shower 6mm or 10mm cable?

avoidtheupsidedownbottles

grumbler said:

Resistance of 20m  of 6mm cable is just 0.13 Ohm 

So about 9 watts per metre?  Not insignificant so I can see why the insulation situation is relevant.

FreeBear

avoidtheupsidedownbottles said:

FreeBear said:

A useful table for you - https://www.tlc-direct.co.uk/Figures/Tab4.7.htm

I wonder why voltage drop is stated as being lower for 3 or 4 cores compared to 2 core  

Simples. 3 or 4 core will be used for a three phase circuit. The current would be shared over the three/four conductors, so more copper to carry the same current, therefor a lower resistance.

sgun

Just an update - and yes, of course I am getting a qualified person to fit, this was just so I knew what to buy/research for my shower choices (my old house actually did have a 3 phase circuit so much for me to learn here as the cables here are quite different from there).
There were no markings on the cable apart from the manufacturer marks, AIE. So I phoned them and they confirmed that a long time ago, perhaps 20+ years, they did in fact manufacture a 6mm cable with a stranded earth core and advised that the cabling would probably be much older than 2010 when the shower was put in. So I'm going with the 8.5kw shower. The cable doesn't appear to run under the insulation, it comes out into the attic from the cavity and then back down into the shower. Our walls aren't insulated so I'm happy to run another 8.5kw shower from it. Just a shame the shower broke before I was getting an electrician in for some small jobs as I would have had that cabling redone too. Husband can't wait any longer for a shower though, its been broken since 1 day after lockdown. Thanks for all the input, I love a good electrical discussion!

Risteard

avoidtheupsidedownbottles said:

FreeBear said:

A useful table for you - https://www.tlc-direct.co.uk/Figures/Tab4.7.htm

I wonder why voltage drop is stated as being lower for 3 or 4 cores compared to 2 core  

That's for a three-phase installation. But essentially it's due to the increased heat dissipation between the conductors and reduced losses.

nofoollikeold

The risk is primarily the heating of the cable.
Using Grumbler's figures above:
1. Heating of 6 mm cable = I x I X R = 39 x 39 x .15 = 198 watts (approx)
2. Heating of 10 mm cable = 39 x 39 x -076 = 116 watts (approx).
Those figures are the heat losses in the cable, and that for 6 mm cable approaches double that of 10 mm cable.  If that heat is substantially under insulation and unable to dissipate it will raise the temperature of the insulation.  In the right (wrong) circumstances could be enough to start a fire.