My son's 7Kw shower has just packed in and we are looking round for a new
one.
There seem to be very few 7Kw ones but loads of 8.5Kw.

First question is can we run an 8.5Kw on his existing 6mm cable?
Length is about 12 meters, some behind plasterboard and some clipped in the
roof.

The fuse is rated 32A so presume that's a bit on the low side.

Other point is, looking at some makes of showers, the water feed seems to be
pushfit
on a lot of them whereas I'm used to seeing the compression elbow input.
Does this mean we have to replace the feed with plastic or will the pushfit
device take
the existing 15mm copper.

When we have more time, we will upgrade the cable to 10mm but for a while
yet,
we will have to make do...
Thanks for any help

On Sun, 15 Jul 2007 21:10:44 +0100, "Philip Thompson"
mused:

My son's 7Kw shower has just packed in and we are looking round for a new
one.
There seem to be very few 7Kw ones but loads of 8.5Kw.

First question is can we run an 8.5Kw on his existing 6mm cable?
Length is about 12 meters, some behind plasterboard and some clipped in the
roof.

Without seeing it I would say that should be OK for 8.5kw.

The fuse is rated 32A so presume that's a bit on the low side.

Maybe, but I don't put anything more than a 32A on any 6mm and I
haven't had a problem yet.

Other point is, looking at some makes of showers, the water feed seems to be
pushfit
on a lot of them whereas I'm used to seeing the compression elbow input.
Does this mean we have to replace the feed with plastic or will the pushfit
device take
the existing 15mm copper.

The existing copper should be fine on the pushfit. Best bet would be
to RTFM.

When we have more time, we will upgrade the cable to 10mm but for a while
yet,
we will have to make do...
Thanks for any help
--
Regards,
Stuart.

Philip Thompson wrote:

First question is can we run an 8.5Kw on his existing 6mm cable?

Possibly, we can do some sums.

Length is about 12 meters, some behind plasterboard and some clipped in
the roof.

What type of earthing does the house have?[1]

Is there a RCD protecting the shower circuit?

[1] http://www.diyfaq.org.uk/electrical/...al.html#system

The fuse is rated 32A so presume that's a bit on the low side.

It is unlikely to blow (since a 32A cartridge fuse will take getting on
for 5A indefinitely), but 40A MCB would be a better choice.

What type of fuse - cartridge or re-wireable? (I presume the former as
you said 32A rather than 30A)

Other point is, looking at some makes of showers, the water feed seems
to be pushfit
on a lot of them whereas I'm used to seeing the compression elbow input.
Does this mean we have to replace the feed with plastic or will the
pushfit device take
the existing 15mm copper.

It will take the copper.

When we have more time, we will upgrade the cable to 10mm but for a
while yet,

There is probably no need.

If you went with a 8.5kW shower at a nominal 230V, that gives 37A

Voltage drop at 37A and 12m should be well inside the 9V limit. The
current rating for the cable is 46A for the installation method you have
described assuming there is no insulating material behind the plasterboard.

With regards to disconnect time there is a very good chance that it will
be within spec since a 40A MCB will only require Zs of 1.2 ohms to
disconnect in 5 secs, and the cable will only contribute 0.13 ohms of that.
We need to check the CPC is of adequate size. Say we take worst case as
being TN-S installation with Ze of 0.8 ohms (this is probably
pessimistically high), and add to that the round trip phase to CPC on a
6mm cable 10.49 mOhms/m we get 0.92 ohms loop impedance, so an earth
fault current of 250A. That should disconnect in 0.1 secs, so we get
sqrt( 250^2 x 0.1 ) / 115 = 0.7mm required minimum CPC size - well
inside the 2.5mm^2 provided by the 6mm^2 T&E. That's not allowing
anything for supply impedance (but is being pessimistic on earth fault
loop), so chances are that will fine. If it does have a RCD then there
is certainly no problem.

(the CPC size is still just ok without a RCD if you redo the sums for a
40A cartridge fuse as well)

--
Cheers,

John.

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Wonderful
Thanks for all the information.
The house is only about 3 years old so the electrics are pretty much
up to standard.

I had got it in my mind that 6mm cable was rated at 32 amps ish and that was
my concern with using an 8.5 KW shower.

Anyway, at least we can get him up and running with a new one fairly
quickly..


Thanks very much guys

In article ,
Philip Thompson wrote:
I had got it in my mind that 6mm cable was rated at 32 amps ish and that
was my concern with using an 8.5 KW shower.

This will give you something to play with:-

http://www.tlc-direct.co.uk/Technica...ltageDrop.html

--
*Why doesn't Tarzan have a beard? *

Dave Plowman London SW
To e-mail, change noise into sound.