I got a water heater which is rated 6kW. The "heating element" consists of 3 heating elements that are connected in series for 1-phase usage.

I would like to run two of those heating elements on one phase and the 3rd on a second phase to reduce the maximum load on a single phase (due to 3 x 25A total house connection). As the heater is rated for 230V, I cannot simply connect the heater to 3-phase electricity.

What kind of electronic module (diode?) can be safely used to switch the 3rd 2kW heating element when the first two are turned on and off? So that I can keep using the original control unit of the heater, but the 3rd heating unit runs on an independent cable + phase.

The installation will be done by a professional electrician, probably he will have his own simple solution, I don't know yet. I just want to have a suggestion for a cost effective solution on hand. Of course, for the electrician the most simple solution is to install a 40A 1-phase power line, but for me this solution is quite expensive, as in this case the main house connection has to be upgraded and a new main cable has to be installed = $$$.

EDIT/UPDATE: the 3 heating elements are connected in parallel, yet the heater is a 230V device, but it's controller has only 4-point input connector( L L N N ). Still I want one of the three heating elements to be connected to a separate phase to reduce the load on a single phase. But this third heating element should still operate according to the controller - so turn on and off when the controller turns on and off the first two elements.

Do I see it right, that I can simply connect two phases to the main input connection it says N N L L(see 2. picture top right corner)? Could this have some effect on the controlling unit?

Edit: Added a diagram from the manual, but don't believe what it says, because this same model is also available as 3-phase model, so I guess the diagram has not been edited for two version.

EDIT: So I want to go the 3phase way. I repositioned some of the cables according to the diagram for 3ph usage(see photo). However a few questions remain:

  1. Do I use the 3phase connection with a single neutral or is it safer to use 3 isolated phases with each having its own neutral?

  2. As this is not a motor, does it matter in which sequence the phases are connected?

  3. I wanted to identify which two heating element connectors belong to each other. I thought, I just measure the resistance and the pairs with resistance lower than infinity will be my pairs - BUT I measure a measurable small resistance between any two of the six heating elements connections. If I measure the resistance between any of the power input and any of the N connectors I get a resistance of approx 28 Ohms, but when I measure the resistance between two power input connectors the resistance is just around 50 Ohms, where I would have expected it to be infinity. Is this all right or is this device badly isolated?

  4. So what is the way to determine which two heating element connections belong together?

Edit: Looked through the manual - found this "choose the proper heater (220V or 380V). The heater and the board will be damaged if 380V were connected to the 220V unit". Is this sure nonsense or could it be that the board has and electronic design that does not allow 3-phase usage?

That brings me back to my original question, is there a ready to use module that lets current through only when on it's input current is applied? That way I could leave the unit with 230V and just connect the two other phases directly to the other two heating elements through such a switching device.

Edit: @Tyler brought contactors to my attention, this is exactly the kind of device I was looking for.

[Heater connections][Controller connection]2[Diagram]3[Backside of the board]4[single phase]5[enter image description here]6[enter image description here]7

  • 1
    \$\begingroup\$ They're connected in series? Are you sure? 3 parallel 2kW elements seems more likely since a 230V 2kW element would be easy to find if one of them needs replacement, while a 77V 2kW element looks like a rare beast to me ... \$\endgroup\$
    – brhans
    Feb 24, 2016 at 20:43
  • \$\begingroup\$ If the installation will be done by "a professional electrician", he will not hook up a home-brew circuit. Unless your original controller has an auxiliary output which can be used to drive a contactor, you are out of luck. \$\endgroup\$ Feb 24, 2016 at 21:12
  • \$\begingroup\$ Why would you connect two heaters on 1ph and the the 3rd on second since you have 3ph source (3x25A), you can connect every element on its own phase. Why do you think they are 3 of them - to connect them on three phases, star connection. \$\endgroup\$ Feb 24, 2016 at 23:17
  • \$\begingroup\$ @WhatRoughBeast I don't want to make my own homebrew circuit, I want to know what options are there, then I can suggest the electrician to go the cheaper way instead of upgrading the power line to my house. \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 6:14
  • \$\begingroup\$ @MarkoBuršič thank you, at this moment it seems to me that I can connect two phases to the input connector. \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 8:15

5 Answers 5


From photo it is seen that each relay has input and output connection, therfore a simple rearangemnt of wires can drive a heater in a star connection with neutral in the middle. This is a part of yours PCB schematics and proper connection to 3ph network 230/400V:


simulate this circuit – Schematic created using CircuitLab

ad 1st) Use 5 wires of 2.5mm^2 cross section. Don't mix the colours, use standard CE colours: black for phase 3 wires, blue for neutral (or you might consider buying a cable 5x2.5mm^2 in this case you get black, brown, grey for phase, blue, yellow/green), yellow/green for earth. Install an overcurrent protection device 3x16A in your main cabinet. Now what you need is L1, L2, L3, N, PE wires (five of them).
ad 2nd) Phase sequence is not important, you can mix them.
ad 3rd) See my schematics: If you measure from N to any element you get resistance of one element R, if you measure between two of them you get a series resistance of two of them 2R. You got 28 and 50ohm which is close to R and 2R.
ad 4th) The free connections are one end from each element (red wires on your heater), while the second ends are connected together in star - the jumper you see that connect three heater ends, where the neutral is attached (black wire).

