Shower question

[Canny]

I need to replace a shower of unknown wattage, the cable is 6mm T&E and has the following approximate run

2m behind plaster
3-4m loose between ceiling and upstairs floorboards
1.5m behind plaster to switch
1.5m behind plaster to loft space
2-3m clipped to upper surface of roof joists (passes over insulation not under)
1m behind plaster down to shower

The question is what should this cable run be treated as and (if someone wants to save me finding the formulae to work it out) what limit do I have on the shower rating without replacing the cable with 10mm.

 

[mapj1]

From the current rating point of view, the most thermally insulated part is that plastered into the wall. So, cable selector, 32A max
by changing for 10mm, in plaster 43 A

I have reproduced the key table extracted below.

Conductor size (mm2) vs Current rating table (amps)

A) Enclosed in a wall
B) Enclosed in conduit
C) Clipped to a surface
D) In Free Air


mm-----A-----B------C-------D
-1------11---- 13---- 15------17
-1.5 ---14 ----16.5 --19.5 ---22
-2.5 ---18.5 --23 ----27----- 30
-4 -----25 ----30 ----36 -----40
-6 -----32 ----38---- 46 -----51
-10-----43-----52---- 63 -----70

Unless the total is more than 10 to 20m then you don't need to worry about voltage drop, and in any case the 4% in the regs is recommended as suitable for situations where the maker doesn't specify something else. However, most modern CE mark showers are specified to work fine on 220V too, and at worst are only a little bit cooler. Cable heating is a much more important consideration, and the rating of the switch.
32A is 7.5kW approx.
cheers M.

 

[Canny]

Thanks, looks like replacing it with 10mm is the best option then.