Conversion from Three Wire to Four Wire

Measure resistance between #1 & all other terminals. Tabulate all the readings.

Measure resistance between #2 & all other terminals. Tabulate all the readings.

Measure resistance between #3 & all other terminals. Tabulate all the readings.

Measure resistance between #4 & all other terminals. Tabulate all the readings.

If it is a 4 terminal 3 phase heater, then you will find one terminal that is common to 3 other live terminals will have identical resistance reading. This terminal then is the common neutral & the rest 3 are the live, phase terminals.

It may even be a 3 terminal device with 4th one being earth, in which case it will have no resistance with other 3 terminals.

Hi Mr. Joshi,

Thanks for comment. The resistance betwen terminals are as follows : 1-2=450, 1-3=565, 1-4=225, 2-3=115, 2-4=225 and 3-4=340 ohms. My supply is 380V, 3phase, 3 wired without nuetral. The heater is not provided with a drawing and is sealed. A label in the eqpt shows it is 2500W and 380/400V.

From your measurements, it looks that:

terminal 1 = L1, 2 = L2 and L3 = 4 will give 2500W at 380V

Configuration looks like: terminal (3)--- 115Ω --- (2) --- 225Ω --- (4) --- 225Ω ---(1)

Since you don't have a Neutral wire, then:

L2 can be connected to either (3), for less heating power, or to (2) for full power.

It all depends on how the different elements are going to be positioned in the oven!!

Your suggestion is absolutely OK. Thanks a lot for the advice. Shorting 1 and 3 solved the problem. Terminals 1, 2 and 4 are connected to L1, L2 and L3 with 1 and 3 shorted.

Regards

Your old heater has two identical resistances of 110 ohms in series. #1 is common, between #1 & #2 there is a resistor of 110 ohms, similarly between #1 & #3 there is a resistor of 110 ohms, the load is on 2 phases only. It is connected to three phases, but draws power from two phases only.

Your new heater does not make sense, since ohmic readings are not rational. In this heater, there are three resistors in series. Between #3 & #2 there is a resistor of 115 ohms (please recheck this reading), between #2 & #4 there is a resistor of 225 ohms, between #4 & #1 there is a resistor of 225 ohms. I think this is for 3 phase heater with #3 & #1 joined together for phase 1, #2 for phase 2, #4 for phase 3. You have to run 3 phase 3 wire supply. Neutral is not to be used. I am not comfortable because one of the odd resistor between #3 & #2.

"It is connected to three phases, but draws power from two phases only."

It will still draw power from the 3 pases.( 110Ω between Ph1 nd Ph2, 110Ω between Ph1 nd Ph3) to make the 2500W~.

On the new Heater: He can bridge terminals 3 and 4 and use that for one of the phases, (Ph3), while 1 to Ph1 and 2 to Ph2. he will get 641 W + 641W + 1255W=2537W.

Now, I don't know how the distribution of the elements is configured within the new range and how the heat will be distributed(?)

2.5kW oven seems a bit anemic to me.

What advantage does OP believe he will gain with this undocumented replacement of unstated origin?

Well, I don't know!

He did not come back to comment!? I hope he is still there.

It is not being used for home, but for a ship. Since the components to be replaced are procured by the company, no choice of merits and demerits. Thank you.

What is you main supply?: 3 phases without ground, or monofasic (actually two phases plus neutral)?.

What voltage do you read accross the three power lines?

You don't mention a ground wire, have that corrected before someone gets hurt or killed.

Be aware that if you need to re-configure the internal connection of the heaters, well, you can't ! since it is a sealed range according to what you say.

Just run another conductor of the appropriate size, plus a ground conductor if it's not already there while your at it, and connect the cooking range in the manner that its designers intended.

"I have a cooking range of 2500W which is working on 380V three wire system".

Why replace If it is working? Anyhow based on your measurements, I believe the old unit is not a 3-phase device! It is powered via a 2-phase 3 wire system, where the 3rd wire as common, with which may also be either connected to a neutral or ground? As dictated by your resistance measurements, terminals 2 and 3 (having 220 ohms between them) are connected across the 380V, while terminal 1, (having 110 ohms with respect to both terminals 2 & 3) being the load center tap and common, can be connected to either earth or neutral.

Using earth as the reference or common to validate the above, you have to measure the power coming out from those 3 wires, but personal Safety always comes first! If you're not sure, get a qualified electrician to do this power supply measurements! Be safe and good luck!!

The fourth wire (neutral) should not matter to the heating elements, as they will all be rated for 380V. Based on your resistance values they are connected open delta if you have 3 phase service. If you have single phase service the elements will be in parallel.

The main difference between 3 wire and 4 wire ranges is the controls. Your old range should have a small transformer inside to reduce the 380V to whatever the controls use (probably 220V). The new range does not have an internal transformer and instead relies on a phase to neutral connection to provide the necessary 220V. If you connect 380V to all 4 connections on the new range you will fry the controls.

If your electric service has a neutral wire with the incoming phase conductors, you can run a neutral wire back to your panel and replace the 3 wire receptacle with a 4 wire. If your service does not have an incoming neutral you will need to install a small 380V/220V transformer in the new range to supply the controls.