Looking to install a 10.8kw unit - therefore 10mm electrical cable and
suitable breaker. Question is, do I need to worry about where this
cable goes? The ideal route is going to be under some floorboards,
into an attic and under some loft insulation.

Being a competent diy'er but not a sparky as such, is this advisable?
Is the cable likely to get hot for example?

Any suggestions much appreciated

wrote:
Looking to install a 10.8kw unit - therefore 10mm electrical cable and
suitable breaker. Question is, do I need to worry about where this
cable goes? The ideal route is going to be under some floorboards,
into an attic and under some loft insulation.

Being a competent diy'er but not a sparky as such, is this advisable?

no

Is the cable likely to get hot for example?

Yes, so simply move the insulation out of it's way!

The ideal route is going to be under some floorboards,
into an attic and under some loft insulation.

Being a competent diy'er but not a sparky as such, is this advisable?

no

Is the cable likely to get hot for example?

Yes, so simply move the insulation out of it's way!

Thanks for the quick response. Are there any other safety issues I
should be aware of?

wrote:

Looking to install a 10.8kw unit - therefore 10mm electrical cable and
suitable breaker. Question is, do I need to worry about where this
cable goes? The ideal route is going to be under some floorboards,
into an attic and under some loft insulation.

On top of the insulation would be greatly preferred.

Any suggestions much appreciated

What length is the cable run?
Will it be fed from a RCD protected supply?
What protective device do you plan on using (fuse / MCB etc)?
What earthing system does your house have?

Given the above you can work out the prospective fault current at the
shower and hence the disconnection times, and you can also check the
earth fault loop impedance. You will also need to check the maximum
voltage drop is not exceeded.

You need an isolation switch - preferably in the same room but not
somewhere that can be reached from in the shower.

You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Wow - thanks John.

What length is the cable run?
up to 15 metres (10mm cable)

Will it be fed from a RCD protected supply?
What protective device do you plan on using (fuse / MCB etc)?
Not well up on this - was going to get a sparky to plumb it into my
fuse box - the highest rated breaker in the box is 45A so I'm guessing
he will fit a separate 60A breaker/isolation switch or whatever is
necessary

What earthing system does your house have?
I have no idea. I thought all houses were earthed the same

Given the above you can work out the prospective fault current at the
shower and hence the disconnection times, and you can also check the
earth fault loop impedance. You will also need to check the maximum
voltage drop is not exceeded.

Okay, now I'm lost.

You need an isolation switch - preferably in the same room but not
somewhere that can be reached from in the shower.
I was going to fit a pull switch isolation switch in the bathroom
which cannot be reached from the shower but through which it's mains
supply will obviously pass.

You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.

Meaning?

Thankyou for the advice - although you've lost me a little on some of
this

wrote in message
...
Wow - thanks John.

What length is the cable run?
up to 15 metres (10mm cable)

Will it be fed from a RCD protected supply?
What protective device do you plan on using (fuse / MCB etc)?
Not well up on this - was going to get a sparky to plumb it into my
fuse box - the highest rated breaker in the box is 45A so I'm
guessing
he will fit a separate 60A breaker/isolation switch or whatever is
necessary

What earthing system does your house have?
I have no idea. I thought all houses were earthed the same

Given the above you can work out the prospective fault current at
the
shower and hence the disconnection times, and you can also check
the
earth fault loop impedance. You will also need to check the maximum
voltage drop is not exceeded.

Okay, now I'm lost.

You need an isolation switch - preferably in the same room but not
somewhere that can be reached from in the shower.
I was going to fit a pull switch isolation switch in the bathroom
which cannot be reached from the shower but through which it's mains
supply will obviously pass.

You suplimentary equipotential bonding will need to be up to
scratch
unless you plan to implement 17th edition style 30mA RCD protection
for
all circuits in the shower room.

Meaning?

Thankyou for the advice - although you've lost me a little on some
of
this

Best get that sparky to do the whole job then, or at least plan it and
(him/her) being willing, allow you to do some of the 'donkey work'.

On Mon, 17 Mar 2008 03:32:05 -0700 (PDT) someone who may be
wrote this:-

Will it be fed from a RCD protected supply?
What protective device do you plan on using (fuse / MCB etc)?

