Another shower cable question. 6mm or external upgrade?



We removed an electric shower a year or so ago (don't know kW of it, long since binned) from an en-suite (now removed) that was in bedroom on the back of the house.
The circuit is still in, but not connected. The circuit is 6mm cable, and has a (currently disconnected) 32A MCB at the consumer unit. From this i'm assuming the old shower was 7 or 7.5kW. There was no RCD incidentally.

We're now doing the bathroom and want an electric shower in there. Problem is our water pressure is erratic and at peak times the old en-suite shower wouldn't work due to low pressure. Solution, triton or mira pumped electric shower fed from cold water tank. Problem - the mira is 9.8kW, and the smallest Triton do is 8.5kW.

The cable run, if i extended the exsiting circuit from where it is currently terminated, to the bathroom, would be approx 18.5 - 19 metres worst case. Routing from CU is between walls, behind skirting, and up to first floor against external wall boxed in behind plasterboard, then across under floorboards to the current point where it terminates below the old shower - im hopefull I can intercept it en-route under the bathroom (not yet removed old bath etc so can't get floorboards up yet to confirm), and knock some 6m off the run. But worst case is i'll have to extend from the existing termination point, meaning my run is say 20m in 6mm. I'd plan on adding 30mA RCD. All the reading i've done on here suggest 10mm is more suitable, but i've also read various places 6mm should be acceptable for 8.5kW.

The Triton 8.5kW pumped tech page says nominal power rating of 8.5kW at 240V and 7.9kW at 230V, and suggests a 40A MCB.
8.5kW at 240 would give me 35.42A which would seem within the limits of the 6mm. But frankly i don't want to take any risks. Whats the general view on this ?

Re-wiring internally, is not really practical, or more to the point cost prohibitive - all the rooms the wiring runs through behind walls etc are newly decorated, floors and walls tiled etc etc - the glorious benefit of hindsight eh.

I've another option if re-wiring is strongly recommended - CU is on an external wall. Can I run a new circuit out through the wall, then externally up to the loft space, back in and down into the bathroom, obviosuly avoding insulation in the loft etc ? Do regs allow this ?
Cable run would then be approx. 10m.
If this is possible, would i be as well going for 16mm giving me the option of higher kW shower now or in future.

Sorry i've waffled on a bit, but thanks for any advice, suggestions.


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i'm no electrician but when i installed my 10.6kW shower, my brother insisted on using 10mm t&e cable and the length of my cable was 14 meters he told me to play safe. i'm not to sure about running cable on the exterior as i'm sure 10mm t&e has not been treated with any kind of uva/uvb protection as well as the effects of cold tempratures, you may find by doing an external job you will limit the usable life of the cable. I work with high speed fibre optic networks and we are limited on what cables we can run outside I also have a fibre optic computer network running between computers at home and 1 of the cables is run outside I noticed just before christmas when I was replacing some of the guttering that it had started to go brittle i'm sure the same is for electrical cable as well.
split concentric may be an idea

its suitable for use externally and you don't have the hassle of needing glands like with SWA

it is quite tricky to get hold of though
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never used the stuff myself so i cant give personal opinions on the stuff

only thing i can think of that might be an issue with a shower cuircuit is getting all the neutrals into the neutral terminal.
The other problem you may face with internal rewiring, unless you're used to the stuff and have taken it into account, that 10mm², and 16mm² even more so, might not run where you think it will because of bends.

External sounds like a good answer, with SWA or S-C. If you use XLPE SWA it has a higher current rating than ordinary, which can help with the size (e.g. 6mm² is good for 60A).

Or just use ordinary PVC singles in conduit outside - a lot less hassle than either S-C or SWA. If you use flexible conduit at either end to assist with the routing if necessary, you won't need to mess around joining it to T/E, you can use singles all the way from CU to shower...
another possibility would be 10mm DI tails in conduit

advantage is you don't need to bother with stuff like flexible conduit. you can just come out of the conduit once you get back indoors.
ban-all-sheds said:

yeah and whats wrong with that its an earth ffs

i cant see how tails and single earth running together are any less protected than T&E.

and a hell of a lot easier to thread through conduit for the outside parts and joist holes for the inside parts than T&E
Yeh - guess you're right - I've never used a mix of DI phase & SI cpc, so I've just got this thing in my head about unprotected singles.
Just bite the bullet..

10mm T&E wired straight up into attic and run it across. You can buy it by the meter from Newy & Ere and CEF..think its 90p per mtr off the roll. Use a 45amp mcb on the rcb side of the board.

Have fun trying to get it into the pull switch, and into the neutral block. On the new boards the 1st few neutral terminals are bigger than the rest.

Why use conduit when you would have to use the 25mm minimum to fit it down, then it looks sightly.

Either reduce the shower size...or crack on..
ban-all-sheds said:
Yeh - guess you're right - I've never used a mix of DI phase & SI cpc, so I've just got this thing in my head about unprotected singles.

surely thats what you have connecting your service head meter and CU

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