My original 8.5 kW nominal shower stopped working due to hard water. I
replaced it and upgraded to 9.5 kW nominal. That gave me a much better
shower. Now that one has stopped working for the same reason. I
installed a water softener but still need a new shower. I want to
replace the shower with another 9.5 kW nominal unit. The house is only
about 7 years old. The shower circuit has its own 40 A fuse on the
board.

My simple arithmetic tells me:
9500 W / 230 V = 41.3 A
9500 W / 240 V = 39.6 A

But some people say 40 amps means 8.5 kW.

1. Which voltage should I use in the calculation: 230 or 240 V?
2. Which voltage do UK shower specifications use when describing it as
9.5 kW?
3. Anything else I should consider?

Thanks in advance

pat wrote:
My original 8.5 kW nominal shower stopped working due to hard water. I
replaced it and upgraded to 9.5 kW nominal. That gave me a much better
shower. Now that one has stopped working for the same reason. I
installed a water softener but still need a new shower. I want to
replace the shower with another 9.5 kW nominal unit. The house is only
about 7 years old. The shower circuit has its own 40 A fuse on the
board.

My simple arithmetic tells me:
9500 W / 230 V = 41.3 A
9500 W / 240 V = 39.6 A

But some people say 40 amps means 8.5 kW.

1. Which voltage should I use in the calculation: 230 or 240 V?
2. Which voltage do UK shower specifications use when describing it as
9.5 kW?
3. Anything else I should consider?

Thanks in advance

Use a 45A fuse, although if the house is only 7 yrs old, you will need a
breaker (mcb)

On 15 Mar 2006 10:05:00 -0800, pat wrote:

The shower circuit has its own 40 A fuse on the board.

Fuse?
--
Nigel M

On 15 Mar 2006 10:05:00 -0800 someone who may be "pat"
wrote this:-

My simple arithmetic tells me:
9500 W / 230 V = 41.3 A
9500 W / 240 V = 39.6 A

But some people say 40 amps means 8.5 kW.

1. Which voltage should I use in the calculation: 230 or 240 V?

The voltage the shower is rated at, which will be marked on the
shower but is very likely to be 230V.

3. Anything else I should consider?

What size of cable runs to the shower? How is it installed?

What sort of fuses are they? You will probably be able to re-use the
carrier as 40 and 45A cartridge fuses are the same size.


--
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

Phil L
if the house is only 7 yrs old, you will need a breaker (mcb)

I don't understand. Are you saying that an mcb would not be required on
older houses?

If I should have mcb's for everything rather than fuses, I could
replace them all.


David Hansen
What sort of fuses are they?

I don't know how to answer that. What types are they? They are just
larger versions of the types you get in a fused plug.


You will probably be able to re-use the carrier as 40 and 45A cartridge
fuses are the same size.

If I upgrade the fuse from 40 to 45 amps, isn't that putting the cable
at risk? Note that I have been running a 9.5 kW shower on a 40 A fuse
for the last two years. Surely that means it is fine?


The consumer unit is made by Wylex.

The shower and socket circuits are covered by an RCD: Wylex WRS80/2
rated at 80 A, 30 mA. The other circuits (e.g. lights) are not covered
by RCD. The whole board has a chunky red on/off switch.

I can't see any part of the cable and I do not know its size or its
route. The consumer unit is at the front door and the shower is in the
centre upstairs. The cable is probably 5 to 10 metres long.

On 15 Mar 2006 16:09:52 -0800 someone who may be "pat"
wrote this:-

What sort of fuses are they?

I don't know how to answer that. What types are they? They are just
larger versions of the types you get in a fused plug.

Then they are cartridge fuses to BS 1361. This will be written on
the fuse itself.

You will probably be able to re-use the carrier as 40 and 45A cartridge
fuses are the same size.

If I upgrade the fuse from 40 to 45 amps, isn't that putting the cable
at risk?

That is why I asked you the questions I did.

Note that I have been running a 9.5 kW shower on a 40 A fuse
for the last two years. Surely that means it is fine?

What it means is that the fuse may eventually get tired of being
abused and operate.

I can't see any part of the cable and I do not know its size or its
route. The consumer unit is at the front door and the shower is in the
centre upstairs. The cable is probably 5 to 10 metres long.

There is no easy answer to this that doesn't involve a little bit of
effort on your part. Nobody else can see your installation. You
should be able to see the cable at the consumer unit or shower ends.
How big is it?


--
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 writes:

What size of cable runs to the shower? How is it installed?
What sort of fuses are they? You will probably be able to re-use the
carrier as 40 and 45A cartridge fuses are the same size.

