6mm vs 10mm cable for electric shower

[Corgigrouch]

I "once" ran a 6mm cable 21 meters, which I was told by an electrician who worked it out on his calculator was the max for an 8.5Kw shower and ok, never again, the cable got shiny and rather hot after 20 minutes of use

Norman PVC cable is designed to run at temperatures at up to 70 centigrade which by my estimation is unnervingly hot..... But within it's design criteria. I have seen electric hobs connected to the ring with 2.5mm Now that was black and crispy

 

[skenk]

But the 10.8Kw shower should be fused/protected at 50A.

And then the protection is beyond the current capacity of the cable, and that is without any external factors applied.

I agree it is borderline, but in the wrong direction. Strictly speaking, the job can''t be done.

Shower power (kw) is normally advertised at 240v so 10.8w = 45A, just possible on a 45A MCB

 

[algas]

Try to get the same shower as you already have no need to alter wiring, plumbing, tiling etc & flog the 10.5 on fleabay!