UK ariston water heater install

johnmann91 · Apr 22, 2021

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

MJPD29 · Apr 22, 2021

Judging from the details you have provided I would say 10mm would be more suited rather than 6mm. I would fit an RCD also.

timhoward · Apr 22, 2021

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.

Dartlec · Apr 22, 2021

+1 for 10mm in a case like this.

Even if 6mm would do, it leaves no future options for replacement/upgrades. If a new floor has gone down and they need to replace the heater in 10 years they'll be thankful they paid the very slight extra for 10mm.

It might mean a larger conduit too of course, but still worth it when there's any doubt.

johnmann91 · Apr 22, 2021

Matthewd29 said:
Judging from the details you have provided I would say 10mm would be more suited rather than 6mm. I would fit an RCD also.

thanks for that ,i had thought that myself

johnmann91 · Apr 22, 2021

Dartlec said:
+1 for 10mm in a case like this.

Even if 6mm would do, it leaves no future options for replacement/upgrades. If a new floor has gone down and they need to replace the heater in 10 years they'll be thankful they paid the very slight extra for 10mm.

It might mean a larger conduit too of course, but still worth it when there's any doubt.

thankyou for the reply ,i had thought 10mm was probably better option ,the floor is not new and is concrete the pipework it will be run in was put in years ago by the aircon guys to get from the cellar to first floor its actually a 6" pipe and theres no chance of potential upgrades or changes to the heater in future ,all i have to do now is find a suitable mcb/rcd combination to install .FYI i can run another set of tails from the henley block to a seperate cu

johnmann91 · Apr 22, 2021

timhoward said:
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.

thankyou for the reply ,i had thought 10mm was probably better option ,the floor is not new and is concrete the pipework it will be run in was put in years ago by the aircon guys to get from the cellar to first floor its actually a 6" pipe and theres no chance of potential upgrades or changes to the heater in future ,all i have to do now is find a suitable mcb/rcd combination to install .FYI i can run another set of tails from the henley block to a seperate cu

johnmann91 · Apr 22, 2021

Matthewd29 said:
Judging from the details you have provided I would say 10mm would be more suited rather than 6mm. I would fit an RCD also.

thankyou for the reply ,i had thought 10mm was probably better option ,the floor is not new and is concrete the pipework it will be run in was put in years ago by the aircon guys to get from the cellar to first floor its actually a 6" pipe and theres no chance of potential upgrades or changes to the heater in future ,all i have to do now is find a suitable mcb/rcd combination to install .FYI i can run another set of tails from the henley block to a seperate cu

Dartlec · Apr 22, 2021

johnmann91 said:
thankyou for the reply ,i had thought 10mm was probably better option ,the floor is not new and is concrete the pipework it will be run in was put in years ago by the aircon guys to get from the cellar to first floor its actually a 6" pipe and theres no chance of potential upgrades or changes to the heater in future ,all i have to do now is find a suitable mcb/rcd combination to install .FYI i can run another set of tails from the henley block to a seperate cu

If going with a separate CU then any of the "shower" units should do fine - 63A RCD and 40 or 45A MCB.

You'll want one with a good amount of space in it to nicely terminate 10mm, but the current metal ones do tend to have more space in now than they used to.

It might pay to get a 2 or 3 way one with blanks to give you the extra space (and the possibility of adding maybe a 6A or 16A in the future.

loz2754 · Apr 22, 2021

A couple of questions if I may...

What type of cable is it going to be run in?
How loaded are the other 3 cables?

littlespark · Apr 23, 2021

Severe case of de ja vu there.
Swear I read the same thing three times...

+1 for 10mm and rcd protection.

johnmann91 · Apr 23, 2021

loz2754 said:
A couple of questions if I may...

What type of cable is it going to be run in?
How loaded are the other 3 cables?

cable will be 10mm T&E.the other cable runs are 2.5mm T&E one of which has 2 double sockets on it ,one of which has microwave plugged in and the other has the washing machine and tumble dryer .and the other cable has a single socket doing nothing and a double rcd socket which will have a dishwasher plugged in.

johnmann91 · Apr 23, 2021

Dartlec said:
If going with a separate CU then any of the "shower" units should do fine - 63A RCD and 40 or 45A MCB.

You'll want one with a good amount of space in it to nicely terminate 10mm, but the current metal ones do tend to have more space in now than they used to.

It might pay to get a 2 or 3 way one with blanks to give you the extra space (and the possibility of adding maybe a 6A or 16A in the future.

thats good news ,ive checked and there is room in the henley block for another set of tails so i can fit a 63Arcd with a 40Amcb shower unit ,wont need expansion in future ,just as long as the 40A mcb will be enough otherwise ive got to find a 45 and it most likely wont be same make ,and i dont want to go up to a 50 for the sake of matching it all

loz2754 · Apr 23, 2021

johnmann91 said:
cable will be 10mm T&E.the other cable runs are 2.5mm T&E one of which has 2 double sockets on it ,one of which has microwave plugged in and the other has the washing machine and tumble dryer .and the other cable has a single socket doing nothing and a double rcd socket which will have a dishwasher plugged in.

Thanks.
I did a Procerts calculation using the info you gave, which points to 10mm minimum.

[ElectriciansForums.net] ariston water heater install

Rockingit · Apr 23, 2021

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!)

UK ariston water heater install

johnmann91

MJPD29

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timhoward

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Dartlec

johnmann91

johnmann91

johnmann91

johnmann91

Dartlec

loz2754

littlespark

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johnmann91

johnmann91

loz2754

Rockingit

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