Shower requirements

11 May 2006
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United Kingdom
Hello All,
I've looked at the faq, but still not sure, apologies in advance then if this is well covered
I'm hoping to get an electric shower in over the bath, and would like to know what it's going to take, (apart from water,10mm2 cable,supplementary bonding, 50 amp pull-switch,and rcd csu ) I'm in Scotland, in a 25 year old house, with a meter in a cupboard outside, the consumer fuse is 100 amp, 6.5mm o.d tails (20mm2 ?),to a meter ( type C11B3a-h, 20-80 amp, 230 volt), then there is an approx. 10 metre run of cable 20x10 o.d, 6.5mm o.d on the wires inside,(20mm2 ?) to an original (vintage) Wylex 8 way 60 amp fuse box in a cupboard at the foot of the stairs. Everything is in good condition and working at present (except me, who is a bit cream crackered) Bathroom upstairs, would be approx. 12 meter cable run through the loft, down through a cupboard, a floor and into the csu cupboard or about 10 metre run to the meter box.
If I were to get a 9.5kw shower:

1. Would I need to get a new meter put in?

2.What would be the highest kw shower with existing set up?

3. What amps for the csu ? 80, 100, I would like a bit of spare capacity for future needs if possible.

4. Do I need to upgrade cable from the meter to the site of the Wylex, if a new csu were to go there?

5.Alternatively, could a shower csu be put in, in the meter cupboard, ie a separate circuit leaving the Wylex in place?

6. A handy run for the cable, if it were ok to have a shower csu in the meter box, would be through the boxed off corner of the kitchen and up to the bathroom, then into the loft, not trunking, just 2 x 2 and plywood, about 150mm x 300mm, where the water pipes and the soil pipe are, plenty of room, not right up to the pipes, airy, only insulation is on the pipes, is sharing a route permissible if no interference is caused to other services?

7. If the cable goes through oval trunking, (like through the cupboards),would 10mm2 be ok, does it have to be uprated ?

I want the job done safely and properly, but am a bit on the skint side, do you sparkies have to do the whole thing, supply the gear, drill the holes, bang the nails, write the theme tune? I would like to do whatever work, if anything, is acceptable in order to have a smaller bill. Is it ok to have some planning and preparation, laying out of cable, done in advance, if you can see it all, and it's done the way you would?
All advice gratefully received,
regards Jim
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6.5mm overall diameter??? if thats the size of your incoming maind I would think they are 10mm ay best, You definitely need to get the main supply upgraded before any kind of electric shower and probably before you do any serious cooking none of your supply ..tails...meter... cu are really up to this load. I would guess(hope) that your main fuse is 45 amp at best
hi cozycats,
Thanks for the response
Yep, incoming tail to the meter is 6.5mm on the outside of the primary insulation, (the red plastic), if the insulation is 1mm thick then the wire is 4.5mm diameter, I make that 15.9sq mm.area cross section It's a 100amp consumer fuse, and a 60 amp fuse box. My cooking is serious enough to need balsamic vinegar,;) never had a fuse blow or any problems. Any idea about the rest?
The tails are probably actually smaller than you have worked out as the copper in cable of this size is not a single solid core, rather it normally is made up of seven strands of copper (to make the cable easier to work with) so there will be some small gaps between / around the strands. It is more likely that, like cosycats said you have 10mm² tails, which is not an unusual size for an older installation, but not sufficient for your proposals.
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A Wylex Standard CU with a sub-100A main switch was never designed for a circuit of more than 32A.

That's why the higher-rated devices have a tab on the back of the fuse shields that keys in with newer units, but does not allow fitment to an older CU.

I would make sure the supply fuse is 100A.(How do you know it is 100?)

The tails (preferably on both sides of the meter, but the mains side is the DNO's responsibility) need to be 25mm2 min. While they're changing the tails, they can fit a new meter, which they normally change on a 20 year cycle anyway, but if you tell them you plan to increase the load, and need a meter to cope with flc.

You need to ensure the main bonding to services (gas, water, oil etc) is at least 10mm2.

As for the existing CU, you have 2 choices; If it were me, I would change the CU for a multi-way split-load affair to suit your supply type. Have you got TN-S, PME or TT?

But if you are on a budget, I suppose you could keep the existing, and add an RCD-fed CU with suitable protection to feed the new shower.

Don't forget to fit the correct-sized MCB, cable and switch. If you are running in a loft, you need to halve the CCC of the cable to take account of the effect of the insulation, unless you can route it entirely away from insulation.

You also need to ensure supplementary bonding within the bathroom (or just outside will do) is adequate, with a cross section of 4mm2 min. You need to link all pipework, metallic baths, trays etc, metallic wastes, and the cpc's of all equipment within the bathroom. Don't forget, all clamps must be made off with an unbroken conductor, or ring crimp terminals.

These clamps need to be accessible, or alternatively, you can sweat the conductors directly onto the pipes.

Don't think I have forgotten anything....
securespark said:
Don't think I have forgotten anything....

If you are are upgrading your tails you will also need to make sure your main earthing conductor is of suitable CSA for the increased CSA of the tails.

Can't think of anything else :D
Thanks RF & securespark also,
the consumer fuse has 100 amps written on it (as well as Series 5 type 11b, 415 volt) I take the point about the multi wire having the smaller area, obvious when you think about it.... Looks like TN-C-S, the earth wire goes to an open block right next to the supply neutral, the block looks like it runs through the plastic to the neutral, but there are no labels etc. not TT anyway
Will I have to pay for the meter? it's the orignal one, so maybe they should be changing it anyway. (A slower one would be great!). Could they put in an isolator, before the meter, so any future work doesnt need the fuse pulled?
If I get the supply upgraded, then I could get a shower csu put in next to the meter,leaving the rest (as my idea first posted)?
So a bit needing doing then... I wondered why all the street lights dimmed whenever I plugged in the new 3 kW Morphy Richards kettle. :eek:

cheers Jim
I might be wrong but I think you are not allowed to mount a C.U. actually within the meter cabinet itself, but you could mount it on the reverse of the wall. I think you are allowed a henley block and a D.P. isolator or a switch fuse (if feeding a submains) also it is often quite limited for space in a meter cabinet. If you fitted the new shower C.U. at the meter position, then at a later date decide to upgrade you wylex C.U. you would still end up with two C.U.s or have to re-route the cable to the position of the existing C.U.

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