RCD and CU for electric 10.8w shower

[StephenStephen]

I'm gathering the bits for a 10.8 kw electric shower installation, which, when I've got everything, I'm then going to ask a part P registered electrician to connect.

So far, I've run the 10mm cable from the consumer unit to the shower room, and will be finding an isolation switch for outside the room, and an RCD

The CU main switch is rated at 65A, and is recent (within the last 8 years or so) as is the wiring of the house. I'm confident in the general quality of the house electrics.

Am I right to assume that the 'trip switches' in the CU are MCB's and I just need to ensure that there is a 45A MCB available within the CU?

I'm looking for a cheap RCD for the shower and am wondering if a 63A RCD would be appropriate or do I need to go for a 45A?
(My thinking is that as long as the cut out rating is low enough, say 30mA, then the current rating needs to be at least high enough, but doesn't matter if it's too high?)

thanks for your help

Stephen

 

[333rocky333]

The CU main switch is rated at 65A,

does such a thing exist

 

[equitum]

10800 watts / 230 volts(Uo) = 46.96 Amps, so you need a 50A MCB.

A 63A RCD would be OK. The current rating of the RCD is how much current it can handle, so it needs to be greater than the maximum current as calculated above. 30mA is the fault current that will cause the RCD to trip.

The MCB is providing the overcurrent protection so it needs to be more than 45A.

The cable has to be able to handle more current than the MCB, so it will need to be 10 sq mm.

To be fair, is you are unaware of this, then your electrician should be designing and installing the circuit. He cannot "sign off" your work anyway .

 

[streetlighter]

Might be better to install a separate shower CU with RCD incomer and 50a MCB then leave the tails for the electrician to test and connect via Henly blocks.

 

[GaryMo]

Might be better to install a separate shower CU with RCD incomer and 50a MCB then leave the tails for the electrician to test and connect via Henly blocks.

If you go this route then make sure you have a main isolator, if not you'll need one.
Also many electricians won't issue an EIC for somebody else's work.

 

[StephenStephen]

Thanks for your help


333rocky333
does such a thing exist?

I'm guessing that you're asking this because the rating is low?
Can you tell me how to check, as your reply suggests I might not be right in my attempt (reading the number off the 'big switch' on the right hand end of the CU)

equitum
Thanks for your reassurance on the RCD - as I thought.
Yes - 10mm cable - ran that.
50A MCB - thanks for that, brilliant
I'm not going to connect anything - just run cables and fix switches and RCD's etc onto walls then ask the electrician to do all the connections - I'm doing this mainly because I want the learning, challenge and satisfaction of doing what I safely can, but also because he's actually busy with something else, and also because I'm trying to install on a very limited budget. Do you forsee problems with this arrangement I should be aware of?

streetlighter
Thanks - yes I was starting to think along these lines - if it turns out that the max load on the CU is insufficient then this seems a good alternative to a new CU.

Thanks for your continuing help

Stephen

 

[333rocky333]

Thanks for your help


333rocky333
does such a thing exist?

I'm guessing that you're asking this because the rating is low?
Can you tell me how to check, as your reply suggests I might not be right in my attempt (reading the number off the 'big switch' on the right hand end of the CU)

65 is an odd rating

You are looking in the right place it sounds like.
60, 63, 80, 100 are more standard

anything lower than 100a may not handle the extra load on the board.

What other circuits are in use on the board

 

[ban-all-sheds]

Do you forsee problems with this arrangement I should be aware of?

Is he happy with the arrangement?

Has he told you exactly where and how to run the cables? And are you doing exactly what he said?

Has he told you what size cable to use, and what rating of MCB? And are you using exactly what he said?

Has he looked at the CU to see if it will take this load?

Has he checked that your earthing and bonding are OK?

Remember he is the one who has to sign certificates and legal declarations to say that he did all the work, so if the answer to any of the above questions is "no", then things might not go how you are hoping when you get to the "ask a part P registered electrician to connect" stage...

