My Mum has decided to have an electric shower fitted.

I had previously, perhaps in error, told her that it would involve a lot
of mess and re-decoration for the cable to be chased into the wall.
Either that or she would have to suffer trunking attached to the wall.

However, she has had an estimate for the job and mentioned my concerns.
The plumber guy said that his sparky mate could drop the cable down the
cavity and straight into the back of the consumer unit... No mess! I
assume that the electrician would be able to self-certify his work under
Part P.

Now, somewhere inside me is the notion that the dropping of cables down
a cavity is considered taboo.

There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be
unsupported, but at about 3m, I don't think that this would be a problem
for 6 or 10mm T/E.

Perhaps I just have this misguided idea that it is a no-no!

Any advice appreciated.

Thanks

Steve.

"Steve" wrote in message
news
My Mum has decided to have an electric shower fitted.

I had previously, perhaps in error, told her that it would involve a lot
of mess and re-decoration for the cable to be chased into the wall. Either
that or she would have to suffer trunking attached to the wall.

However, she has had an estimate for the job and mentioned my concerns.
The plumber guy said that his sparky mate could drop the cable down the
cavity and straight into the back of the consumer unit... No mess! I
assume that the electrician would be able to self-certify his work under
Part P.

Now, somewhere inside me is the notion that the dropping of cables down a
cavity is considered taboo.

There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be unsupported,
but at about 3m, I don't think that this would be a problem for 6 or 10mm
T/E.

Perhaps I just have this misguided idea that it is a no-no!

Any advice appreciated.

Thanks

Steve.

The cable can be dropped down the cavity no problem. As to whether the
electrician is correctly qualified, well that's another matter. Don't
forget it will need some form of isolation switch between the shower and the
consumer unit.
Do your mum a favour and watch she isn't ripped off by cowboys. They
usually lie and say the consumer unit can't accommodate the cable or there
isn't enough space. They will offer to fit a very expensive one. Then they
will try other tricks of the trade to get more money. Cowboy plumbers are
bad. I have inspected work they have done. One charged £200 to fit a cold
water pipe through a wall from an airing cupboard to a shower - all of 18
inches!
Check the company details with the online Companies House website to see if
they are really a company. Never use one with a mobile phone number only,
then check the registered office is correctly stated by using Google and/or
a business directory.

You really do have to watch out. Usually cowboys rip people off with
fascias, guttering, roof parts and TV aerials!

Norm wrote:

The cable can be dropped down the cavity no problem.
[...]
Do your mum a favour and watch she isn't ripped off by cowboys.

An electrician who runs twin & earth cable in wall cavities *is* a
cowboy, IMHO. It certainly isn't a recommended practice:

- the cable could be damaged by sharp intrusions into the cavity: mortar
snots, wall ties and so on, also by the fauna and flora that could be
present in the wall cavities of older buildings, also by some wall
thermal insulation materials - particularly polystyrene, although I
don't think that's used for cavity wall insulation (CWI) any more;

- the cable cannot be secured against one leaf of the wall. If CWI is
subsequently introduced you'd have to assume that the cable is
completely surrounded, so the current rating is reduced by 50%. Thus
even with 16 mm^2 cable for your shower (that's the largest T&E made)
you would be limited to 42.5 A (9.7 kW). Moreover that size cable is
likely to be unmanageable in the wiring accessories that you would
normally use in conjunction with a shower;

- the cable cannot be secured in place and must support its own weight.
This can lead to damage, particularly for long vertical drops;

- the cable could easily touch both leaves of the wall, thus bridging
the cavity and leading to penetration of damp.

--
Andy

Andy Wade wrote:
Norm wrote:

The cable can be dropped down the cavity no problem.
[...]
Do your mum a favour and watch she isn't ripped off by cowboys.


