intention of regurgitating them.

I do know, so your point is moot.

Ah.
I have explained why such heaters can be dangerous. Such heaters can be safely installed by DIYers, but those who do not understand earthing are best advised to practice their electrical skills on other projects first. What is likely to be the highest rated appliance in the house and one which involves standing naked, perhaps in a metal bath, in a stream of water which has passed over an uninsulated heating element, is not a good project to practice on.

Excellent, a personal attack. Do keep it up.
--
David Hansen, Edinburgh
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wrote:

No, you haven't. You've explained the dangers of a different type of heater.

Except that there isn't an uninsulated element.
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Thank you all - I do intend to get a sparky to do the wiring into the fusebox, but regardless of regulations etc. etc. I intend to cable it through myself, and if appears simple enough, to the isolator switch in the bathroom and then the shower. The only work I can really see for a sparky is attaching the cable for me to a fuse box (which itself looks about 30 years old - highest breaker is 45A which I'm fairly certain feeds the oven)
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On Mon, 17 Mar 2008 08:59:33 -0700 (PDT) someone who may be snipped-for-privacy@googlemail.com wrote this:-

I suggest you pick your electrician first and discuss the job with them. You should then be able to agree a division of labour you can both live with. If you do what you want to do first and then ask an electrician to wire up the last bit you may find they are not keen.
The same goes for gas work.
--
David Hansen, Edinburgh
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David Hansen wrote:

You're presumably referring to regulation group 554-05 in the 16th ed. This does not apply to ordinary domestic electric showers since they have _insulated_ elements.
And the 17th ed. does have similar requirements - sections 554.1 for electrode boilers and heaters and 554.3 for uninsulated immersed elements - all quite irrelevant to domestic shower units.
554.2 - headed "Heaters for liquids or other substances having immersed heating elements" - does apply though. This section only has one regulation:
544.2.1 Every heater for liquid or other substance shall incorporate or be provided with an automatic device to prevent a dangerous rise in temperature.
--
Andy

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David Hansen wrote:

If you insist.
The argument over how electic showers work is entirely irrelevant. It's reasonable to assume that any water heater sold for domestic water heating will be safe with the appropriate earthing as specified by current regulations.
What I know or don't know about this particular subject isn't that relevant to my point that you seem to have fogotten that this is uk.d-i-y. I think you meant to post in uk.don'tevenfeckingthinkaboutit!
Unless you're prepared to say that it *is* rocket science and well beyond the competance of a DIYer, I'll continue to maintain that your comments were entirely of the kind that people come here to get *away* from.
Tim
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On Mon, 17 Mar 2008 16:25:23 -0000 someone who may be "Tim Downie"
There is very little science involved in rockets. There is a lot of engineering though.

Nice try. However, those who read all my comments will note that I have been quite clear about the original poster splitting the work with someone who knows what they are doing. I have not suggested they do nothing and leave it all to an electrician. If, despite my warnings, someone decides they can do it all themselves then they may succeed in killing themselves or someone else if they don't follow the appropriate regulations. That is their lookout.
--
David Hansen, Edinburgh
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David Hansen wrote:

Nope, you've offered little advice other than to say, "Get an electrician in, you can be the monkey who pulls the wire through if you like".
I'd be surprised if that was this kind of help the OP was looking for.
Tim
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David Hansen wrote:

Hmm, you either don't understand the use of the term "glorified" in that context or you're using it as an excuse for some sort of point scoring. As I assume you're smart enough to know the former I have to assume you're attempting the latter.
To coin a very tired phrase touted around here by someone, "nice try, do keep it up."

You've adopted the typical tradesman ploy of exaggerating the dangers in an apparent attempt to scare someone off of doing something that should be within the capabilities of a competant DIYer. That's not what this group is here for. Not only that, you seem not to understand how they work.
Tim
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snipped-for-privacy@googlemail.com wrote:

OK, we probably have enough info to try a design exercise...
(see end)

An RCD is a device that greatly reduces the danger posed by an electric shock. See http://wiki.diyfaq.org.uk/index.php?title=RCD

A 45A type B MCB will be OK. Depending on circumstances, you may want to use a separate enclosure with a RCD, or you could add the MCB to the RCD protected side of your CU is it is a split load type, or you could add a RCBO (combined MCB and RCD) to the CU.
Types of CU explained here:
http://wiki.diyfaq.org.uk/index.php?title=House_Wiring_for_Beginners#CU

No, there are two common ways for properties in towns, and a third common way for properties out in the sticks. See:
http://www.diyfaq.org.uk/electrical/electrical.html#earthing

"Prospective fault current" is a figure that tells you how much current can flow in a circuit when things go badly wrong and you end up with a short circuit at the end of it. It will be limited only by the resistance of the cables in the circuit and the resistance the supply itself and also of the path to earth (in the case of a fault to earth). It needs to be large enough to cause the protective device (i.e. fuse / MCB) to open quickly to minimise the risks of shocks to you, and also to prevent damage to cables from overheating.

Yup, that's fine. Get a decent quality one, because 10mm^2 cable is a pig to work with and can put lots of strain on plastic accessories - so a strong one with good quality terminals is worth buying.

The traditional way of mitigating the danger of electricity in bathrooms etc is via equipotential bonding. This is done by joining together all the bits of metal that have the potential to introduce a voltage into the room. So pipes, the earth wires on all circuits, and a few other things are all wired together with heavy gauge earth wires. It means that if during a fault end up with a dangerous voltage on something, it is duplicated on *everything* in the room that conducts. This means that you can't simultaneously touch two things at a markedly different potential, and hence get a shock.
The new edition of the wiring regs (the 17th edition) does away with the need for bonding, and instead insists that all circuits in a bathroom are protected by RCDs with trip currents not exceeding 30mA. The RCD(s) will instead limit any potential shock duration to a safe limit. Note following the 17th edition is not mandatory until the summer this year.

