What contactor size for a 9kw electric boiler

Brendo · Jun 1, 2018

Hi all.
Don't have much experience on this topic.

Called to a job with an electric boiler. It has a dedicated 10mm supply from a 50A MCB. I narrowed down the fault the the contactor which was only feedig 1 of tge 3 elements and had scorching marks around the other 2 terminals. And is currently a 20A. The boiler has 3 x 3kw elements.
My query is surely the 20A is insufficient for the 9kw load from the elements and shouldn't I fit a 40A contactor in it's place.

Thanks

elsparko · Jun 1, 2018

ohms law is your buddy.

Darkwood · Jun 1, 2018

Firstly is this 1ph or 3ph and if the latter is this 9Kw per phase or 3Kw a phase would be my first few questions, have you clamped the load to see what 1 element pulls if you are in doubt?
I guess 1ph but that would rely on the fact the installer got it right and we should never assume in this game.

Brendo · Jun 1, 2018

darkwood said:
Firstly is this 1ph or 3ph and if the latter is this 9Kw per phase or 3Kw a phase would be my first few questions, have you clamped the load to see what 1 element pulls if you are in doubt?

It's a single phase supply. I've clamped the 1 load which is working at 13.68A.

elsparko · Jun 1, 2018

a contactor is just a switch essentially, rate it correctly for the loads and thats about all there is to it

Darkwood · Jun 1, 2018

Your contactor is 20amp per pole so it is sufficient for a resistive load a 3kw element per pole, the problem is likely to be poor termination, this is a high current load, have the cable ends been suitable dressed with rated crimps or lugs?
What is the brand of contactor - is it a recognised reliable brand or cheap import.

Brendo · Jun 1, 2018

darkwood said:
Firstly is this 1ph or 3ph and if the latter is this 9Kw per phase or 3Kw a phase would be my first few questions, have you clamped the load to see what 1 element pulls if you are in doubt?
I guess 1ph but that would rely on the fact the installer got it right and we should never assume in this game.

There is usually an electronic control board which is installed with his system but it's blew in the past and someone has hard wired it up with this 20A contactor and a 16A timer

Brendo · Jun 1, 2018

darkwood said:
Your contactor is 20amp per pole so it is sufficient for a resistive load a 3kw element per pole, the problem is likely to be poor termination, this is a high current load, have the cable ends been suitable dressed with rated crimps or lugs?
What is the brand of contactor - is it a recognised reliable brand or cheap import.

I've attached a photo of the contactor which is a Chint NC1-0910

[ElectriciansForums.net] What contactor size for a 9kw electric boiler

elsparko · Jun 1, 2018

Brendo said:
I've attached a photo of the contactor which is a Chint NC1-0910

View attachment 42565

chint...and where is the housing etc? that looks a right mess lol, i know you didnt install it

elsparko · Jun 1, 2018

i wouldnt be surprised if there is "undue strain" on those connections if its just left hanging like that in a cupboard

Strima · Jun 1, 2018

Brendo said:
I've attached a photo of the contactor which is a Chint NC1-0910

View attachment 42565

Erm, that's a bit of a mess. Is the contactor just hanging on the cables?

Brendo · Jun 1, 2018

elsparko said:
chint...and where is the housing etc? that looks a right mess lol, i know you didnt install it

No I would never install this.
The black box/housing above the contactor usually would contain the main pc control board with digital display. Someone just removed that and fitted.this instead

Darkwood · Jun 1, 2018

Looks like the plumbers done that, either explain you will have to upgrade it or walk, that is an appalling installation.
The cheap contactor brand says a lot about the installer too.

Brendo · Jun 1, 2018

Strima said:
Erm, that's a bit of a mess. Is the contactor just hanging on the cables?

It was stuffed into the housing and closed up so wasn't hanging. I just took it out to get a clear photo of it

elsparko · Jun 1, 2018

Brendo said:
No I would never install this.
The black box/housing above the contactor usually would contain the main pc control board with digital display. Someone just removed that and fitted.this instead

if i was you i wouldnt try and improve upon this install without starting from fresh

id say the "fault" is down to poor installation than the boiler or contactors themselves