An electrician who runs twin & earth cable in wall cavities *is* a
cowboy, IMHO. It certainly isn't a recommended practice:

- the cable could be damaged by sharp intrusions into the cavity: mortar
snots, wall ties and so on, also by the fauna and flora that could be
present in the wall cavities of older buildings, also by some wall
thermal insulation materials - particularly polystyrene, although I
don't think that's used for cavity wall insulation (CWI) any more;

- the cable cannot be secured against one leaf of the wall. If CWI is
subsequently introduced you'd have to assume that the cable is
completely surrounded, so the current rating is reduced by 50%. Thus
even with 16 mm^2 cable for your shower (that's the largest T&E made)
you would be limited to 42.5 A (9.7 kW). Moreover that size cable is
likely to be unmanageable in the wiring accessories that you would
normally use in conjunction with a shower;

- the cable cannot be secured in place and must support its own weight.
This can lead to damage, particularly for long vertical drops;

- the cable could easily touch both leaves of the wall, thus bridging
the cavity and leading to penetration of damp.


Thanks Andy.

This more or less confirms my feelings and you make a couple of
additional very valid points against it that I hadn't considered.

I think Mum will either have to have walls chased or surface mounted
trunking after all!

Cheers

Steve

"Steve" wrote in message
. uk...
Andy Wade wrote:
Norm wrote:

The cable can be dropped down the cavity no problem.
[...]
Do your mum a favour and watch she isn't ripped off by cowboys.


An electrician who runs twin & earth cable in wall cavities *is* a
cowboy, IMHO. It certainly isn't a recommended practice:

- the cable could be damaged by sharp intrusions into the cavity: mortar
snots, wall ties and so on, also by the fauna and flora that could be
present in the wall cavities of older buildings, also by some wall
thermal insulation materials - particularly polystyrene, although I don't
think that's used for cavity wall insulation (CWI) any more;

- the cable cannot be secured against one leaf of the wall. If CWI is
subsequently introduced you'd have to assume that the cable is completely
surrounded, so the current rating is reduced by 50%. Thus even with 16
mm^2 cable for your shower (that's the largest T&E made) you would be
limited to 42.5 A (9.7 kW). Moreover that size cable is likely to be
unmanageable in the wiring accessories that you would normally use in
conjunction with a shower;

- the cable cannot be secured in place and must support its own weight.
This can lead to damage, particularly for long vertical drops;

- the cable could easily touch both leaves of the wall, thus bridging the
cavity and leading to penetration of damp.


Thanks Andy.

This more or less confirms my feelings and you make a couple of additional
very valid points against it that I hadn't considered.

I think Mum will either have to have walls chased or surface mounted
trunking after all!

You could start off with the trunking and chase in as you decorate. Not
ideal I know.

Adam

ARWadsworth wrote:


I think Mum will either have to have walls chased or surface mounted
trunking after all!

You could start off with the trunking and chase in as you decorate. Not
ideal I know.

Adam

Indeed, but better than a cowboy job.

Steve

Steve,

Be careful, a shower unit can draw up to 80 amps and needs to be wired
directly to a properly fused consumer unit with an unused fuse rated
for 80 amps

I suggest you visit this site if you want to understand the pitfalls

http://www.focusdiy.co.uk/stry/diy19...,diycatego ry

You can copy and paste it into your browser.
Though I probalbly shouldn't advertise commercial outlets.


Steve wrote:
My Mum has decided to have an electric shower fitted.

I had previously, perhaps in error, told her that it would involve a lot
of mess and re-decoration for the cable to be chased into the wall.
Either that or she would have to suffer trunking attached to the wall.

However, she has had an estimate for the job and mentioned my concerns.
The plumber guy said that his sparky mate could drop the cable down the
cavity and straight into the back of the consumer unit... No mess! I
assume that the electrician would be able to self-certify his work under
Part P.

Now, somewhere inside me is the notion that the dropping of cables down
a cavity is considered taboo.

There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be
unsupported, but at about 3m, I don't think that this would be a problem
for 6 or 10mm T/E.