Not well up on this - was going to get a sparky to plumb it into my
fuse box - the highest rated breaker in the box is 45A so I'm guessing
he will fit a separate 60A breaker/isolation switch or whatever is
necessary

10.8kW at 230V comes to a little under 47A, according to Mr Gates'
calculator and my fingers, so a 50A MCB sounds likely.

There is also the question of whether the consumer unit/supply can
take this. What does the 45A MCB feed? What rating is the
"electricity board" fuse?

What earthing system does your house have?

I have no idea. I thought all houses were earthed the same

Then it is a good idea for you to get someone else to design and fit
the installation. They may allow you to do some of the donkey work
however, such as drawing through cables.

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element. The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.

You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.

Meaning?

It is a good idea for you to get someone else to design and fit the
installation. That is the best advice you will receive here.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element. The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.




Un-insulated - No - it is mineral insulated. The water is not in contact
with the electricity.

John wrote:
Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element. The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.




Un-insulated - No - it is mineral insulated. The water is not in contact
with the electricity.


Many, many years ago was there not a Tomorrow's World item on an
electric water heater in which it was implied (or maybe even stated)
that the electricity *did* go directly through the water? I have a
feeling that they laboured the "electricity and water don't mix" in the
lead into the item.

--
Rod

Hypothyroidism is a seriously debilitating condition with an insidious
onset.
Although common it frequently goes undiagnosed.
www.thyromind.info www.thyroiduk.org www.altsupportthyroid.org

"David Hansen" wrote in message
...
snip

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element. The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.


Ignorant ****!

On Mon, 17 Mar 2008 12:52:18 -0000 someone who may be ":Jerry:"
wrote this:-

Ignorant ****!

Excellent, proof by assertion.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

"David Hansen" wrote in message
...
On Mon, 17 Mar 2008 12:52:18 -0000 someone who may be ":Jerry:"
wrote this:-

Ignorant ****!

Excellent, proof by assertion.


Attack being the best form of defence and all that, no doubt...

On 17 Mar, 12:21, Rod wrote:

Many, many years ago was there not a Tomorrow's World item on an
electric water heater in which it was implied (or maybe even stated)
that the electricity *did* go directly through the water? I have a
feeling that they laboured the "electricity and water don't mix" in the
lead into the item.

Such electrode heaters do exist, but they aren't used in domestic
electric showers.

On Mon, 17 Mar 2008 12:07:34 GMT someone who may be "John"
wrote this:-

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element. The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.

Un-insulated - No - it is mineral insulated. The water is not in contact
with the electricity.

"Water heaters having immersed and uninsulated heating elements" is
the phrase used to describe such heaters in the 16th edition. No
idea about the 17th as I have yet to need to study it.

There are even water heaters where the electricity passes through
the water. These are called electrode water heaters and boilers.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On 17 Mar, 13:36, David Hansen
wrote:

"Water heaters having immersed and uninsulated heating elements" is
the phrase used to describe such heaters in the 16th edition. No
idea about the 17th as I have yet to need to study it.

There are even water heaters where the electricity passes through
the water. These are called electrode water heaters and boilers.

"Water heaters having immersed and uninsulated heating elements" and
electrode boilers are the same thing. Normal electric showers are
classed as "instantaneous water heaters".

David Hansen wrote:

10.8kW at 230V comes to a little under 47A, according to Mr Gates'
calculator and my fingers, so a 50A MCB sounds likely.

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.

What earthing system does your house have?
I have no idea. I thought all houses were earthed the same

Certainly not - see
http://www.wiki.diyfaq.org.uk/index....Earthing_Types

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element.

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.

You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.
Meaning?

It is a good idea for you to get someone else to design and fit the
installation. That is the best advice you will receive here.

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

--
Andy

David Hansen wrote:

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element.

Nope. They're just glorified electric kettles.

The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.

You suplimentary equipotential bonding will need to be up to
scratch unless you plan to implement 17th edition style 30mA RCD
protection for all circuits in the shower room.

Meaning?

It is a good idea for you to get someone else to design and fit the
installation. That is the best advice you will receive here.

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...ngbathroom.htm

Tim

wrote:
On 17 Mar, 13:36, David Hansen
wrote:

"Water heaters having immersed and uninsulated heating elements" and
electrode boilers are the same thing.

