3 phase to single phase

[treyp3]

The tech support for this steamer said changing this 3 phase 208v unit to single phase 240v is very complicated and that they could not provide a diagram. Shouldn't I be able to rewire the heating elements in series for single phase?

 

[jim dungar]

Right now each resistor sees L-L voltage across its terminals.
If you put the resistors in series each resistor will only see 1/3 of L-L voltage. Among other issues, you will notice a tremendous loss of heat output.
If you put them in parallel each resistor will see L-L voltage. Among other issues, you will notice a substantial increase in the current draw.

Two methods with different considerations. Only you can decide which is best.

 

[GoldDigger]

View attachment 12622 The tech support for this steamer said changing this 3 phase 208v unit to single phase 240v is very complicated and that they could not provide a diagram. Shouldn't I be able to rewire the heating elements in series for single phase?

I hope you mean wire them in parallel....
There could be a problem supplying the control voltages, and they may not want to take responsibility for applying 240 to elements designed for 208. The current requirement for the circuit will also be a lot higher.
The control transformer has input taps for both 208 and 240, so that is not a priblem. But the heater strips show different part numbers for 208 and 240.
Rewiring the strips and line connections to the three pole contactor should not be difficult. But I would not take responsibility for supplying a diagram without a hands-on look at the unit.

 

[Tony S]

There?s more of a knock on effect than you may think with this little conundrum.

Changing from 208V 3Ph delta to 240V Sp has quite a bearing on the total power and current.

208V 3Ph delta
9KW
24.97A

240V Sp
11.98KW
49.92A

 

[kwired]

View attachment 12622 The tech support for this steamer said changing this 3 phase 208v unit to single phase 240v is very complicated and that they could not provide a diagram. Shouldn't I be able to rewire the heating elements in series for single phase?

They want you to send it in to them so they can make $$$ converting it. It is not that hard. As mentioned put the individual elements in parallel across a 240 volt single phase input.

One problem you do have is if it actually has the 208 volt elements installed they will run hotter on 240 and it will likely shorten their expected life. (I may or may not run them until they fail if it were something of my own, but would inform a customer they may have to change elements sooner then expected if they don't put in the recommended elements.) It tells you on the lower left corner it is 3000 watts and gives part numbers for both 208 and 240 volt elements.

It also appears to be set up for heavy enough incoming wiring for handling more then it would draw should you convert it to single phase 30 amp breaker/30 amp plug/ 35 amp contactor? This 3000 watt unit would only draw 12.5 amps plus what little bit is drawn by controls if it were connected to single phase input.

Other thing is to change the input tap on the control transformer - it is shown on the drawing as well as having a tap for 208 and a tap for 240.

 

[Smart $]

First, to convert to 240V 1?, you should get the 240V heater elements. However, that is not all.. You have to change out the plug, cord, breaker, [most, if not all] internal power wiring, and likely the contactor [i.e. if it's only rated for three phase current levels].

 

[kwired]

First, to convert to 240V 1?, you should get the 240V heater elements. However, that is not all.. You have to change out the plug, cord, breaker, [most, if not all] internal power wiring, and likely the contactor [i.e. if it's only rated for three phase current levels].

I mentioned some of this already and it seems to have components capable of handling more then what is connected - even if connected for single phase. Appears it has option of either a 35 or 60 amp mercury contactor a L15-30 cord cap and 30 amp circuit breaker according to the drawing posted and only a load of 3000 watts.

Unless I am misreading it - is possible there are three 3000 watt heaters in which case the 30 amp breaker is actually slightly undersized if considered a continuous load and using on 208 volts.

 

[Tony S]

I mentioned some of this already and it seems to have components capable of handling more then what is connected - even if connected for single phase. Appears it has option of either a 35 or 60 amp mercury contactor a L15-30 cord cap and 30 amp circuit breaker according to the drawing posted and only a load of 3000 watts.

Unless I am misreading it - is possible there are three 3000 watt heaters in which case the 30 amp breaker is actually slightly undersized if considered a continuous load and using on 208 volts.

I took it as 3 x 3KW.

I blame it on my eyesight.

 

[Smart $]

I took it as 3 x 3KW.

I blame it on my eyesight.

Eyesight correct. Each channel heater is 3kW, for a 9kW total.
25A@208V,3?
21A@240V3?
37.5A@240V1?
...not counting control load.

