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

 

[brianmoooore]

Every shower I've ever dealt with has had the power quoted at 240V on the box, sometimes with the 230V power in smaller writing underneath.
The reason is obvious - a 10.3kW shower looks better value than an equally priced 9.5kW one.
When we fit these, we should be using 230V for our calculations, so we really should first calculate the power at 230V, if not stated in the instructions.

 

[DefyG]

I think many fall into the trap of thinking a 10.5kW shower will give them a better shower experience than a 9.5k one when if the water pressure is towards the low side it wont make any difference.

The one constant we need to work out the power is resistance of the element, but this is never quoted and as the voltage quoted at 240v (#6) the nominal value (IET Regs) 230v it could be anything but!
During EICR and general testing I've found voltages from 224 upto 252v a 28v difference which will clearley affect the 'actual' power (kW) of the shower.

That said in designing a shower circuit using 230 or 240 volts probably wont make much difference between choosing a 6mm or 10mm cable!