10mm or 6mm, RCBO or MCB

[r.bob]

New 8.5kW shower, about 18m from CU. Cable mostly surface mounted.

I know that 10mm cable would be a sound bet, but it's expensive and more tricky to work with. Is 6mm safe and acceptable?

The CU was new this year, therefore up to date. Is a slot in 40A MCB safe and acceptable or should a RCBO be fitted?

Would appreciate your advice, as I can't find a definitive answer via Google! Thanks.

 

[ricicle]

Cable mostly surface mounted. .

Where else will the cable run?

The CU was new this year

There should be a 30mA RCD already covering some of the circuits which can be used for the shower which means an MCB is ok.If RCBOs have been use then another is the order of the day.

This work is notifiable.

 

[r.bob]

Thanks. The cable will also run through two single brick walls and possibly about 2m of plastic sheath between shower and isolating switch.

 

[ricicle]

6mm² should be fine as long as it is nowhere near any thermal insulation or grouped with other circuits otherwise more calcs are necessary

As a footnote though it might be worth the trouble of running in 10mm² as this will enable a greater rated shower in the future when you get fed up of the dribble from 8.5kw

 

[r.bob]

Thanks ricicle...I assume you are not the one that goes POP!?

Are 8.5kW showers really that bad? Or put it another way, how much better are 9.5 or 10.5kW? Cheers.

 

[ricicle]

Thanks ricicle...I assume you are not the one that goes POP!?

Are 8.5kW showers really that bad? Or put it another way, how much better are 9.5 or 10.5kW? Cheers.

It depends how you like your showers, with a higher rated one you will get a faster flow of water for the same temperature as a lower one due to the fact that the water has to spend less time in the heater.

 

[Cremeegg]

Always go for the bigger size - 6mm maybe £2 a metre 10mm maybe £3 a metre. In the average set up a difference of maybe a tenner. I'd go straight for 10.5kW - yes you do notice the difference upgrading from 8.5kW especially in the winter when the incoming supply is that much colder. Just wait for one of the heating cans to go in the 10.5kW and you'll really notice the difference - they never seem to last that long.

 

[Pensdown]

As rcicle has pointed out, 8.5kW should be fine in the summer but in the winter when the mains water is colder you may have to dance around a bit to get wet.