I was surprised some time ago to find that the IEE specs. (fusing
curves) indicate that a 45 A. cartidge fuse should blow in 100 seconds
at 150 amps, while a rewirable 45 A. fuse should blow in 100 seconds at
100 amps.

Kind of gives the lie to the idea that cartridge fuses are safer
than rewirable (except that they make it more difficult for people to
do silly/dangerous things when replacing blown fuses).

If the original wiring installation was sensibly done, it's a pretty
safe bet that it could handle 45 amps (ISTR that B&Q's 6mm. is rated for
47A. though that would be derated if the cable ran through a conduit or
under insulation - in which case a proper installation would use 10mm.
cable which can handle about 60 amps.)

I've noticed that most devices use a little less that the spec says,
probably to be safe, so it wouldn't greatly surprise me if a nominal
9.5 Kw. shower used less than 40 A.

Very likely you could just continue to use the 40 A. fuse; they seem to
take a long time to blow on slight overloads, so unless you took 30
minute showers you'd probably be OK. Too large a fuse is dangerous, but
too small a fuse is at worst annoying when it blows.

(But some Nazis might make a big issue about it if any kind of problem
were ever to develop).

--
Windmill, Really t m i l l
@ O n e t e l
. n e t
. u k

On 15 Mar 2006 16:09:52 -0800, pat wrote:

The consumer unit is made by Wylex.

Explains a lot. Replace the 40A fuse with a 40A MCB, all will be well.

--
Nigel M

pat wrote:
Phil L
if the house is only 7 yrs old, you will need a breaker (mcb)

I don't understand. Are you saying that an mcb would not be required
on older houses?


I'm saying that he won't have re-wireable fuses in and they will probably be
MCB's....you also might have MCB's if your house has been rewired in the
last 15-ish years.

Nigel Molesworth wrote:

On 15 Mar 2006 16:09:52 -0800, pat wrote:


The consumer unit is made by Wylex.


Explains a lot. Replace the 40A fuse with a 40A MCB, all will be well.

You seem to be changing stuff for the sake of it. Without more
information on what is actually installed it is not possible to say
accurately what changes (if any) are required.

There is nothing intrinsically wrong with a cartridge fuse so there may
be no need to replace it with a MCB. In fact in some cases a fuse will
be preferable since it will have a higher current braking capacity.

Regarding the load, A 40A fuse or a 40A MCB will run with a 41A load
indefinitely without tripping or blowing. The real issues here are the
cable - i.e. given the length and the way it is installed will it have
the current carrying capacity, and will it meet the voltage drop
requirements.

Also I see no mention of a RCD on the shower feed - this may also be OK
since it is not absolutely required so long as you can meet the required
disconnection times in the event of a fault etc. Again we come back to
cable size and earth fault loop impedance.



--
Cheers,

John.

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

On Wed, 15 Mar 2006 10:05:00 -0800, pat wrote:


2. Which voltage do UK shower specifications use when describing it as
9.5 kW?

The problem is that the 9.5kW (is in big letters on the packaging box).
The real rating is less and specified at 230V and is the one for the
calculations.




--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Thu, 16 Mar 2006 19:55:43 +0000 someone who may be John Rumm
wrote this:-

Again we come back to
cable size and earth fault loop impedance.

Indeed. The OP appears to want someone to say that their proposed
installation is fine. Someone may do so eventually, but one may
think that anyone who says so, on the information given, is a fool
or a liar.


--
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 Thu, 16 Mar 2006 12:27:19 GMT someone who may be
(Windmill) wrote this:-

I was surprised some time ago to find that the IEE specs. (fusing
curves) indicate that a 45 A. cartidge fuse should blow in 100 seconds
at 150 amps, while a rewirable 45 A. fuse should blow in 100 seconds at
100 amps.

I would quibble with the value of the currents a little, but at that
particular point on the curves the rewirable fuse may operate more
quickly. However, that is a carefully chosen point, as a glance at
the time/current curves shows. That the curves overlap in places is
not news.

Kind of gives the lie to the idea that cartridge fuses are safer
than rewirable (except that they make it more difficult for people to
do silly/dangerous things when replacing blown fuses).

Not in the least. These things have to be looked at in the round.
The cartridge fuse does not age like a rewirable fuse, has a higher
breaking capacity, is easier to replace and operates rapidly enough
to limit high fault currents.


--
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

Thanks for the interesting replies. I did indeed want somebody to say
it is within spec. Now I know it is more complicated. I have decided to
get an electrician.

For info, the cable is 6 mm2. I will convert to mcb's for convenience.
I will make any other changes to the consumer unit that are necessary.
Cost is not the top issue.