210.9/215.11 Autotransformer question

[ciupol]

Can somebody help me interpret article 210.9. Does this article says that the branch circuit can't be derived from autotransformer if circuit supplying it does not have the grounded conductor or that if the circuit supplying autotransformer have grounded conductor the branch circuit grounded conductor have to be connected to it.
The question is for the following scenario: I have 480/240 - 120/24V transformer. On the secondary side I am using both 120 and 24 voltages. Transformer is fed from two legs of a 480V 3-phase system. Transformer have middle tap on primary side. Can I derive 240V from this transformer?

 

[templdl]

Are you aware that an autotransformer nor a buck/boost transformer for or that matter do not provide isolation as a common transformer would? As such they are not able to derive a new system. NEC art 450 should show a section and a diagram regarding it.
But it is good that you are asking the question as providing protection for them is unique also.

 

[Smart $]

... Can I derive 240V from this transformer?

You can, but you're not going to power up much load. The transformer is just not designed for a 240V load.

 

[ciupol]

Are you aware that an autotransformer nor a buck/boost transformer for or that matter do not provide isolation as a common transformer would? As such they are not able to derive a new system. NEC art 450 should show a section and a diagram regarding it.
But it is good that you are asking the question as providing protection for them is unique also.

Thanks for the comment. I fully understand isolation concerns and I am aware about section 450 and protection requirements but that is not what I was asking about.

You can, but you're not going to power up much load. The transformer is just not designed for a 240V load.

Setting the size of the load aside you are saying that such wiring would meet all the code requirements and is allowed by code?

 

[templdl]

Thanks for the comment. I fully understand isolation concerns and I am aware about section 450 and protection requirements but that is not what I was asking about.

Sorry, I was going by your reference to the term autotranforner.
Can you take a picture of the nameplate/wiring diagram of the subject transformer which would be helpful in addressing your question?
But I'm taking a guess at what you are getting at now. Since the primary has (2) 240v windings H1,H2,H3, and H4 wher H2and H3 and checked together in seieies for a 480v source and if you can get 240 from the pimarty since the secondary only is only 120/24v.

 

[ciupol]

Here you go from manufacturers website.



On secondary side I have grounded neutral at X3, X2 and X1 are 24V and 120V respectively going to the fuse.
On primary side I have supply 480V between H1 and H4. I would have to derive 240V from H2 and H4.

Again all I am asking is if such wiring will comply with code. I was in discussion with other person over article 210.9 as it mentions grounded conductor on supply side which we do not have in this case.

EDIT:
Here I have actually a drawing showing what one of my suppliers want me to do. Is that allowed by code.

 

[templdl]

Here you go from manufacturers website.

View attachment 15380

On secondary side I have grounded neutral at X3, X2 and X1 are 24V and 120V respectively going to the fuse.
On primary side I have supply 480V between H1 and H4. I would have to derive 240V from H2 and H4.

Again all I am asking is if such wiring will comply with code. I was in discussion with other person over article 210.9 as it mentions grounded conductor on supply side which we do not have in this case.

EDIT:
Here I have actually a drawing showing what one of my suppliers want me to do. Is that allowed by code.

View attachment 15381

That's what I thought you were suggesting to do. I've never seen that tried before. I'll have to think of what the implications would be on that one.

 

[Smart $]

.... Setting the size of the load aside you are saying that such wiring would meet all the code requirements and is allowed by code?

No, I'm not saying that. I said what I said, and I'm sticking to it.

 

[Smart $]

...
Again all I am asking is if such wiring will comply with code. I was in discussion with other person over article 210.9 as it mentions grounded conductor on supply side which we do not have in this case.
...

And the other person is correct. It is not compliant with 210.9.

Now if you connected 208V from H3 to H4, you could derive 240V compliantly per 210.9 Exception No. 1.

 

[ciupol]

And the other person is correct. It is not compliant with 210.9.

Now if you connected 208V from H3 to H4, you could derive 240V compliantly per 210.9 Exception No. 1.

Actually I am the person that thinks it is incorrect. The wiring is suggested by the manufacturer of the equipment that I bought. And obviously fully agree with you about the 208V comment.
Thanks for the comments.

