new electric shower installation

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.
Ive had a look around and I think the following items would work.
For splitting the cable: http://www.allaboutelectrics.co.uk/60a-3-terminal-junction-box.html
For the consumer unit: http://www.allaboutelectrics.co.uk/combined-rcbo-shower-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.
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geoffr wrote:

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).

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)

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?title bles#Cable_Sizes

to the CPCs of the shower radial, and also probably the lighting circuit in the room.
http://wiki.diyfaq.org.uk/index.php?title rthing_and_Bonding#Supplementary_bonding
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Cheers,

John.

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John Rumm wrote:

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).

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...)

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

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Thanks John and Andy for your replies.

The cable is T&E.

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.
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snipped-for-privacy@hotmail.com wrote:

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

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.

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.

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)

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

John.

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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
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snipped-for-privacy@hotmail.com wrote:

Yes. So long as the cable to the socket is 2.5mm^2 or larger, it would just become a spur from the ring (taken at the circuit origin rather than an accessory).

Indeed you would.

You could, but I would not advise it since you would lose any discrimination - a fault that trips the RCD would hence turn *off* all the circuits[1]. A better solution might be to install a RCBO for the new circuit if you can fine on that will fit the existing CU.
[1] Even basic 17th edition installations tend to split the circuits over a pair of RCDs such that you don't lose lighting in the same location as another fault.
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Cheers,

John.

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In article

Why not just change the CU for one that does what you need - they're not that expensive and would be the better way.
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*Why is the time of day with the slowest traffic called rush hour?

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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