Wiring for 10.8kW shower

[Miguel]

I am putting in a new bathroom and have installed a new 10.8kW electric shower. I have upgraded the old t&e to 10mm.

Currently, a separate 30A fuse box is located next to the consumer unit. This is fed by separate l&n cables from the CU.

My plan is to replace this fuse box with a 45A box.

I was wondering...

1. Is it worth the extra money putting in an RCBO?

1. What kind of cable should I use to hook up the new fuse/rcbo to the CU?

2. I have 4mm earth cable, is that big enough?

Thanks in advance, Miguel Sasso

[Christian McArdle]

What do you mean "from the CU"? You would expect the separate fuse box to be fed from a Henley block. If the wires are indeed coming from the CU, can you see where they are connected? Perhaps they are connected to the output from the consumer unit RCD, or to a circuit MCB.

Personally, I would always RCD protect a shower, despite the fact that it has a heating element that can lead to nuisance tripping.

That depends on the configuration. If the new consumer unit is being fed via a Henley block, which I would recommend if replacing the old consumer unit with a bigger one is not practical, then I would use 25mm for the live conductors (phase+neutral).

Big enough for what? If you mean supplementary equipotential bonding, then yes.

Christian.

[Guy King]

The message from "Miguel" contains these words:

Does it already have an RCD in the CU?

[Miguel]

My CU uses mcb's and does not have a built in RCD. The 30amp 'fuse' for the shower is the older style fuse that uses fuse wire.

Three individual wire's (two thicker ones l/n and a much thinner earth go behind a wooden panel, into the CU then onto a 45amp mcb.

As I understand it, I have to have this separate fuse for the shower, so am I just as well fitting a new 'box' and ftting an RCBO?

Should I use 25mm single core? this seems quite large? What about the earth?

Thanks for your advice

Miguel

Christian McArdle wrote:

[Guy King]

The message from "Miguel" contains these words:

Bloody hell, that's 8mm thick!

[Miguel]

Thats what I thought..

"That depends on the configuration. If the new consumer unit is being fed via a Henley block, which I would recommend if replacing the old consumer unit with a bigger one is not practical, then I would use 25mm for the live conductors (phase+neutral)."

Miguel

Guy K> The message

[John White]

Am I right in thinking that your existing shower cable comes from the shower to the 30A wire fuse, and that the 30A fuse box is then connected to the 45A MCB in your main fuse box?

Like this: +-----+ +-----+ | | Shower ====| OXO |====| OXO | 45A MCB +-----+ | | 30A CU

If this is the case then you can connect your new 10mm cable straight into the 45A MCB, and disconnect and throw away the 30A fuse box.

Like this: +-----+ | | Shower ===============| OXO | 45A MCB | | CU

John

[Guy King]

The message from "Miguel" contains these words:

That's meter-tail sort of thickness.

[Miguel]

Thanks John. Thats exactly right. It goes from the

-->shower -->45a isolating switch (pullcord in bathroom), -->30amp fuse (fuse wire type) -->45A mcb

All using 10mm t&e

I had heard from a few people that I needed to have the separate fuse next to the CU, and have indeed seen it done in a number of houses. However after doing more reading, a lot of people say it is not necessary. The installation instructions recommend it but do not demand there is one.

screwfix direct sell a shower unit with a 50A RCBO with a 30mA RCD for =A329.50. I dont mind paying the money if it is worth doing.

Miguel

[John White]

In that case dump the 30A fuse box and wire the shower cable directly into the consumer unit.

OK there's a mix up here. You DO need to have a shower on its own fuse or MCB, but you DO NOT need to have a separate fuse box for it. The

45A MCB in you consumer unit is fine.

If you did not have a MCB available in the consumer unit, then it would have been necessary to fit a second consumer unit for the shower's MCB.

There is no requirement in the regulations that an electric shower has to have a RCD. Some people would however prefer to fit one.

If you wish to add a RCD to the circuit then I would suggest putting in a unit such as the Screwfix 90026-82 RCD shower kit. This fits in the 10mm cable between the consumer unit and the shower where the 30A fuse is at the moment.

John

[Christian McArdle]

Indeed. The connection between the Henley block and an additional consumer unit would be a meter tail.

Christian.

[Christian McArdle]

No. You don't need the separate fuse. The 45A MCB is all you need. The only reason to have a separate box is if you want to provide RCD protection. The box would need only an RCD, not an RCBO.

Alternatively, you could swap out the 45A MCB with a single width 45A RCBO, although these are hard to find.

Christian.

[Miguel]

Thanks to John & Christian for your replies. Crystal clear now. I could not find a combined RCBO which would fit in my CU. It is made by MEM and the slots for the MCB's look to small to accomodate one.

I can get either a 40A or 50A shower unit with combined MCB and RCD for bout =A320 tops off eBAY. While fitting the 50A MCB would clearly be pointless, it is still cheaper tha going for an RCD only. I take it this would be ok?

Thanks, Miguel

[Christian McArdle]

Yes. You can just stick the supplied MCB in a cupboard somewhere. No need to actually use it.

Christian.

[David Hansen]

On 15 Aug 2006 05:24:03 -0700 someone who may be "Miguel" wrote this:-

indicates that one can fit an RCBO up to 40A to one of the modern MEM consumer units and any existing MCB can be fitted with a Field Fittable Residual Current (RC) Unit.