I have an old Wylex consumer unit, like this:
http://www.tlc-direct.co.uk/Products/WY804.html

I've bought an 9.5kW electric shower, but I've just discovered that
Wylex don't make 45A MCBs for these consumer units anymore, the highest
rated they produce is 40A. At 230v, 9.5kW draws 41.3A. Will I get away
with a 40A MCB or will it be forever tripping? (I have 10mm cable, by
the way, so 40A won't be a problem there).

In article .com,
"Martin Pentreath" writes:
I have an old Wylex consumer unit, like this:
http://www.tlc-direct.co.uk/Products/WY804.html

I've bought an 9.5kW electric shower, but I've just discovered that
Wylex don't make 45A MCBs for these consumer units anymore, the highest
rated they produce is 40A. At 230v, 9.5kW draws 41.3A. Will I get away
with a 40A MCB or will it be forever tripping? (I have 10mm cable, by
the way, so 40A won't be a problem there).

The reason is most likely that the fusebox is only rated 60A in
total, and you may find your main fuse is also 60A in that case.
Sounds like you need to check if your existing supply can
support the additional 9.5kW load you are intending to add.
You may need the supply (or at least the main fuse) upgrading
to 100A and a new fusebox.

--
Andrew Gabriel

Andrew Gabriel wrote:

The reason is most likely that the fusebox is only rated 60A in
total, and you may find your main fuse is also 60A in that case.
Sounds like you need to check if your existing supply can
support the additional 9.5kW load you are intending to add.
You may need the supply (or at least the main fuse) upgrading
to 100A and a new fusebox.

You're right, the main fuse is a 60A one. The cooker is gas though, and
the water is heated by a combi, so there's not that much demand for
electricity. It's a three bedroom terrace. The shower is for occasional
use only, predominantly in case the combi breaks down.

Can upgrading the supply be as simple as getting the main fuse changed?
Presumably the cable from the meter to the CU has to be heavy enough to
take 100A. I've no idea what size the current cable is, except that it
looks bloody thick! The only problem with changing that is that it the
fusebox is boxed in in a place that makes access to the incoming cable
very difficult.

In article .com,
"Martin Pentreath" writes:
Andrew Gabriel wrote:

The reason is most likely that the fusebox is only rated 60A in
total, and you may find your main fuse is also 60A in that case.
Sounds like you need to check if your existing supply can
support the additional 9.5kW load you are intending to add.
You may need the supply (or at least the main fuse) upgrading
to 100A and a new fusebox.

You're right, the main fuse is a 60A one. The cooker is gas though, and
the water is heated by a combi, so there's not that much demand for
electricity. It's a three bedroom terrace. The shower is for occasional
use only, predominantly in case the combi breaks down.

Can upgrading the supply be as simple as getting the main fuse changed?
Presumably the cable from the meter to the CU has to be heavy enough to
take 100A. I've no idea what size the current cable is, except that it
looks bloody thick! The only problem with changing that is that it the
fusebox is boxed in in a place that makes access to the incoming cable
very difficult.

You must have upgraded the fusebox to a 100A one before they will
upgrade the fuse, and make sure earthing is up to current regs,
etc. I haven't had a fuse upgraded for over 10 years, but it used
to be free (at least if supply cable was OK for 100A). Someone
here may have had it done more recently and might be able to
confirm if it's still free or not -- with increasing split-up of
the supply industry, it may well not be.

--
Andrew Gabriel

Martin Pentreath wrote:

[snip]

Just a reminder of the importance of the supplementary bonding, when
you're fitting an electric shower, especially if it's over a metal bath.
At the very least make sure you have a 4 mm^2 bond between the
shower's earth terminal and the bath.

Without bonding, if your shower heater happens to develop an earth fault
the shower metalwork may rise to over 160 volts[*], relative to other
'earthy' objects such as the bath. This voltage will persist until the
fuse blows or MCB trips, and that could take up to 5 seconds, during
which time you could be receiving a particularly nasty shock. The local
supplementary bonding will greatly reduce this fault voltage and thus
keep you safe.
[*] 10 mm^2 cable has a 4 mm^2 CPC, so the resistance in the CPC is 2.5
times as high as in the phase conductor. So with a phase-CPC s/c at the
shower and no bonding you'll have 230 * 2.5 / 3.5 V = 164 V developed
across the CPC - nasty!

--
Andy