  • \$\begingroup\$ So you would touch the powerswitch for my device on 3ph connection, OK, so my electrician has nothing to worry, just connect and turn on :) \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 10:19
  • \$\begingroup\$ @AndyZ Why are you so afraid to connect it to a three phase supply? Judging from the pictures the water heater was clearly designed with that as an option, and I'm pretty sure that even the installation manual has a picture similar to the above. \$\endgroup\$
    – jms
    Feb 25, 2016 at 12:23
  • \$\begingroup\$ @jms I updated my question with two more tables from the manual, both tables suggest that this is totally a single phase model. No option for 3phase. Also while I was waiting for the delivery the sales person confirmed that it is a single phase device and it cannot be used with 3phase connection. My single concern now is if there could be some overload on the mainboard due to non-calculated 3phase power. But obviously all users here are sure that's not a problem, so my electrician should have the same opinion. Thank you! \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 19:31
  • \$\begingroup\$ @AndyZ Just don't be so scared:) The device is obviously designed for 3ph/1ph connection. If you rearange the wires it can accept 3ph, the device can't feel the difference, as I depicted P9 and N is the control voltage (I can see the PCB traces), without it it won't work, the rest are the other two phases. You already have the heater connected in star: Don't believe everything what sealesman says, even the table showing power and cross sections is ridicolus 14A@4mm^2, for your app. a cable 5x1.5mm^2 is enough, you can put 5x2.5mm^2 if you want. \$\endgroup\$ Feb 25, 2016 at 23:50
  • \$\begingroup\$ @MarkoBuršič thank you, but "don't be scared" is probably not the best way to deal with 230V/400V ;-), but yeah, as to me I am convinced, so everything depends on the electrician and I hope he will agree. \$\endgroup\$
    – AndyZ
    Feb 26, 2016 at 5:53

No way that they are in series. Sounds like you live in a country with 230 V 50 Hz mains, and your water heater is set up with the heating elements in parallel:


simulate this circuit – Schematic created using CircuitLab

Contrary to what you said, you could simply connect it to a three phase supply. While the nominal voltage between any two phases of your three phase supply is 400 V, the voltage between any of the phases and neutral is just 230 V.

There are two balanced ways to connect three phases to three loads: wye and delta. In wye (AKA Y or star) the three loads share a common connection with each other. The neutral wire is usually connected to this neutral point for rendudancy and load balancing, but doesn't strictly have to be.


simulate this circuit

In delta the tree loads would be directly across the phases, forming a triangle. Unlike in a wye connection, the heaters would experience 400 V and would become grossly overloaded.

Wye vs delta

The installation should be done by a professional electrician, and the control circuitry is likely to require modifications.

  • \$\begingroup\$ Thank you for your detailed explanation. My bad, it was a late evening, I saw that three of the heater connectors are connected by a steel plate and I was sure they are in series, BUT, it's only the ground that is "in series", whereas the power comes from a separate cable to each element. Please see my update with pictures. Can I directly connect two phases as incoming power? \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 8:13

As it is pointed out jms, star connection is the best solution to reduce loading on a particular phase. Star connected load share the all three phases equally.

Most water heater elements are thrystor/triac controlled by phase chopping. The photo you have posted shows 3 independent relays for each heaters but thrystor cannot be seen. So I conclude there is no phase control but the elements are simply on and off by the relays. I also assume the switching circuits are independent from each other and the heater elements are electrically isolated from each other.

As such, feed the 3 input lines to the relays from 3 different phases (L1,L2, L3), connect the outputs from the relays to the respective heater elements, terminate the other ends of the elements to the Neutral.

In this mod, your professional electrician will need to cut and splice some wires with connectors. It should cost you minimum.

