Installing electric shower from scratch

[JamesMaher]

Firstly I have done a few small electrical alterations (lights, sockets, etc) but have not done anything with the consumer unit. (I did study electronics so have a grasp of the basics). I am going to install an electric shower, but am not sure on a couple of points. I intend to install a 9.5 KW shower. I am going to use 10mm cable and a 45 or 50 amp circuit breaker, problem here is that cu is old blade fuse type, I see I can get a mcb of this type would this be ok? How essential is it that I fit a rcd to the circuit (could I do it without one?). I have gone for a double pole wall mounted switch located outside the bathroom on the wall to isolate the shower. Also there is no rating printed on the outside of the cu, but it has two free slots and four used (45A - central heating/immersion, 30A ring main, 15A - Marked "pay cable" but haven't a clue what this is and a 5A lighting circuit) I am reluctant to fit an rcd because of the amount of time the things spend tripping out i.e. when a bulb blows (wasn't funny in my last place that had one fitted) would the shower circuit be dangerous without one? (assuming the install is done correctly). I have read a bit on opening up the consumer unit and am reasonably confident on that front. Any advice would be appreciated, cheers.

 

[Big_Spark]

Try this

 

[Inspector]

You need an RCD fitted

Take the supply from a 60A fuse in the CU to a separate 45A 30mA RCD unit and then run the 10mm T&E cable to the shower. Clip the cable clear of being enclosed in insulation.

 

[Big_Spark]

You need an RCD fitted

Take the supply from a 60A fuse in the CU to a separate 45A 30mA RCD unit and then run the 10mm T&E cable to the shower. Clip the cable clear of being enclosed in insulation.

Inspector, if your going to talk shiite, post elsewhere.

First of all it should be determined what size supply he has coming in as it may not be capable of handling such a large loading from an individual circuit.

As for putting a 60A fuse in a domestic CU..ARE YOU EFFING MAD.

I think you should stick to blowup paddling pools and wendy houses fella, cos you know shiite about electricity.

The maximum MCB that should ever be installed in a STANDARD domestic Consumer unit is 50A, and even then that is pushing it, most manufacturers do not recommend it. You just bang in, state FUSE..well a fuse could be a piece of wire (BS3036) or a ceramic type (BS88, BS1361 etc) and such a huge fuse should be installed in a domestic unit EVER, PERIOD, END OF STORY.

 

[JamesMaher]

Thanks fwl, I have bought a 40A MCB. (39.5A max current for 9.5KW right?). I'm not going to put a rcd in (can't stretch to the extra 50 quid at the moment) but I plan to add one later. Only thing I'm not sure of is if the main fuse can handle the 9.5kw shower, any ideas?

 

[BR]

Just to clarify things, and sorry FWL if I am echoing your comments. (Also please feel free to advise of any mis representation of the british regs.)

James, Although 16th edition (Wiring Regulations) does not require an RCD fitted to a shower circuit, it is in my opinion advisable. As for your comments on RCD’s constantly tripping, these are unfounded. Lights should not be protected with an RCD and I would imagine that this combined with the use of poor quality bulbs was partly the cause of nuisance tripping in your previous accommodation.

I note you are using 240V for your calculations. You should really use 230 as voltage was (in theory) standarised at this right across Europe in 1995.

Below is the way I would install this shower.

10mmsq tails from old CU to new small (4 way) CU mounted along side old CU.
40A RCBO fitted in new CU.
Run 6mmsq T & E to 45A DP ceiling switch
Run 6mmsq T & E to shower unit

RCBO's (MCB and RCD combined) are considerably more expensive than MCB's but you are left with a safe and well designed system at the end.

Please disregard the above post by 'inspector' which is quite simply "Shiite" (extra i or else it wont appear!)

 

[JamesMaher]

br, thanks for the reply, yes, I agree about the rcd, but to fit a second cu or add a stand alone rcd is just too expensive at the moment. Do you think that 6mm cable will be ok? I thought it had to be 10mm for a 9.5kw? I will fit a rcd in a few months time when I renew the cu cos mine is quite old wylex blade fuse type. I think I will use a new cu with a "built in" 100mA rcd for all house circuits, if I did this do you think it would still be worth fitting a dedicated 30mA rcd to the shower circuit or will the 100mA one do the same job just as effectively? thanks, James.

 

[Inspector]

FWL - your inexperience lets you down on this one. Not up for debate and sorry about your shortcomings.

