I wish to install a 8.5KW electrical shower in the top floor of a
house. The house wiring is slightly unusual in that it is divided into
three separate zones each with its own consumer unit. The top floor is
unit is supplied with 16mm cable which I assume is fine for the
shower. There are no other heavy duty electrical items on the top
floor (there is just a couple of ring circuits and a lighting circuit)

However the consumer unit has no spare fuse ways and there is no RCD
protection on the top floor circuit. Therefore I would have to buy a
separate consumer unit for the shower and split the incoming 16mm
cable so that I can feed both the original unit and the new shower
consumer unit.

I’ve had a look around and I think the following items would work.

For splitting the cable: http://www.allaboutelectrics.co.uk/6...ction-box.html

For the consumer unit: http://www.allaboutelectrics.co.uk/c...-unit-40a.html

I assume that I would then need to run either 6mm or 10mm cable to the
shower. The length of the run from the new unit to the shower, via an
isolator pull cord switch, would only be about 5m.

As to earth bonding in the bathroom the only metal is the pipes so
they are the only items that need to be bonded.

I would be grateful if anybody can confirm that this is all OK and
whether there is any cheaper/easy alternatives with regard the
consumer unit and junction box.

I am aware of part P but having installed two showers in the past feel
competent enough to do it.

geoffr wrote:

However the consumer unit has no spare fuse ways and there is no RCD
protection on the top floor circuit. Therefore I would have to buy a
separate consumer unit for the shower and split the incoming 16mm
cable so that I can feed both the original unit and the new shower
consumer unit.

I’ve had a look around and I think the following items would work.

For splitting the cable: http://www.allaboutelectrics.co.uk/6...ction-box.html

That would work, however there is a fair chance there will be enough
terminal capacity on the main incomer switch on the existing CU to add
an extra cable from there (they are built to take at least 35mm^2 normally).

For the consumer unit: http://www.allaboutelectrics.co.uk/c...-unit-40a.html

Yup, that's the sort of thing. You can do it without an RCD if you can
be certain that the earth fault loop impedance is going to be low
enough. However the RCD eliminates the doubt (not that it hurts to
design as if it were not there)

I assume that I would then need to run either 6mm or 10mm cable to the
shower. The length of the run from the new unit to the shower, via an
isolator pull cord switch, would only be about 5m.

6mm^2 ought to be ok, assuming the cable is not run in such a way as it
needs to be de-rated too much.

http://wiki.diyfaq.org.uk/index.php?...es#Cable_Sizes

As to earth bonding in the bathroom the only metal is the pipes so
they are the only items that need to be bonded.

to the CPCs of the shower radial, and also probably the lighting circuit
in the room.

http://wiki.diyfaq.org.uk/index.php?...tary _bonding


--
Cheers,

John.

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

John Rumm wrote:

For splitting the cable:
http://www.allaboutelectrics.co.uk/6...ction-box.html

That would work, however there is a fair chance there will be enough
terminal capacity on the main incomer switch on the existing CU to add
an extra cable from there (they are built to take at least 35mm^2
normally).

Or it might be worth replacing the whole top-floor disboard, given that
there are only three other circuits. An RCBO for the lighting and an
RCD group with three MCBs (2 rings and the shower) would bring things up
to 17th ed. standard and give a neater-looking job for not that much
more cost. Then if this lighting circuit feeds the shower room the need
for supplementary bonding is eliminated (provided that the main bonding
for the house is OK and there are no other circuits in this shower room).

[...] You can do it without an RCD if you can be certain that the
earth fault loop impedance is going to be low enough.

That's not an option under the 17th. All circuits feeding bath/shower
room circuits must now be 30 mA RCD protected. (I'm sure you knew that,
really...)

However the RCD eliminates the doubt (not that it hurts to
design as if it were not there)

Zs needs to be watched here, given that the feeding distribution circuit
is 16 mm^2 - twin & earth, quite possibly, although no specific cable
type was mentioned - so possibly only a 6 mm^2 incoming CPC to the
board. I'd want to take a loop reading at the board before going too
much further.

--
Andy

On 19 Aug, 16:58, Andy Wade wrote:

Thanks John and Andy for your replies.

Zs needs to be watched here, given that the feeding distribution circuit
is 16 mm^2 - twin & earth, quite possibly, although no specific cable
type was mentioned - so possibly only a 6 mm^2 incoming CPC to the
board. *

The cable is T&E.


