UK ariston water heater install

[johnmann91]

hi ,ive been asked to install an ariston tankless water heater ,the manual states its 9.5kw at 240v with min cable size of 6mm and that it will pull 39.6 amps .the cable run is 17 meters partly enclosed in an under concrete floor pipe along with 3 other 2.5mm cable runs and the rest of the run back to the board is clipped direct ,there is currently no mcb available to use and no rcd protection either .
my questions are > is 6mm good enough
can i use a 40A mcb with no rcd
should i install a seperate CU which would have 63A rcd and 40A mcb
many thanks for any advice
john

 

[loz2754]

The pedant in me needs to point out that Uo is 230, not 240....... (although you're better off using 240-243 for real-life purposes!)

Indeed. I was also going to point this out. Using 230 volts results in 41.3 amps, so a 45 or 50 amp OCPD would be needed.

 

[Rockingit]

Indeed. I was also going to point this out. Using 230 volts results in 41.3 amps, so a 45 or 50 amp OCPD would be needed.

Which in itself raises an interesting question...... should we actually ignore the 'official' nominal voltage when doing cable calcs and instead use what we all know is realistic in order to err on the side of caution??

 

[MJPD29]

John with all due respect should you not be capable of designing this circuit yourself if you are carrying out the work?

 

[loz2754]

John with all due respect should you not be capable of designing this circuit yourself if you are carrying out the work?

This I was also thinking

 

[johnmann91]

thankyou all for the advice ,i will be running 10mm T&E from a seperate CU which will have a 2 pole 63A 30mA RCD device and a 50A MCB Type B fed from a seprerate set of 16mm tails from the henley block

 

[brianmoooore]

The manual said 9.5kw at 240v, drawing 39.6amps but worth remembering that it only draws 39.6 amps if you have an exactly 240v supply.
As a silly and extreme example if you actually had the UK minimum permitted voltage (230v - 6%) and had 5v further volt drop over 17m of 6mm T+E it's now pulling 45 amps. Unlikely of course.

10mm csa cable would permit a higher value MCB and suffer less volt drop, and as above be more suitable in the first place.

Indeed. I was also going to point this out. Using 230 volts results in 41.3 amps, so a 45 or 50 amp OCPD would be needed.

I've just read this thread for the first time, and then read it again. Can't believe that no one picked up on the fundamental error in the first quote, and then found it repeated in the second!

The manufacturer's spec. for this heater is 9.5kW at 240V. I calculate that as a resistance of 6.06 ohms.
At 230V - 6% -5V (=211.2V) the heater will draw 34.9A NOT 45A, and at 230V it will draw 37.9A NOT 41.3A
It's the resistance of the element that's fixed, not the heating power in kW.
To draw 45A, the voltage at the heater's terminals would need to be 273V

 

[loz2754]

Oops. Schoolboy error on my part...
@brianmoooore you are of course correct.

 

[Dartlec]

I've just read this thread for the first time, and then read it again. Can't believe that no one picked up on the fundamental error in the first quote, and then found it repeated in the second!

The manufacturer's spec. for this heater is 9.5kW at 240V. I calculate that as a resistance of 6.06 ohms.
At 230V - 6% -5V (=211.2V) the heater will draw 34.9A NOT 45A, and at 230V it will draw 37.9A NOT 41.3A
It's the resistance of the element that's fixed, not the heating power in kW.
To draw 45A, the voltage at the heater's terminals would need to be 273V

Isn't it about time that manufacturers were made to quote their power rating at 230V? Or at least all standardised. Not like they are using 240V elsewhere since we "changed" are they?

I know it means they can give a bigger number on their box, but does anyone actually buy based on that anyway? Half the time the sites selling it don't even give the figure anyway.

 

[timhoward]

@brianmoooore thanks - seems so obvious! Stupid mistake on my part.

 

[Jim_e_Jib]

The manual said 9.5kw at 240v, drawing 39.6amps but worth remembering that it only draws 39.6 amps if you have an exactly 240v supply.
As a silly and extreme example if you actually had the UK minimum permitted voltage (230v - 6%) and had 5v further volt drop over 17m of 6mm T+E it's now pulling 45 amps. Unlikely of course.

10mm csa cable would permit a higher value MCB and suffer less volt drop, and as above be more suitable in the first place.

If the supply voltage goes down, the current will go down rather than up - the heating element is a resistive load.

Edit: well and truly beaten to it!

 

[timhoward]

If the supply voltage goes down, the current will go down rather than up - the heating element is a resistive load.

Edit: beaten to it!

Indeed and the stupid thing is that when writing that I was thinking about a commercial dishwasher on a site prone to over voltages drawing more than 13 amps and yet still managed to write a load of rubbish...
Oh well I doubt I’ll do that one again.

 

[johnmann91]

well thats another spanner in the works then !!

 

[brianmoooore]

well thats another spanner in the works then !!

Not at all. Makes things easier for you.
Manufacturer says 9.5kW at 240V, but the declared voltage in the UK is 230V, and this is what is used for design. Your heater is 8.73kW, not 9.5kW, in spite of what it says on the box, so the current you design for is approx. 38A

 

[johnmann91]

Not at all. Makes things easier for you.
Manufacturer says 9.5kW at 240V, but the declared voltage in the UK is 230V, and this is what is used for design. Your heater is 8.73kW, not 9.5kW, in spite of what it says on the box, so the current you design for is approx. 38A

ahhh theres my brain fart then ..of course yes ,so i can use a 40A with a 63A rcd