I had an electric shower, rated at 7.5 KW, put into my bathroom around 10
years ago. IT was on its own fuse box with a 30 AMP fuse. I replaced the
shower 2 years ago and took advice which was to keep the shower under a 10
KW rating. My new shower is now 9.5 KW. I have had to replace the ceiling
switch and had a qualified electrician do this last year. Every thing has
been fine. All of a sudden the fuse has blown on two occasions in the same
day. Can someone explain what maybe the problem. Is the fuse too small and
if so why has it worked for a year without a problem? I noticed on the fuse
packaging that a 45 AMP fuse should be used for a 9-10KW shower, but I am
surprised it has worked for this long. If it is the case that the fuse is
too small do I need to new fusebox as a 45 AMP fuse will not fit in the box
I have? Would a circuit breaker be a better bet? Any help would be greatly
The fact of the matter is that a 10KW shower on full will draw 41.6A
(10000/240) so it's little surprise your fuse is blowing. If it's only just
started blowing then perhaps your local supply voltage has risen recently
maybe from 220 to 240 perhaps, remember 10KW is not a constant, it all
depends on the voltage and current (V x I) and I depends on the resistance
of the element and the voltage (I = V/I).
Also it's possible your only now using your shower on full power due to
the time of year.
Incidentally you should be using 10mm cable for a 10KW shower check that
6mm wasn't installed for your smaller shower as this cable is only rated at
Whilst I agree that 10mm is the the best solution, 6mm cable is rated at 32
amps on the worst case scenario when inside conduit inside insulation. If
the cable is clipped direct or clipped and plastered over then it should
carry 47 amps.
What is more worrying is a 30amp fuse blowing when 40 amps is been drawn.
The fuse should withstand 45 amps for one hour before blowing. Either the
shower is on its way out or there is some damage to the cable. The OP did
not state how long the shower was running before the new fuse blew.
Adam it's in his house so it's going to be method 3 isnt it in all
proberbility, ie installed directly in an insulated wall. It's not going to
be clipped direct and remember you need to size for the worst part in the
6mm for a 10Kw shower? It would hardly be professional to say this is ok
Too many probables. We have no idea how or where the cable runs. I would
have said if the cable is installed directly in an insulated wall then it is
installation method 15 not reference method 3 (3 may be OK but 15 would not
be), but there is also a good chance the cable is embedded in plaster which
case use reference method 1
I never said it was. We are refering to a 9.5kW shower not a 10kW shower. If
the shower is 9.5kW @ 240 volts (another unknown) and if the cable is
clipped or embedded in plaster the cable and then passes through some
insulation then table 6B applies and the cable may still be sufficient.
7500 W / 240V = 31.25 A
9500 W / 240V = 39.58 A
When you are putting in a shower you need to ensure that the fuse will
melt before the wire melts, and that the fuse and the wire are both
"thick enough" to avoid melting under normal use.
I reckon that either the shower is drawing more power (perhaps due to
colder water coming in over the winter), or the wiring is buggered,
causing increased current and the fuse to blow.
If possible, I'd put in a 45A fuse and possibly re-run the job with
I'm not an electrician BTW.
No that can't happen. A resistive element simply works or it doesn't. It
can't react to water temperature diference and adjust the output to achieve
the heating effect. The OP may select the hotter setting if the water is
colder though. This would add additional elements creating a higher current.
or the wiring is buggered,
agreed OP must check the cables are 10mm not 6mm or go back to a 7.5KW
Well, sort of. The new figure of 230V was brought in a few years ago to
bring us into line with Europe. The tolerance however is +- 10% of 230V or
216.2V - 253.0V. In reality the voltage didn't really change only the
nominal voltage 'tag' did.
Thanks for the comments. The cable is 10mm and is in a conduit embedded in
the wall. When I replaced the shower two years ago I was told that I would
be OK if I kept the new shower below 10 KW hence the reason I went for 9.5
KW. The surprise to me is that has worked fine for over a year since I last
replaced the fuse. Why would it all of sudden decide to blow fuses?
Theory 1. your voltage has risen significantly within the range we
mentioned. The increased current doing the damage.
Theory 2. you may have a faulty shower
Theory 3 you may have faulty wiring
get the above checked but with 10mm cable there is no problem uprating your
fuse if you physically can
The fuse box is downstairs from the shower but there has been no noise when
it does blow. When I replace it the old fuse is just a but hot. I am getting
an electrician to look at it so hopefully he will get to the bottom of it.
It always seems to be at this time of year as it did it last year at this
time and the year before. That is why I suspected that it may have had
something to with the water temperature that is feeding the shower. Today
was a milder morning and everyone in the house had a shower successfully
without the fuses blowing. It beats me what is going on, but hopefully the
electrician will get to the bottom of it.
No way. If they dropped the voltage they drop the power. That means they
also drop their income from us bill payers. They ain't gonna go for that.
Interestingly though their are manufacturers now rating appliances at 230V.
This means that in a lot of cases they are running over powered. A 3KW fire
may be running at 3.25Kw foe example.
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