Sizing of a cable for a shower.

haf1963 · 2025-03-15T09:46:36+0000

Hi, am looking to upgrade my electrical shower from 9.5 to 10.5kw and am unsure if my cable is upto the job as its 6mm attached to a 40A fuse. The length of the run from fusebox to shower is approx 12-15m so I am trying to check if its a viable project as I have no desire to upgrade the cable to 10mm given the route.
If there is a formula for this calculation then that would be a big help.
Thanks

buzzlightyear · 2025-03-15T11:22:06+0000

The 10.5kw shower would take 45Amp.
So your cable that is installed. Will not cut it, so your 10.5 shower cable needs upgrading, plus mcb.

DefyG · 2025-03-15T13:44:19+0000

What do the manufacturers instructions say? I would expect they would say 10mm cable as 10.5kW (45.6Amps) and the instructions would/should override any calculations.

I really depends on how the cable is installed - for example from the IET Wiring Regulations, if run in insulated stud wall its current carrying capacity (CCC) could be as low as 27.5A . Enclosed in a plastered wall - 32A so you see way below what a 10.5kW shower would draw.