 

[StephenStephen]

333rocky333
Thanks

so, starting at the bottom:

1) Big fat wire coming in
black box (danger 415 volts)
digital prepayment meter

2) There are 3 large sealed fuses (?) - 1 of which is labelled 60A and which is connected to the CU (the other 2 are labelled 100A)

3) CU unit has a double switch on the right hand side (2 switches joined together) whic has 63A written above one side of it (sorry - I misread it - 65A was a red herring)

There are approx 9 circuits coming out of the CU, the switches don't have ratings written on them. Including 3 power circuits, 3 lighting circuits, cooker.

So - current usage (if this helps?): - cooker is gas with electric oven, 3 small T.V's 2 freeview boxes, 1 stereo, 1 PC, 1 pump for central heating, 1 washing machine, low energy lights, no electric heating or water heating. bill's under £20 a month (sorry - I'm just throwing out random facts now - I'd better go to bed and be more sensible tomorrow)

reading this - I'm thinking that my guess is that we would almost certainly manage our usage through this board, even with the shower, but if we move on then new owners might hit the board's limit if their usage is high, so I really ought to think in terms of separate provision for the shower - taking it off one of the other 2 big fat 100A fuses (?) under the CU. Ho hum more stuff to buy - so I guess that means:
"install a separate shower CU with RCD incomer and 50a MCB then leave the tails for the electrician to test and connect via Henly blocks." (Streetlighter) except that it would connect to the 100A fuse (?) rather than henley block?


cheers

Stephen

 

[333rocky333]

The other two 100amp fuses may not be yours to tamper with

Do not touch them yourself, they may be different live phases, and proberly not going through your meter
They are if still live,going to have 415volts between them and also between them and the consumer unit so.
DO NOT TOUCH THEM YOURSELF

 

[StephenStephen]

ban-all-sheds

Thanks.

Yes, he's happy with the arrangement, though we've not discussed it in detail.

What I'm doing here is to attempt to gather and position all the parts so that he can see everything, be happy with everything, connect everything (including cross bonding etc) and make it safe.

I'm hoping, with your advice, to put him in the position where he's happy to do that, so that the circuit is safe.

However - I do agree that it would have made my life easier to have had him over to take specific advice beforehand.
Also, you've made me realise that there will be a short cable run (no joints) that he won't be able to see before connecting everything, so I'll either ask him to have a look before tiling, or take photos to show him and keep my fingers crossed that I won't be tiling twice!

cheers

Stephen

 

[StephenStephen]

333rocky333

Thanks for your concern - I do appreciate it

But don't worry - when I say I'm going to ask an electrician to do all the connections, I mean that, and I also won't be disconnecting deconstructing or otherwise interfering with anything live. My plan is to learn lots (as I am) buy the gear, run and label cables, screw boxes, switches etc to walls, and then leave the electrician to open disconnect test connect etc

cheers

Stephen

 

[StephenStephen]

333rocky333

damnit you're right! - they're not wired through the meter.

So I guess that means that if I want more than 60amps in the house I need to contact the supply company?

Did we establish that I almost certainly do ned more than 60amps?

cheers

Stephen

 

[333rocky333]

Ask the electrician

He will need to advise

I feel you may be pushing it with a 60 amp fuse and 63amp main switch.

Even not connected to the old board, it would be near the 60 amp fuse limit if that is what it is.

You may if the other two are 100 amp, be able to get your 60 amp uprated to 100amp.

He may advise a new 100amp board, do you know the make and type of the old one ,or of the mcb's

If it is still made you may be able to reuse the old mcb's from that into the new one, if they are ok and buy what you need to connect the shower properly
The cost of your shower rcd , mcb,enclosure, tails ,henley blocks? and isolater, can all be deducted from the cost of the new board etc.

 

[StephenStephen]

Thanks for all your help - I'm much clearer in my mind what's going on, and what needs to be done.

I'll give the electrician a ring and see if he's happy to expand his work beyond the fairly quick job we originally imagined.

Cheers all

signing off for now

Stephen