I'd want to take a loop reading at the board before going too
much further.

The last earth loop impedance reading was taken about 2 years ago and
the reading was 0.3 ohm. Is this OK or should I get it checked again?

I have gone down the route of puchasing a separate RCD shower consumer
unit. However there is insufficient vertical space where I would like
to place the new unit and therefore can these consumer units be fixed
horizontally so that in effect it will be on its side. Will this
effect the operation of the MCB or the RCD?

Failing that are the any regulations requiring the unit to be a
certain height from the floor?

Many thanks again for any replies.

wrote:
On 19 Aug, 16:58, Andy Wade wrote:

Thanks John and Andy for your replies.

Zs needs to be watched here, given that the feeding distribution circuit
is 16 mm^2 - twin & earth, quite possibly, although no specific cable
type was mentioned - so possibly only a 6 mm^2 incoming CPC to the
board.

The cable is T&E.

So 6mm^2 it is then....

I'd want to take a loop reading at the board before going too
much further.

Note that the answer here is somewhat moot in the sense that you have an
RCD so the actual impedance is not critical. My comment about designing
as if it was not there is really just a belt and braces thing - if you
can show that there would be sufficient fault current to operate the MCB
quickly enough then you have the reassuring knowledge that your circuit
is protected even if the RCD should fail.

The last earth loop impedance reading was taken about 2 years ago and
the reading was 0.3 ohm. Is this OK or should I get it checked again?

Where was that reading taken? Is it the the supply earth impedance
measured at the main distribution board near where the supply enters the
property, or is it the complete earth loop measured at the auxiliary CU
position (i.e. including the 16mm^2 T&E run to the CU)?

The 5 to 0.1 sec opening time on a 40A type B MCB is 200A. You can
therefore afford a 1.15 ohms total ELFI to play with.

I have gone down the route of puchasing a separate RCD shower consumer
unit. However there is insufficient vertical space where I would like
to place the new unit and therefore can these consumer units be fixed
horizontally so that in effect it will be on its side. Will this
effect the operation of the MCB or the RCD?

You would have to consult the manufacturers data sheets here. My only
concern would be if adequate cooling of the MCB will be maintained in
the "wrong" orientation. (while it is rare to see domestic CUs arrange
the breakers horizontally, its quite common in industrial ones)

Failing that are the any regulations requiring the unit to be a
certain height from the floor?

It ought to be accessible without a ladder etc. Other than that, use
common sense.


--
Cheers,

John.

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

On 25 Aug, 20:29, John Rumm wrote:
wrote:

I have gone down the route of puchasing a separate RCDshowerconsumer
unit. However there is insufficient vertical space where I would like
to place the new unit and therefore can these consumer units be fixed
horizontally so that in effect it will be on its side. Will this
effect the operation of the MCB or the RCD?

You would have to consult the manufacturers data sheets here. My only
concern would be if adequate cooling of the MCB will be maintained in
the "wrong" orientation. (while it is rare to see domestic CUs arrange
the breakers horizontally, its quite common in industrial ones)

Failing that are the any regulations requiring the unit to be a
certain height from the floor?

It ought to be accessible without a ladder etc. Other than that, use
common sense.

Thanks again John for your reply.

I've had a re think with regard the consumer unit as allthough there
are no spare fuse ways
in the unit that's currently installed one of the fuse ways is just
for a single double socket
in the attic which is never used. Therefore would it be ok to simply
wire this directly into the adjacent
ring main fuse way in the cosumer unit?

I then would then have a spare fuse way to use for the shower.

The other issue is can I just swap the main switch on the unit for a
RCD. The switch that is
currently there is rated at 100A whilst the RCD I've purchased is
rated at 63A. The shower will be 8.5KW
otherwise there are no major loads, just a ring main, bathroom
extractor fan and light circuits so I assume its OK.

Thanks, Geoff

In article
,
geoffr wrote:
However the consumer unit has no spare fuse ways and there is no RCD
protection on the top floor circuit. Therefore I would have to buy a
separate consumer unit for the shower and split the incoming 16mm
cable so that I can feed both the original unit and the new shower
consumer unit.

Why not just change the CU for one that does what you need - they're not
that expensive and would be the better way.

--
*Why is the time of day with the slowest traffic called rush hour?

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