Perhaps I just have this misguided idea that it is a no-no!

Any advice appreciated.

Thanks

Steve.

wrote in message
oups.com...

Steve wrote:
My Mum has decided to have an electric shower fitted.

I had previously, perhaps in error, told her that it would involve a lot
of mess and re-decoration for the cable to be chased into the wall.
Either that or she would have to suffer trunking attached to the wall.

However, she has had an estimate for the job and mentioned my concerns.
The plumber guy said that his sparky mate could drop the cable down the
cavity and straight into the back of the consumer unit... No mess! I
assume that the electrician would be able to self-certify his work under
Part P.

Now, somewhere inside me is the notion that the dropping of cables down
a cavity is considered taboo.

There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be
unsupported, but at about 3m, I don't think that this would be a problem
for 6 or 10mm T/E.

Perhaps I just have this misguided idea that it is a no-no!

Any advice appreciated.

Thanks

Steve.

Steve,

Be careful, a shower unit can draw up to 80 amps and needs to be wired
directly to a properly fused consumer unit with an unused fuse rated
for 80 amps

I suggest you visit this site if you want to understand the pitfalls

http://www.focusdiy.co.uk/stry/diy19...,diycatego ry

You can copy and paste it into your browser.
Though I probalbly shouldn't advertise commercial outlets.

80 amps, you sure about that - I haven't seen a domestic shower that draws
anything like that!

40A maybe, 80A no way!

An 80A shower would give a great flow though!

Sparks...

wrote:

Be careful, a shower unit can draw up to 80 amps and needs to be wired
directly to a properly fused consumer unit with an unused fuse rated
for 80 amps

That might actually be a quite decent shower at getting on for 20kW.

The reality is that most electric showers you can buy for domestic use
consume no more than 12kW - and most somewhat less than that (7 - 11kW
being most common). So say upto 45A is a more realistic limit.


--
Cheers,

John.

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

On 3 Nov 2006 14:46:01 -0800 someone who may be
wrote this:-

Be careful, a shower unit can draw up to 80 amps and needs to be wired
directly to a properly fused consumer unit with an unused fuse rated
for 80 amps

I would be interested in a domestic consumer unit that can take an
80A protective device. There may be one, but I suspect there are
very few. It is more the territory of industrial/commercial boards.

The cabling to the shower would be interesting, as would the size of
the supply cable to the house.


--
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

In article ,
Steve writes:
There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be
unsupported, but at about 3m, I don't think that this would be a problem
for 6 or 10mm T/E.

You should assume the cavity will become insulated at some time,
so ask the electrician to pick a cable size suitable for the
current load when buried in insulation, which means derating by
50%. 16mm² T&E will derate to 42.5A, which is about a 10kW shower.
Might want to knock down to 10mm² from the switch to the shower
as it's easier to handle, if not buried in insulation.

--
Andrew Gabriel

In article , Andrew Gabriel
writes
In article ,
Steve writes:
There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be
unsupported, but at about 3m, I don't think that this would be a problem
for 6 or 10mm T/E.

You should assume the cavity will become insulated at some time,
so ask the electrician to pick a cable size suitable for the
current load when buried in insulation, which means derating by
50%. 16mm² T&E will derate to 42.5A, which is about a 10kW shower.
Might want to knock down to 10mm² from the switch to the shower
as it's easier to handle, if not buried in insulation.


Better still do away with that piece of pox known as an Electric
"shower" which it is, and get a power shower much, much, better )
--
Tony Sayer

Andrew Gabriel wrote:
In article ,
Steve writes:

There is, at present, no cavity insulation so it would be a relatively
straightforward job and I can't really think of many reasons against it
except that any future cavity insulation would degrade the current
carrying rating of the cable. Also the vertical drop would be
unsupported, but at about 3m, I don't think that this would be a problem
for 6 or 10mm T/E.