Sorry about that - there is a fair bit of complexity when designing and installing circuits like this. In many cases even if you don't know all the detail there is a fair chance you will get away with it and end up with something that is safe, but, the proper way to do it is to do a design exercise first and prove this is the case. (Note you also ideally need copies of BS7671 and the On Site Guide to be able to look up all the cable and protective device performance data (a fair bit of what you need is reproduced in our wiki))
For example, using what you have told us so far we can do the following:
(design calcs)
Your shower is a 10.8kW one. That is probably the spec at 240V, so that suggests an actual current of 10800 / 240 = 45A. We can compute the resistance of its heating element as 10800 = 240^2 / R, so R = 5.3 ohms.
For design purposes we work at 230V. So the likely power output at that voltage will be 230^2 / 5.3 = 9981W. The design current is hence 9981 / 230 = about 43A.
So a 45A breaker would be OK.
The cable (10mm^2 cable clipped direct), is good for 64A, so we can safely say that is adequate.
Voltage drop
see table : http://wiki.diyfaq.org.uk/index.php?title bles#Voltage_Drop
is 4.4mV / A / m, and so that gives us 0.0044 * 43 * 15 = 2.8V which is less than the 4% allowed, so that is ok.
Lets look at disconnect time for a phase to earth fault at the shower. We need to open the protective device within 5 seconds (radial circuit supplying fixed equipment inside an equipotential zone)...
First thing we need to know is the maximum current that might flow under fault conditions. For this we need the total resistance of the circuit and its supply and earth connections. Some bits we can calculate:
http://wiki.diyfaq.org.uk/index.php?title=Electrical_Circuit_faults#Wire_resistance_table
So the round trip (phase + earth) resistance of the cable is 6.44 mOhms / meter or 0.00644 * 15 = 0.0966 ohms. If we assume for a moment that your house has a TN-S[1] earthing system, we can use a worst case earth fault loop impedance at your consumer unit of 0.8 ohms, so we have a total loop impedance of 0.8996 ohms. Now that assumes normal operating temperature; to allow for running at close to cable maximum capacity when the fault occurs we introduce a correction factor of 1.2, giving a resistance of 1.07952 ohms.
The fault current would therefore be 230 / 1.07952 = 213A. A look at figure 3.4 in the wiring regs shows that a 50A[2] type B MCB will open "instantly" (i.e. in 0.1 secs) for fault currents of 250A or more, and a 40A device at 200A or more. That suggests your 45A breaker is going to be borderline. So you may need a RCD in there[3] to ensure disconnection on an earth fault. In reality there is a fair chance that your actual earth fault loop impedance will be less than the 0.8 ohm figure we assumed (you would need test equipment to measure this), and the voltage and therefore fault current will be higher than that calculated - so you would probably get away without without a RCD. With a TN-C-S supply you would be fine also since the worst case earth loop impedance is 0.35 ohms.
[1] See http://www.diyfaq.org.uk/electrical/electrical.html#earthing
Assuming TN-S is more pessimistic than assuming TN-C-S - if you actually have TT then you will have to use a RCD regardless.
[2] No curve shown for a 45A device - but we can safely assume that will trip instantly at a lower current than a 50A device. We can also interpolate between the figures given for the 40 and 50A devices.
[3] Many electricians will fit a RCD on a shower feed as a mater of course these days, and also the coming 17th edition will mandate it.
Lets assume (in the absence of real measurements), that we are ok on disconnection time. We need one final check that the earth wire in the cable will withstand that magnitude of fault current for long enough to allow the circuit breaker to operate. So we compute the minimal cross sectional area "s" of the earth wire using the adiabatic equation s sqrt( I^2 x t ) / k. For PVC clad cable, k = 115, so we get sqrt( 213^2 x 0.1 ) / 115 = 0.59mm^2 which is comfortably less than the 4mm^2 earth wire in the 10mm^2 cable. So that is ok.
(note its late, and I wrote that quickly - so don't rely on it as gospel without checking!)
--
Cheers,

John.

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wrote:

Alternatively the circuit length is short enough to select a conventional circuit from the OSG and comply with the installation method and overcurrent device selection. I agree that 10.8kW is very close to the 45A MCB rating but that's not by accident. The bit of cabling under the insulation was always the installation method risk. Personally I'd suggest a 9kW 40A route and not worry about the cable run. If you accept electric showers it will still be adequate, just less adequate.
Jim A
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Jim Alexander wrote:

perhaps that should be, it will still be inadequate, just slightly more hopelessly so! ;-)
--
Cheers,

John.

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John Rumm wrote:

Now that we're supposed to have a metre or so depth of rockwool in the roofspace, laid perpendicular to the joists, how is this normally reconciled with the need to have cables attached to joists rather than (eg) lying on top of the insulation?
(My parents have just told me that they are about to have their roofspace insulation topped, costing them about 1.34 GBP as they are 'old' - I do hope the installers bear this issue in mind. Yeah, right.)
David
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Lobster wrote:

Most cables in lofts are lighting cables, and even with 1.0mm^2 T&E there is enough overrating with a 6A MCB for it to not usually cause a problem. Power circuits obviously need more care.
--
Cheers,

John.

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