@kwired: the 30A breaker is not under NEC rule for continuous load factoring.

 

[treyp3]

First, to convert to 240V 1?, you should get the 240V heater elements. However, that is not all.. You have to change out the plug, cord, breaker, [most, if not all] internal power wiring, and likely the contactor [i.e. if it's only rated for three phase current levels].

There is no power run to the unit now and changing out the cord is a given. Wouldn't the controls see the same current draw, they are only using two hot legs? Also, I've forgotten how to determine what the new draw will be. Something to do with the square root of three, although it would be super easy to temp power it and put an amp probe on it.

 

[Smart $]

There is no power run to the unit now and changing out the cord is a given. Wouldn't the controls see the same current draw, they are only using two hot legs? Also, I've forgotten how to determine what the new draw will be. Something to do with the square root of three, although it would be super easy to temp power it and put an amp probe on it.

The control wiring before the CPT should actually see less after switching primary voltage tap. Everything on the low voltage side of the CPT will remain the same.

As far as current draw, what does the nameplate say? Compare to current level for 208V in my last post. Should be slightly more. Take that difference and add to the amount for 240V 1?.

 

[iwire]

I think you should put aside the way you would do this for moment and look at should you do this.

No doubt this is a listed appliance and these modifications would be a violation of 110.3(B).

I would not modify this appliance for a customer without factory support.

 

[GoldDigger]

The control wiring before the CPT should actually see less after switching primary voltage tap. Everything on the low voltage side of the CPT will remain the same.

As far as current draw, what does the nameplate say? Compare to current level for 208V in my last post. Should be slightly more. Take that difference and add to the amount for 240V 1?.

If you replace the elements with the correct part number for the voltage, the current per element should be lower for the 240V model.

 

[Smart $]

If you replace the elements with the correct part number for the voltage, the current per element should be lower for the 240V model.

That's not what I'm saying in that post. I'm saying the nameplate current for the existing 208V model should be slightly higher than the current draw of just the 3x3kW heaters to account for control power draw... and the difference will be the same no matter how the heater elements are configured. So simply add that difference to the currrent draw of 3x3kW 240V elements wired in parallel for 1? operation (37.5A) for total draw.

 

[kwired]

I think you should put aside the way you would do this for moment and look at should you do this.

No doubt this is a listed appliance and these modifications would be a violation of 110.3(B).

I would not modify this appliance for a customer without factory support.

It may very well be supported by the factory to convert to single phase. Would be a good reason why the schematic mentions both a 35 amp and a 60 amp contactor being possible.

 

[GoldDigger]

It may very well be supported by the factory to convert to single phase. Would be a good reason why the schematic mentions both a 35 amp and a 60 amp contactor being possible.

Or it may be that support only factory selection of the configuration with no field modification possible and they just economized by providing one schematic that showed the parts but not the wiring for the 240 configuration?

 

[kwired]

Or it may be that support only factory selection of the configuration with no field modification possible and they just economized by providing one schematic that showed the parts but not the wiring for the 240 configuration?

I kind of wondered when I was thinking the 60 amp contactor was probably what is needed if single phase - but if they are going to mention it then why not draw alternate wiring - plus it wouldn't need to be a three pole contactor.

 

[iwire]

It may very well be supported by the factory to convert to single phase.

The tech support for this steamer said changing this 3 phase 208v unit to single phase 240v is very complicated and that they could not provide a diagram.

That does not sound like the factory supports the idea. But maybe.


With the equipment I work with that can be configured for single or three phase the information needed to do so is included in the diagram.

Most recently (within the last month) it was a 10 KW unit heater.

 

[GoldDigger]

I kind of wondered when I was thinking the 60 amp contactor was probably what is needed if single phase - but if they are going to mention it then why not draw alternate wiring - plus it wouldn't need to be a three pole contactor.

:thumbsup: If you would use only one pole.
But if you rewire the contactor to interrupt only one L conductor you could put one element on each pole.

 

[kwired]

That does not sound like the factory supports the idea. But maybe.


With the equipment I work with that can be configured for single or three phase the information needed to do so is included in the diagram.

Most recently (within the last month) it was a 10 KW unit heater.

They may not support it but it is not that complicated for any electrician that is used to hooking up or troubleshooting controls. Those that don't do much but bend pipe, pull conductors and connect simple devices and fixtures maybe have a harder time with it.