Fitting an electric shower to an old cu help

Simon_ · Sep 20, 2011

I have been asked to fit an 10.5kw electric shower with separate breaker for the shower connected to an old Dorman Smith CU in a block of flats. The CU consists of 2 100a main iso switches, on one side is 5 old mcbs (4 x 20a/1x 16a) they are/were for the Economy 7 heaters in the flat. The other side consists of 3 mcbs 30a cooker, 30a Ring, 5a lights. Tails to the switch are fed from the main supply downstairs in a cupboard with a 60a main switch and a separate supply labelled 'off peak' with 100a which i think controlled the Economy 7 heaters. The tails also have an earth terminated into a connector block. Does anyone have experience with this old CU and would an electric shower of this size be compatible with this installation? I think for a start the fuse and tails need upgrading. Any help would be appreciated.

malcolmsanford · Sep 20, 2011

To be honest I would be very surprised if that board would be able to take a 45amp load, as often as not they were not rated for such loads.

Also you are in the realms RCD protection here and so will never get an RCBO to fit that, if the CU was rated for it.

My advice would be to fit a small shower unit enclosure easily available from the wholesalers. You could disconnect the supply feeding the 100amp switch and put into a set of Henley's and then 2 sets of tails one into the CU and other into the shower enclosure.

that is the easy part the difficult part is,

1. The shower is 10.5kw you say and the flat is fed from 60amp switch/fuse? With that shower being a 45amp load, even with diversity you are tight.

2. What size are the tails coming up to feed the CU, as it is a 60amp switch/fuse? I'm betting 16mm max most likely 10mm, so are they big enough.

3. Is the economy 7 side still in use, or have the heaters been removed. If they are not in use then you maybe able to utilize that by transferring the tails over to the normal tariff and feed the shower enclosure direct with them

Simon_ · Sep 20, 2011

Thanks for the advice Malcolm. The main fuse is definately 60a and the economy 7 heaters are being removed. The CU is shown in the pic at the bottom of the post, I think the tails are 10mm.

Engineer54 · Sep 20, 2011

If their was ever a case for replacing a CU ...this would be it!! American style breakers are crap with a capital ''C'' The CU looks like it's a home-made job, made up from a plastic box and from bits and bobs to resemble a CU!!

Paul.M · Sep 20, 2011

For the first time in a long time I totally agree with E54.

Get rid of that shoe box filled with bits n bobs and install a new cu + tails + earthing (ill assume its needed) + a full PIR of course and then fit the shower.

nickblake · Sep 21, 2011

I Agree here , you have said the tails look like 10mm as malcome has said if they are the tails are not big enough to handle the load , if you look at the de rating factor for the cables , realy you should design for the worst case scenario you dont know how the tails are installed they could be surrounded by insulation , when you look at the costs involved in adding a seperate CDU and upgrading the tails then i feel as the guys have said it realy requires a EICR then CDU upgrade hope that makes sence

Simon_ · Sep 22, 2011

The CU box is actually metal believe it or not! An upgrade is not an option at the moment which is why I've decided to step away. There are some other flats in the building that have had them fitted on the same type of installation and frankly I don't wanna take the chance.

gooner85 · Sep 22, 2011

good choice. really needs a CU change before any additions are made

GLENNSPARK · Sep 22, 2011

Engineer54 said:
If their was ever a case for replacing a CU ...this would be it!! American style breakers are crap with a capital ''C'' The CU looks like it's a home-made job, made up from a plastic box and from bits and bobs to resemble a CU!!

"Art attack" or "blue peter" eng...lol......:crowngrin: