12KW three phase immersion heater

[Harmonise82]

Hi all, good afternoon.
New on here, so here goes!

Quick question and some advice really!

I have a 12KW three phase immersion heater installed into a hot water cylinder (for commercial premises).
The immersion heater enables and disables via an on site building management system so therefore the immersion heater enables via a Schneider (3ph) ICT 25A 400v contactor. Complete with 230v coil.

The building in question (their maintenance man) is explaining to me that he thinks the contactor is 'under rated' as the 12KW immersion heater will be drawing more load?? Also he seems to think the contactor is changed every 12 months or so??

above the contactor there is a Schneider (3ph) C60HB B20 415v MCB installed.

What kind of load 'potentially' could a 12KW three phase immersion draw? Is it simple ohms law?

12000 watts (12KW) / 400v = 30 amps or is there (back to my college days) a route3 equation in there somewhere + a power factor? I feel a clip on clamp meter coming on soon!

Feedback appreciated

Thanks

 

[Harmonise82]

Or, thinking about it (after getting the text books out) is it -

Amps = 12000 / (1.73x400) = 17.34amps

Is this correct for a 12KW immersion heater?

 

[davesparks]

Right equation, wrong voltage I think.
Check the rating of the immersion heater, it's almost certainly going to be rated 12KW at 415V not 400V

Do you know why the contactor is being regularly replaced? Is it the coil burning out or the actual contacts at fault?

 

[Richard Burns]

The question of why the contactor is being changed is very relevant.

From a design point of view the contactor and circuit breaker are rated OK for the load and 16-17 A is about right for the load.
I cannot think of a contactor type that should not be able to handle a pure resistive load, though the contacts could be wearing out if it is switched on and off a lot.

 

[Harmonise82]

Rich/Dave many thanks.

I will confirm with Santon the immersion heater people to see if its 12KW at 415v or 400v.

Also you make a good point regards the coil burning out and not the contacts - When I attend I will check this.

Could you work it the other way also? Just curious! so -

12÷3=4

12kw ÷ by 3 phases = 4kw. 4kw is 4000 watts

Amps = 4000/(1.73x415)=5.57amps per phase

5.57 x 3 = 16.71 Total?

 

[Deleted member 72666]

Have you checked to see if the heaters are star or delta connected?

 

[snowhead]

Rich/Dave many thanks.

I will confirm with Santon the immersion heater people to see if its 12KW at 415v or 400v.

Also you make a good point regards the coil burning out and not the contacts - When I attend I will check this.

Could you work it the other way also? Just curious! so -

12÷3=4

12kw ÷ by 3 phases = 4kw. 4kw is 4000 watts

Amps = 4000/(1.73x415)=5.57amps per phase

5.57 x 3 = 16.71 Total?

Nope, approx;
4000 / 240 = still 17amp per phase.

Typical real world clamped current on a 12kw is 16.5 amp per phase, I've checked quite a few over the years.

And they'll be Star connected and 415v rated

 

[sinewove]

Nope, approx;
4000 / 240 = still 17amp per phase.

Typical real world clamped current on a 12kw is 16.5 amp per phase, I've checked quite a few over the years.

And they'll be Star connected and 415v rated

Would they not be 240v rated elements if connected in star?
sv

 

[snowhead]

Would they not be 240v rated elements if connected in star?
sv

Well spotted, that's why I used 240 in the calculation.

 

[Knobhead]

Would they not be 240v rated elements if connected in star?
sv

Because you can get 115V, 240V, 380V, 415V, 440V, 550V and 660V elements. They’re just the ones I’ve come across over the years.

Never assume anything, check the rating plate and manufacturer’s destructions.

I was called to a product tank heater that reportedly wasn’t working. It was supposed to be 30KW, the product was turning to sludge because it was cold. Whoever connected the heater assumed it was star and connected it as such, it was only giving 9.2KW.

 

[Besoeker]

Have you checked to see if the heaters are star or delta connected?

In either case, it's the 12kW that determines the line current.
The three-phase voltage in this country is 400V - at least nominally.
Harmonisation is wonderful...................

 

[telectrix]

it's 400 because the frogs can't do sums with numbers that don't end in 00.

 

[Besoeker]

it's 400 because the frogs can't do sums with numbers that don't end in 00.

It's a fudge. And not a very good one at that.

 

[Knobhead]

In either case, it's the 12kW that determines the line current.
The three-phase voltage in this country is 400V - at least nominally.
Harmonisation is wonderful...................

That’s strange, I recall a conversation on a site outside the UK where you backed me on UK voltage levels ;-)

The IET and EU forgot to tell UKPN, if they had they would get a resounding **** ***!

It’s not feasible and never will be to change from 433/250V. What annoys me are calculations based on a purely fictitious value. They are inherently 7.6% inaccurate.

I’m just waiting for the Belgians to step in with 380/220V as the new harmonised voltage.

 

[Besoeker]

That’s strange, I recall a conversation on a site outside the UK where you backed me on UK voltage levels ;-)

Which part of "at least nominally" did you miss?