Sorry that's completely wrong. An electrode boiler passes current
directly through the medium to be heated. See, for example,

http://www.collins-walker.co.uk/summary.html

(should you happen to need a 3.5 MW 'immersion heater' :~) ).

OTOH an immersed and uninsulated heating element is just that - ever
seen an Australian electric kettle?

--
Andy

On 17 Mar, 14:25, Andy Wade wrote:
wrote:
On 17 Mar, 13:36, David Hansen
wrote:

"Water heaters having immersed and uninsulated heating elements" and
electrode boilers are the same thing.

Sorry that's completely wrong. An electrode boiler passes current
directly through the medium to be heated. See, for example,

http://www.collins-walker.co.uk/summary.html

(should you happen to need a 3.5 MW 'immersion heater' :~) ).

OTOH an immersed and uninsulated heating element is just that - ever
seen an Australian electric kettle?

No, I haven't. Would you care to explain further? And, surely, an
electrode heater fits the description perfectly?

On Mon, 17 Mar 2008 14:25:25 +0000 someone who may be Andy Wade
wrote this:-

"Water heaters having immersed and uninsulated heating elements" and
electrode boilers are the same thing.

Sorry that's completely wrong.

Indeed, which is why just before the section entitled, "Water
heaters having immersed and uninsulated heating elements", there is
one entitled, "Heaters for liquids or other substances, having
immersed heating elements", and just before that section there is
one entitled, "Electrode water heaters and boilers".


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Mon, 17 Mar 2008 13:31:01 -0000 someone who may be ":Jerry:"
wrote this:-

Ignorant ****!

Excellent, proof by assertion.

Attack being the best form of defence and all that, no doubt...

No matter how many words are used, your original proof by assertion
remains incorrect.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Mon, 17 Mar 2008 14:19:42 -0000 someone who may be "Tim Downie"
wrote this:-

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element.

Nope. They're just glorified electric kettles.

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.

It is a good idea for you to get someone else to design and fit the
installation. That is the best advice you will receive here.

It's not rocket science though, is it? If you don't know,

I do know, so your point is moot.

The *rules* may be a little complex (and I admit to not
knowing them myself)

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.

but I'm sure anyone with a bit more eloquence and
patience than you could explain them simply enough.

Excellent, a personal attack. Do keep it up.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On 17 Mar, 15:49, David Hansen
wrote:

I have explained why such heaters can be dangerous.

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

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

Except that there isn't an uninsulated element.

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)

wrote:

[...] ever seen an Australian electric kettle?

No, I haven't. Would you care to explain further?

Well when I was there in the 1970s the kettles just had bare nichrome
wire elements directly in the water - i.e. "immersed and uninsulated
heating elements". Some current doubtless flows through the water, but
the main heating effect is still from the passage of current through
resistance wire.

And, surely, an electrode heater fits the description perfectly?

An electrode boiler has no element as such - it just has electrodes.
Current is persuaded to flow directly though the water (or other working
medium), presumably by the use of chemical treatment to increase the
conductivity. Thus the medium is heated directly instead of by the
conduction and convection of heat from a hot metal element.

--
Andy

On Mon, 17 Mar 2008 08:59:33 -0700 (PDT) someone who may be
wrote this:-

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)

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
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

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

and just before that section there is
one entitled, "Electrode water heaters and boilers".

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."


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:
On Mon, 17 Mar 2008 14:19:42 -0000 someone who may be "Tim Downie"
wrote this:-

Nope. They're just glorified electric kettles.

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.

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

David Hansen wrote:
On Mon, 17 Mar 2008 14:19:42 -0000 someone who may be "Tim Downie"
wrote this:-

but I'm sure anyone with a bit more eloquence and
patience than you could explain them simply enough.

Excellent, a personal attack. Do keep it up.

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

On Mon, 17 Mar 2008 16:25:23 -0000 someone who may be "Tim Downie"
wrote this:-

Unless you're prepared to say that it *is* rocket science

There is very little science involved in rockets. There is a lot of
engineering though.

I'll continue to maintain that your comments were
entirely of the kind that people come here to get *away* from.

