Is there such thing as a

[steelersman]

120/240 Volt 3-phase transformer and no it's not a high leg (if it exists)? I've only heard of 120/208 but maybe 120/240 is a delta?

 

[beanland]

Delta

Delta

Yes, you can purchase 3-phase padmount 120/240V high-leg delta transformers. I believe Square D offers them in the 480V-240/120V version.

 

[LarryFine]

Is there such thing as a 120/240 Volt 3-phase transformer and no it's not a high leg (if it exists)? I've only heard of 120/208 but maybe 120/240 is a delta?

Outside of 208/120, the only way to have no high leg, i.e., have three equal voltages to earth, is for there to be no bonded center tap, which implies no bond at all and all three lines capacitively bonded.

You might want to double-check your voltmeter just to be sure.

 

[winnie]

The physics of the situation means that if you have a 240V delta transformer with a center tap on one leg, then you get 120/240V on the leg that has the center tap, and the third leg will be a high leg.

I suppose that you could have some sort of intentionally unbalanced transformer, where the 'base' of the delta gives 120/240V, but where the 'high' leg is also 120V to 'neutral'. The 'other' line-line voltages would only be 170V. I can think of no good reason to do this; you would bear the cost of three phase service without being able to run a normal motor.

-Jon

 

[steelersman]

Outside of 208/120, the only way to have no high leg, i.e., have three equal voltages to earth, is for there to be no bonded center tap, which implies no bond at all and all three lines capacitively bonded.

You might want to double-check your voltmeter just to be sure.

I was arguing with a buddy at work today and he was saying that a couple of schools that he wired a while back had a 120/240 volt 3 phase no high leg service. I said I've never heard of 120/240 3 phase no high leg are you sure and he said yes that in that area (Leesburg) that that's what all the commercial services were or are. But then again he also thinks that amps and watts are both the same and that they both measure current. Even after I showed him the Ohm's law formulas he still thinks he's right. I told him to go look online tonight and see for himself.

 

[steelersman]

The physics of the situation means that if you have a 240V delta transformer with a center tap on one leg, then you get 120/240V on the leg that has the center tap, and the third leg will be a high leg.

I suppose that you could have some sort of intentionally unbalanced transformer, where the 'base' of the delta gives 120/240V, but where the 'high' leg is also 120V to 'neutral'. The 'other' line-line voltages would only be 170V. I can think of no good reason to do this; you would bear the cost of three phase service without being able to run a normal motor.

-Jon

no he said it was 240 between A B and C phase and 120 to ground on A B and C. Besides if you had a high leg it would be 240 to ground from B phase not 120 to ground.

 

[Dennis Alwon]

no he said it was 240 between A B and C phase and 120 to ground on A B and C. Besides if you had a high leg it would be 240 to ground from B phase not 120 to ground.

He could be incorrect, you know. I am not saying it doesn't exist but I would question whether he is correct. He may never have measured the high leg to ground.

 

[steelersman]

He could be incorrect, you know. I am not saying it doesn't exist but I would question whether he is correct. He may never have measured the high leg to ground.

Dennis that is what I'm implying. I don't think it exists but then again I'm not that experienced with transformers and commercial switchgear and all that jazz.

 

[LarryFine]

no he said it was 240 between A B and C phase and 120 to ground on A B and C. Besides if you had a high leg it would be 240 to ground from B phase not 120 to ground.

Definitely incorrect. He's either wrong about the high leg, or it's an ungrounded Delta.

 

[steelersman]

Definitely incorrect. He's either wrong about the high leg, or it's an ungrounded Delta.

so it could be an ungrounded delta? That is possible then? To have 240 on all legs and 120 from all legs to ground with no high leg?

 

[Strahan]

Definitely incorrect. He's either wrong about the high leg, or it's an ungrounded Delta.

In the case of an ungrounded delta is it safe to say that all phases would have to be balanced in order to see 120 to neutral (center tap)?