You should assume the cavity will become insulated at some time,
so ask the electrician to pick a cable size suitable for the
current load when buried in insulation, which means derating by
50%. 16mm² T&E will derate to 42.5A, which is about a 10kW shower.
Might want to knock down to 10mm² from the switch to the shower
as it's easier to handle, if not buried in insulation.


Thanks Andrew

I am going to persuade Mum against the idea... If I can ;-)

Steve

On Sat, 04 Nov 2006 13:16:02 GMT someone who may be Steve
wrote this:-

I am going to persuade Mum against the idea... If I can ;-)

Assuming she has mains pressure cold water and gravity pressure hot
water then perhaps you could persuade her to install a venturi
shower instead. Not many types are available but
http://www.screwfix.com/app/sfd/cat/...97330&ts=49804 is one
of them.

It only needs two pipe connections and may well be rather less
disruptive, as the only work will be in the bathroom and perhaps the
loft.


--
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:


Assuming she has mains pressure cold water and gravity pressure hot
water then perhaps you could persuade her to install a venturi
shower instead. Not many types are available but
http://www.screwfix.com/app/sfd/cat/...97330&ts=49804 is one
of them.



Are these things any good ? My gut reaction is to think that they
aren't such a good idea.

Do they regulate temperature properly ? The manufacturers claim that
they do, but they also mention that certain types of poor operation can
only be sorted by tweaking the temperature of the hot water tank. To me,
that suggests that the range of temperature adjustment that's possible
is very limited.

Is the flow good and strong ?

I'm sceptical, but if they work well in reality, it could be a useful
option.

On Mon, 06 Nov 2006 01:03:24 GMT someone who may be
(Roly) wrote this:-

[venturi shower]

Do they regulate temperature properly ?

Within limits. As the cold supply fluctuates so will the amount of
hot. However, they will not compensate for a cold cylinder.

Is the flow good and strong ?

As strong as the mains is.

I'm sceptical, but if they work well in reality, it could be a useful
option.

Provided they are installed in accordance with the manufacturers
instructions they seem to do well. I'm sure those with problems with
them would be complaining loudly if there was a major flaw.


--
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

Andrew Gabriel wrote:
/snip/

You should assume the cavity will become insulated at some time,
so ask the electrician to pick a cable size suitable for the
current load when buried in insulation, which means derating by
50%.

is that correct?

IIRC the relevant IEE regs table (regrettably not to hand at the
moment) only gives derating factors up to 500mm. Inspection of the
factors & consideration of the physics behind them suggests that
extrapolation to longer runs would imply much larger derating factors.

The problem surely is that if the cable is embedded in (good) thermal
insulation then there is nowhere for heat to escape except along the
length of the cable & there's clearly going to be a limiting point
beyond which the cable heats up to 70deg C (the limiting operational
temperatiure for normal FTE).

IOW 500mm is the maximum length the IEE reg authors thought worth
specifying factors for, not the maximum derating factor for longer
cable runs.

In article .com,
"ironer" writes:

Andrew Gabriel wrote:
/snip/

You should assume the cavity will become insulated at some time,
so ask the electrician to pick a cable size suitable for the
current load when buried in insulation, which means derating by
50%.

is that correct?

From memory, yes -- I don't have the regs on me.

IIRC the relevant IEE regs table (regrettably not to hand at the
moment) only gives derating factors up to 500mm. Inspection of the
factors & consideration of the physics behind them suggests that
extrapolation to longer runs would imply much larger derating factors.

IIRC, the table is for lengths up to 500mm. I think the text
says 50% derating for anything longer, but again, that's from
memory.

The problem surely is that if the cable is embedded in (good) thermal
insulation then there is nowhere for heat to escape except along the
length of the cable & there's clearly going to be a limiting point
beyond which the cable heats up to 70deg C (the limiting operational
temperatiure for normal FTE).

Insulation isn't a perfect insulator, and with the power dissipated
being the square of the current, 50% derating is just 25% of the
power.

