Sizing of a cable for a shower.

[haf1963]

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]

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]

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.

 

[haf1963]

Thanks for the feedback and I will stick with the existing 9.5Kw - seems way to much work to redo everything for 1 extra kw.

 

[Teddy Lop]

Careful when swapping out shower heaters of this capacity on 40a devices, check the voltage at the shower terminals.

A 9.5kW 240v heater will draw 39.5a.
A 9.5kW 230v heater will draw 41.3a

But.

A 9.5kW 230v heater connected to a 240v supply will draw 43a, and is in fact operating as a 10.3kW heater.

Usually it seems to pass unnoticed as most people take 5-10 minute showers and the 40a device will tolerate that on its slow-to-act thermal overload, but occasionally you'll get a plumber wapping' in a "like4like" and get confused when it trips. the safer option is go for a 9kW, which in the above over- volts scenario is still 9.8kW/41a