 

[LarryFine]

so it could be an ungrounded delta? That is possible then? To have 240 on all legs and 120 from all legs to ground with no high leg?

Let's see: In order for the voltage from each phase to whatever point you're calling ground to be equal, the system's connection to that point must be of a wye configuration.

That the phase-to-phase voltages are equal is inhertent to any normal three-phase system, but let's keep the 240v in mind. You state having 120v line to ground all the way around.

That rules out having a high leg, which you confirm. A 240v wye system should have 138.5v (half of 277 from a 480/277) line to ground. 120v line to ground calls out for 208v line to line.

This could either be an error in voltage measuring, or the ground reference is not solidly tied to the system, which is where my ungrounded-Delta educated guess came from. That's my take.

 

[LarryFine]

In the case of an ungrounded delta is it safe to say that all phases would have to be balanced in order to see 120 to neutral (center tap)?

Okay, to answer the quesstion more literally, if you mean the center tap of one secondary, then there must be a high leg.

Of course, all of these discussions presume no unintentional opens or resistances in any conductors or connections.

It would be helpful to eyeball the service, and look at the neutral pathway, if there is one.

 

[infinity]

Given the possibilities I would say that he's definitely wrong.

 

[Strahan]

Okay, to answer the quesstion more literally, if you mean the center tap of one secondary, then there must be a high leg.

Of course, all of these discussions presume no unintentional opens or resistances in any conductors or connections.

It would be helpful to eyeball the service, and look at the neutral pathway, if there is one.

Ok so with an ungrounded delta lets say 240V between all phases there is no way you could reference 120V(to what?) right? I guess I'm confused with the readings that are taking place.

 

[steelersman]

Ok so with an ungrounded delta lets say 240V between all phases there is no way you could reference 120V(to what?) right? I guess I'm confused with the readings that are taking place.

Just so you know there weren't any readings taken. This was just a discussion between a guy at work and myself. He said that a long time ago meaning probably 10 years ago he took part in wiring a school that was 3 phase 120/240. I then asked oh so it had a high leg? He said no. I said I think you mean 120/208 then. He said no. I said ok well I doubt it but......whatever. So here I am with this thread. I do believe that he's mistaken as I said earlier he doesn't seem to know the differences between amps and watts. It's a long story. Also he never went to electrical school so in his defense he just doesn't know any better perhaps. But I'm sure now he's wrong and I and all of you guys are correct.

 

[Besoeker]

Definitely incorrect. He's either wrong about the high leg, or it's an ungrounded Delta.

Couldn't it be a specially made transformer 120V to 240V delta star or star star?
We routinely get large and small transformers made to non-stardard voltages and with unusual vector arrangements so I can't see a problem in making a 120V to 240V unit.

 

[infinity]

Couldn't it be a specially made transformer 120V to 240V delta star or star star?
We routinely get large and small transformers made to non-stardard voltages and with unusual vector arrangements so I can't see a problem in making a 120V to 240V unit.

I don't see how it's possible to actually make a Delta transformer, without a high leg, and still end up with the voltages outlined in the OP.

 

[steelersman]

Couldn't it be a specially made transformer 120V to 240V delta star or star star?
We routinely get large and small transformers made to non-stardard voltages and with unusual vector arrangements so I can't see a problem in making a 120V to 240V unit.

I don't think this would be a specially made transformer as it was the service for an entire school. He says that all commercial buildings in that area are this way. I say "yeah right".

 

[Besoeker]

I don't see how it's possible to actually make a Delta transformer, without a high leg, and still end up with the voltages outlined in the OP.

I wasn't suggesting a delta secondary.
But there would be no difficulting in making such a transformer with a 120V primary and a 240V delta secondary. We have a test transformer in our works that is 400V star primary and 3300V delta secondary. It's not a high leg arrangement.

As I understand it, a high leg is just a delta winding with the addition of a center tap on one of the delta windings that is usually taken to ground.