IOW 500mm is the maximum length the IEE reg authors thought worth
specifying factors for, not the maximum derating factor for longer
cable runs.

I'll check on Monday unless anyone beats me to it.

--
Andrew Gabriel

Andrew Gabriel wrote:

IIRC, the table is for lengths up to 500mm. I think the text
says 50% derating for anything longer, but again, that's from
memory.

Yes, that's correct:

"For a single cable likely to be totally surrounded by thermally
insulating material over a length of more than 0.5 m the current
carrying capacity shall be taken, in the absence of more precise
information, as 0.5 times the current carrying capacity for that cable
clipped direct to a surface and open (Reference Method 1)."
[523-04-01 (part of)]

The same regulation first requires that "the cable shall wherever
practicable be fixed in a position such that it will not be covered by
the thermal insulation."

--
Andy

Andrew Gabriel wrote:
In article .com,
"ironer" writes:

Andrew Gabriel wrote:
/snip/

You should assume the cavity will become insulated at some time,
so ask the electrician to pick a cable size suitable for the
current load when buried in insulation, which means derating by
snip
IIRC, the table is for lengths up to 500mm. I think the text
says 50% derating for anything longer, but again, that's from
memory.

The problem surely is that if the cable is embedded in (good) thermal
insulation then there is nowhere for heat to escape except along the
length of the cable & there's clearly going to be a limiting point
beyond which the cable heats up to 70deg C (the limiting operational
temperatiure for normal FTE).

Insulation isn't a perfect insulator, and with the power dissipated
being the square of the current, 50% derating is just 25% of the
power.

It also helps that the wire is very small, generally, in relation to the
insulation - the apparent effectiveness of the insulator drops off as
you go out.

Considering the simple case of a 1cm diameter cable in the middle of an
infinite fiberglass insulator.
Imagine the insulator split into millimeter thick cylinders.
The first millimeter of the insulation has a perimeter of 32mm or so.
The 50th mm has a perimeter of about 340mm.

So, the 50th millimeter adds 1/10th as much as the first one.

As an example, with fiberglass, 0.04W/m/K, that's 40w/mm/k for a 1mm
thick 1m square sheet.

Or about 1.5w/m/k for 1m of flex insulated with 1mm of fiberglass.
10mm gets you about .21w/m/k, 50mm about .1, and 100mm 0.08.

A whole meter only about halves this.

1mm^2 cable has a resistance of 1.7*10^-2 ohms (.017 ohms).
2.5mm^2 cable, for both directions has about .013 ohms per meter.

Taking 30A through this dissipates 12W or so, or allows at most 10mm of
insulation or so (temp rise of 60C ish).

Derating to half gives a power per meter of 3W.
With 10mm, a rise of 14C, even 100mm is quite safe, with a rise of only
40C.
You have to get pretty near a meter of insulation on each side of the
wire, to get close to being dangerous.

A simple metal heat spreader, next to the wire, even if it's buried in
insulation can help lots.
Actually, it doesn't even need to be metal, it just needs to be a
comparatively poor insulator. Timber helps lots.

(If you use this as the basis for running cables through insulation, and
your house catches on fire, I reserve the right to point and laugh)

Ian Stirling wrote:

/snip/

It also helps that the wire is very small, generally, in relation to the
insulation - the apparent effectiveness of the insulator drops off as
you go out.

/snip/


(If you use this as the basis for running cables through insulation, and
your house catches on fire, I reserve the right to point and laugh)

interesting calculations. I usually acept and rely on calculated
results but in this case I regret I'm not convinced enough. The
derating factors in the table in the IEE regs just don't level off in
the way you normally get when a limiting value is near. On top of that
I just don't trust FTE in close proximity to good insulation either.

One question I always consider before doing any work is 'If this job is
done that way, what's the chance it will have to be done again while
I'm still around?" In this case the answer is too near Yes for me to
risk it.