  • \$\begingroup\$ thank you for your reply. My main concern is if it could be possible that the 3 isolated phases get combined to real 3-phase connection somewhere in the mainboard just by chance because of 1-phase design which does not take into account the possibilities of 3-phase powersupply (I hope you get my idea) :) \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 9:20
  • \$\begingroup\$ Flip your board, take some photos and post up. The copper trace run behind the PCB will tell us if three phase connection is possible or not. \$\endgroup\$ Feb 25, 2016 at 9:23
  • \$\begingroup\$ thanks, I updated my question with a picture of the backside of the board \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 10:11
  • \$\begingroup\$ Looks like you got no problem at all..!! Your unit us ready made for three phase. \$\endgroup\$ Feb 25, 2016 at 12:33
  • \$\begingroup\$ thank you, I hope the electrician will see it the same way, then I got no problem at all \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 19:32

Judging by the photos & diagram you've posted, you should have no trouble at all in running one of your heater elements from a separate phase, or even running each element from its own phase.

From the diagram we can see that this same controller board can be wired for single-phase or 3-phase operation.
So you have 2 options to consider.
1. Leave the controller board as it is (single-phase configuration), but run 1 element from a 2nd phase.
2. Rewire the screw-terminal block to match the 3-phase configuration in the diagram and run your elements from 2 or even all 3 phases.

Option 1 is certainly the simplest. All you need to do is look to find where the 2 red incoming wires come from and move one of them over to a different phase.

Option 2 is slightly more complicated, but gives you the option to split the load evenly over all 3 phases if you choose.
You will need to disconnect the white wire and move it one slot down to sit on top of the black wire below it, then remove the incoming black wire (extra neutral) completely.
Now you have an empty row on your screw terminal block.
Next you take one of the 2 red wires from the top-left terminal and move it down to the empty row. This gives you the 3-phase configuration shown in the diagram.
So now you can connect 3 incoming phases to the 3 red wires, or you can parallel 2 of them together to run from 1 phase while the 3rd runs from another phase.
If you choose the 3-phase route, you will need to run another red wire from the screw-terminal block along with the existing 2 red wires to connect to the 3rd phase. Otherwise you can link one of the existing incoming phases from one of the incoming red wires to the open screw-terminal.

  • \$\begingroup\$ Thank you for your answer. I will present the device to the electrician and hope that he will be as sure as you and all other users here, that the device is perfectly capable of 3ph operation. \$\endgroup\$
    – AndyZ
    Feb 25, 2016 at 19:34

On your last questions.

  1. There's no reason for using 3 neutral wires except for the amount of current passing through. You can connect only one neutral wire if it is rated for the sum of all the currents that will be flowing from the 3 phases. If it IS rated for this higher current, you don't need any more neutral wires. The power distribution system in your country probably use only one neutral wire for the 230VAC system. Isolation between neutrals is worthless.

  2. Nope, perfect heater elements should be resistive components only. They might have some parasitic inductance, but it doesn't matter. BTW when using motors, if you swap any pair of phases the only thing that's going to happen is the rotation direction will change.

  3. I want to believe your device controls the heater elements with normally-open relays. If that's true, the heater elements will not be connected to both power inputs when the device is not working. They might be directly connected to neutral, but not to any of the phases. Thus, measuring resistance on the power connections will not provide useful information. About the 50 ohm, you're probably kinda measuring the controller board resistance, which useless. Of course this answer will be BS if the relay is NC.

  4. I couldn't understand your question.

Also, the 3-phase connection seems aimed at 380VAC systems. Be aware of that. You can have 380VAC between two 230VAC phases, but it's not the case as it seems to be 3-phase + neutral.
I might be wrong. The schematic might be leading me to misunderstand this last part.

  • \$\begingroup\$ thank you, I've been measuring the resistance between pairs of the heater connectors from first picture. Before measuring the resistance, I disconnacted the cables from the mainboard. Is this still the wrong way to measure the resistance of the single heating elements? \$\endgroup\$
    – AndyZ
    Feb 27, 2016 at 18:39
  • \$\begingroup\$ ad 1) And what is the sum of currents trough all thre phases? Answ: if the load is symetric (heater is) then the sum is zero. If one heater is disconnected then I_neutral=I_phase, if only one heater is connected I_neutral=I_phase, so neutral current is never bigger than phase current. \$\endgroup\$ Feb 27, 2016 at 18:51
  • \$\begingroup\$ @Filipe well, Marko opened my eyes, I was measuring two heating elements in series, because the neutral connectors are metal plate connected which I did not disconnect before measuring. 2000W per heating element = 8.7A = 27Ohms \$\endgroup\$
    – AndyZ
    Feb 27, 2016 at 18:57
  • \$\begingroup\$ @MarkoBuršič is right. I forgot to think about that sum and just pointed it out without realizing. You don't have to worry about the neutral. \$\endgroup\$ Feb 27, 2016 at 19:25
  • \$\begingroup\$ @AndyZ, still, the best way to measure any component is to remove it from the circuit. It's enough to disconnect one lead of the heater element and measure it between its leads or connectors on the board. \$\endgroup\$ Feb 27, 2016 at 19:31

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