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
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:
On Mon, 17 Mar 2008 14:19:42 -0000 someone who may be "Tim Downie"
wrote this:-

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element.

Nope. They're just glorified electric kettles.

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.

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."


It is a good idea for you to get someone else to design and fit
the installation. That is the best advice you will receive here.

It's not rocket science though, is it? If you don't know,

I do know, so your point is moot.

The *rules* may be a little complex (and I admit to not
knowing them myself)

Ah.

I have explained why such heaters can be dangerous.

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

David Hansen wrote:
On Mon, 17 Mar 2008 16:25:23 -0000 someone who may be "Tim Downie"
wrote this:-

Unless you're prepared to say that it *is* rocket science

There is very little science involved in rockets. There is a lot of
engineering though.

I'll continue to maintain that your comments were
entirely of the kind that people come here to get *away* from.

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.

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

wrote:

What length is the cable run?
up to 15 metres (10mm cable)

OK, we probably have enough info to try a design exercise...

(see end)

Will it be fed from a RCD protected supply?

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

What protective device do you plan on using (fuse / MCB etc)?

Not well up on this - was going to get a sparky to plumb it into my
fuse box - the highest rated breaker in the box is 45A so I'm guessing
he will fit a separate 60A breaker/isolation switch or whatever is
necessary

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 he

http://wiki.diyfaq.org.uk/index.php?...r_Beginners#CU

What earthing system does your house have?
I have no idea. I thought all houses were earthed the same

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/....html#earthing

Given the above you can work out the prospective fault current at the
shower and hence the disconnection times, and you can also check the
earth fault loop impedance. You will also need to check the maximum
voltage drop is not exceeded.

Okay, now I'm lost.

"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.

You need an isolation switch - preferably in the same room but not
somewhere that can be reached from in the shower.
I was going to fit a pull switch isolation switch in the bathroom
which cannot be reached from the shower but through which it's mains
supply will obviously pass.

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.

You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.

Meaning?

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.

Thankyou for the advice - although you've lost me a little on some of
this

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?...s#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?... istance_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/....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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"John Rumm" wrote in message
...
wrote:

What length is the cable run?
up to 15 metres (10mm cable)

OK, we probably have enough info to try a design exercise...


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

"Owain" wrote in message
...
Rod wrote:
Un-insulated - No - it is mineral insulated. The water is not in contact
with the electricity.
Many, many years ago was there not a Tomorrow's World item on an electric
water heater in which it was implied (or maybe even stated) that the
electricity *did* go directly through the water?

That would be an electrode heater which are widely used for bulk water
heating in industry etc but not AFAIK used domestically.

Owain

Certainly not in the UK; but in Australia ....?

I recall my shock (=astonishment) on first encountering an electric kettle
in Australia, opening the lid on
the plastic kettle to fill it, I saw two plates separated by an inch (or so)
which formed the 'heater'!

The kettle worked .... (what more can one say?).

--

Brian

John Rumm wrote:
wrote:

Looking to install a 10.8kw unit - therefore 10mm electrical cable and
suitable breaker. Question is, do I need to worry about where this
cable goes? The ideal route is going to be under some floorboards,
into an attic and under some loft insulation.

On top of the insulation would be greatly preferred.

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

In article , Brian Sharrock
scribeth thus

"Owain" wrote in message
...
Rod wrote:
Un-insulated - No - it is mineral insulated. The water is not in contact
with the electricity.
Many, many years ago was there not a Tomorrow's World item on an electric
water heater in which it was implied (or maybe even stated) that the
electricity *did* go directly through the water?

That would be an electrode heater which are widely used for bulk water
heating in industry etc but not AFAIK used domestically.

Owain

Certainly not in the UK; but in Australia ....?

I recall my shock (=astonishment) on first encountering an electric kettle
in Australia, opening the lid on
the plastic kettle to fill it, I saw two plates separated by an inch (or so)
which formed the 'heater'!

The kettle worked .... (what more can one say?).


Wonder how the impurities in the water affected the "time to boil".....
--
Tony Sayer

Jim Alexander wrote:

and not worry about the cable run. If you accept electric showers it will
still be adequate, just less adequate.

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

--
Cheers,

John.

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Lobster wrote:

(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.)

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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