Replace electric shower - what permitted wattage?

[RrogerD]

I am aware that I have to have the correct size of supply cable for an electric shower, that they draw a lot of power. What are the specific cable sizes for each kw of supply?

Am I correct in that, if I currently have a 7.5kw shower then I can go up to 10kw before needing to upgrade the wiring and obvs the supply fuse? Assumed 10mm cable currently.

I've found this:
https://www.commercialwashroomsltd....s/what-size-cable-for-an-electric-shower.html

 

[EFLImpudence]

You cannot assume the cable size.


You divide the shower's stated Wattage by the stated Voltage to determine the Amperage (current).

E.g. 7500W / 240V = 31.25A
If the voltage rises, the Wattage and Amperage will rise; you do not just divide by the larger number.

T&E cable clipped to the surface or buried in masonry has the following current capacity - less if installed in material which prevents cooling.
2.5mm² - 27A
4mm² - 37A
6mm² - 47A
10mm² - 64A

 

[RrogerD]

You cannot assume the cable size.


You divide the shower's stated Wattage by the stated Voltage to determine the Amperage (current).

E.g. 7500W / 240V = 31.25A
If the voltage rises, the Wattage and Amperage will rise; you do not just divide by the larger number.

T&E cable clipped to the surface or buried in masonry has the following current capacity - less if installed in material which prevents cooling.
2.5mm² - 27A
4mm² - 37A
6mm² - 47A
10mm² - 64A

Presumably, I can go up into the loft and check the size of the cable going down into the shower and, as long as I do not go above the permitted wattage for that cable size then I'm good to go?

 

[Adam_151]

What are the specific cable sizes for each kw of supply?

There are no specific sizes, any table you find will be making various assumptions and you need ensure that they are valid in your case. The nominal current for a given loading is equal to I = P/V, you then need to pick a cable that has a nominal rating in excess of that, for the installation method in use, taking into account any further factors that have to be applied for grouping, ambient temperature, or thermal insulation. Then volt drop and disconnection times would need to be checked that they are in spec.

That said, generally you can make 8.5kw comply on a 6mm^2 cable in most domestic installations (that doesn't mean it will always, and it does require care on the instalaltion method)

Am I correct in that, if I currently have a 7.5kw shower then I can go up to 10kw before needing to upgrade the wiring and obvs the supply fuse? Assumed 10mm cable currently.

Another generalisation is that 10mm should be sufficient for most domestic showers, and if it is not, then generally you have got something else wrong in your design and should address that if possible rather than going upto 16mm (never say never thouugh). It would be unusual to find a 7.5kw shower on 10mm cable though, so fiver says it is smaller than this

 

[RrogerD]

Thanks both.

I've just checked and it is 10mm cable. So, I guess I'm ok up to 10kw. I suspect the current shower is 9.5kw, although it's pretty cr*p tbh. Any way of knowing before I take the front off?

 

[conny]

There may be a label underneath or on top of the shower unit giving manufacturer, model type, serial number, voltage etc

 

[DetlefSchmitz]

Actually the size of fuse or MCB powering this shower is at least as important. Haven't seen that mentioned.

 

[endecotp]

Presumably, I can go up into the loft and check the size of the cable going down into the shower

I've just checked and it is 10mm cable.

Did you find that printed / embossed on the cable sheath in the loft? Or did you measure it?

 

[flameport]

I suspect the current shower is 9.5kw, although it's pretty cr*p tbh

So are all electric showers.

The difference in water flow between 9.5 and 10.5 is minimal, so if you are expecting a dramatically improved shower you will be disappointed.

Other forms of water heating are significantly better. Do you have a combination boiler or a hot water cylinder?