Cable size for 9kW shower

[Dickiem]

Hi,

Same project as detailed in my other post (I want to run shower cables to the CU whilst the kitchen ceiling is disturbed).

I'm going to install a 9kW shower, cable length will be 12m max from CU to shower switch. Cables will run in the cavity between between floors (between floor joists) then up into the loft through a stud wall.

The various online calculators show 10mm cable is required, however, when Googling 'required cable size for 9kW shower', the results pretty much agree on "showers up to 9kW can carry 6mm cable as long as the shower is within 18m of the fuseboard", anyone able to confirm if 10mm is a bit overkill or that 6mm is a bit under rated please

 

[SJD]

Irrespective of whether you can get away with 6mm2 or not, why would you not just run 10mm2 and you can then be reassured if you decide to uprate the shower to 10.5kW some day, you won't need to rewire it? The cost saving of using 6mm2 instead of 10mm2 for a 12m run I'd estimate at no more than £20 and peanuts in the overall scheme of a refurb.

 

[Dickiem]

Irrespective of whether you can get away with 6mm2 or not, why would you not just run 10mm2 and you can then be reassured if you decide to uprate the shower to 10.5kW some day, you won't need to rewire it? The cost saving of using 6mm2 instead of 10mm2 for a 12m run I'd estimate at no more than £20 and peanuts in the overall scheme of a refurb.

Hi thanks for you reply and fair point. The showers will be wired via a Garo unit so limited to 9kW each. I guess I was mainly wondering why the difference between calculated size (10mm) and 'specified' size (6mm)

 

[littlespark]

It’s not just the length of cable that can sway the decision between 6 or 10mm
If the cable is run through insulation, then that can derate the current carrying capacity quite a bit.

10 would be preferable, so I would advise a wall switch in a deep 2 gang box for an isolator rather than a pull cord. Much easier to work on.

 

[Dickiem]

It’s not just the length of cable that can sway the decision between 6 or 10mm
If the cable is run through insulation, then that can derate the current carrying capacity quite a bit.

10 would be preferable, so I would advise a wall switch in a deep 2 gang box for an isolator rather than a pull cord. Much easier to work on.

Thanks for the explanation, appreciated. I understood the effect of insulated cavities but the available ‘environments’ on their selection list a don’t seem to include things like dot n dab walls, buried in plaster etc. I guess they’re the equivalent as some of the others they list but it’s unclear which. I’ll definitely use 10mm tho given the advice received.

 

[brianmoooore]

You say it will go into the loft, so will almost certainly encounter insulation there.