 

[AdamW]

10mmsq tails from old CU to new small (4 way) CU mounted along side old CU.
40A RCBO fitted in new CU.
Run 6mmsq T & E to 45A DP ceiling switch
Run 6mmsq T & E to shower unit

Surely you want a 45A RCBO and 10mm2 cable throughout? I realise you ideally want the ceiling switch rated to a higher current than the RCBO but 40A x 230V = 9.2kW. Now seeing as tests I have read on the net seem to put mains voltage as coming out at 240V still (well within tolerances!) and thus under 40A, all we need is a straight-banana regulation from Brussels to cause problems here. 6mm cable might be suitable for a very short run but 10mm cable is a safer bet.

The method suggested by Inspector sounds to me like a big-a** version of an FCU! Which in terms of logic might appear a safer way. Is the only thing you have against it the 60A blade-type fuse, or is there something else I am missing here?

One other thing James: it might seem like a good idea to just get the shower in and working now, and buy the RCBO at a later date but as someone said on here a week or so back "there is nothing so permanent as a temporary job". If I were you, wait until payday if you can and get the RCBO then.

Now, who knows what this "pay cable" James mentions is?

 

[breezer]

Now, who knows what this "pay cable" James mentions is?

perhaps the letters T.V were missing (note from other half, "pay cable tv"
or perhaps it is b not a P
or a typo and the p was pressed instead of the L (look at your keyboard)

then again i think i could be totaly wrong

 

[BR]

James 6mmsq cable is rated at 40A, so it is acceptable to use it here, although there is nothing wrong with using 10mmsq, which is rated at about 45A (rough measurements).

You can still fit a 40A MCB alongside the CU in a 1way box (inexpensive) and use this to feed your shower for the time been.

As regards your new CU. this needs to be thought about carefully. You have two options:
1 Fit a split-load board
This is the cheapest option and probably the most common in a domestic installation. As some circuits in your should not be RCD protected this allows some circuits to be RCD protected and some not (e.g. lights, freezer etc) One of the disadvantages of this board is that if one circuit trips the RCD all RCD protected (i.e. not lights etc.) circuits are de-energised.

2. Do not use a split load board and use RCBO's (see my last post) for all circuits you want to RCD protect and MCB's for those you do not want to. This is without doubt a better setup but is also considerably more expensive.

In a split load board you will require 30Ma RCD protection on at least your socket circuits {{(excl fridge/freezer socket(s)}} and also shower, immersion, cooker.... although there is debate on this sockets are a must.

You should think carefully before fitting a main switch incorporating an RCD. In industrial situations these have advantages but in domestic........? I would advise against it, unless you go for a 300ma type. And bear in mind you still need your 30Ma RCD or RCBO's as the case may be.

 

[AdamW]

Or possibly a double typo; it should say "lay table".

The previous ower of the house was obviously Dr. Frankenstein and this was where he built his abominations of nature. They were sewn together, lying on a table hence "lay table". Then he needed a hefty power supply to bring them to life. So a 15A circuit was installed.

 

[BR]

I actually say this labelling before, several years ago. Some very smart woman (i'm not joking) died. She had two fuse boxes. The circuits feeding into one and looping into the other.

One box was labelled 'pay' and the other was labelled 'free'
I couldn’t figure out what this was. Turns out the second board was connected upstream from the meter in the basement. All she had to do was turn off the main switch on 'pay' and turn on free to give her 'free' electricity.

The poor woman was as odd as bedamed. Her bill worked out at about IR£11 every two months. Looks like she alternated between the two so as it would not look to suspicious

 

[plugwash]

and what did you do?

did you remove the "free" box quietly without telling anyone
did you report it to the electricity company
or what?

 

[JamesMaher]

Thanks all for your advice, I'll go ahead with the install without a rcd for now (showered for about 15 years in my parents house which didn't have one and I'm still alive!) but will fit one with new cu. I have 2 spare bays in the existing cu so I'll install the shower circuit into one of them.

BR - the cable run is going to be about 6 metres, is it definately going to be safe to use 6mm cable?

The reason I'm using a wall mounted double pole switch is that I live in a flat so don't have access to the celing and don't want to have to chase it out or have trunking everywhere.

Also, I don't see the wiring as a temporary job, it just makes more sense to me to "upgrade" the circuit to incorporate an rcd when I replace the cu. Probably by using a rcbo on the shower circuit.

BR - About that cable?