 

[templdl]

Actually I am the person that thinks it is incorrect. The wiring is suggested by the manufacturer of the equipment that I bought. And obviously fully agree with you about the 208V comment.
Thanks for the comments.

That schematic that you posted certainly illustrated what you are trying to do. It was a big help.

 

[GoldDigger]

I would assert that what you are deriving from the two line inputs of the source wye does not have a grounded conductor and cannot have a grounded conductor with this wiring scheme. Nor does it consist of two line of an output wye. It is a nonstandard voltage output with two hot lines with nonstandard voltages to ground and it would be a nightmare for anyone working with it later without knowledge of how it was set up. I would think using the output to drive an isolation transformer that could then supply a grounded 0-240 or 120-0-120 would be OK, but in that case what is the point of the autotrnansformer connection in the first place.
JMO

 

[dkidd]

I don't see where it is being connected as an autotransformer. Could someone please point me in the right direction? It looks like a two winding transformer.

 

[Smart $]

I don't see where it is being connected as an autotransformer. Could someone please point me in the right direction? It looks like a two winding transformer.

480V primary power connected H1 and H4.

H2 and H4 used as a source of 240V. Diagram just shows the 240V fusing but no loads connected. Once a load is a connected, the primary winding will be functioning as an autotransformer.

 

[wwhitney]

Hello,

At first I didn't understand the point of 210.9, but then I considered the example of a 20 amp, 240V only circuit from a 120V/240V service and using an autotransformer to rederive the neutral to serve a 120V/240V piece of equipment. Then 20 amps on the autotransformer input could provide 40 amps hot-to-neutral on the autotransformer output, right? So the autotransformer output conductors are no longer protected at 20 amps.

The solution, of course, would be to add 20 amp OCPD on the autotransformer output, and then the autotransformer is no longer serving a branch circuit, but a feeder. So 210.9 doesn't apply.

Does that sound right?

Cheers, Wayne

 

[ciupol]

I would assert that what you are deriving from the two line inputs of the source wye does not have a grounded conductor and cannot have a grounded conductor with this wiring scheme. Nor does it consist of two line of an output wye. It is a nonstandard voltage output with two hot lines with nonstandard voltages to ground and it would be a nightmare for anyone working with it later without knowledge of how it was set up. I would think using the output to drive an isolation transformer that could then supply a grounded 0-240 or 120-0-120 would be OK, but in that case what is the point of the autotrnansformer connection in the first place.
JMO

Of course I can do a lot of modifications to make it simpler. The thing is that the panel and drawings come from manufacturer of equipment that I bought for my facility. If such wiring is not allowed by code, which I think is not, I will just require from manufacturer to redesign the panel. I am not going to spent my time and money to redo it.
I am aware that it is non standard but from the very beginning my question was if such installation is allowed by code. As I said in my first post I think there are two ways of interpreting 210.9:
1. branch circuit can't be derived from autotransformer if circuit supplying it does not have grounded conductor.
2. if the circuit supplying autotransformer have grounded conductor the branch circuit grounded conductor have to be connected to it.

Please correct me if my understanding is incorrect.

Hello,

At first I didn't understand the point of 210.9, but then I considered the example of a 20 amp, 240V only circuit from a 120V/240V service and using an autotransformer to rederive the neutral to serve a 120V/240V piece of equipment. Then 20 amps on the autotransformer input could provide 40 amps hot-to-neutral on the autotransformer output, right? So the autotransformer output conductors are no longer protected at 20 amps.

The solution, of course, would be to add 20 amp OCPD on the autotransformer output, and then the autotransformer is no longer serving a branch circuit, but a feeder. So 210.9 doesn't apply.

Does that sound right?

Cheers, Wayne

Not really following your thinking. But even if we consider it to be feeder not branch circuit article 215.11 says exactly the same about feeders.

By the way per 450.4 for autotransformers you need protection only on primary (input) side.

 

[wwhitney]

But even if we consider it to be feeder not branch circuit article 215.11 says exactly the same about feeders.

Whoops, I missed that. So now I have to ask--why are 210.9 and 215.11 in the NEC? What is the safety reason?

Edit: also, can 240V - 120V/240V autotransformers, such the ones below, be used in an NEC compliant fashion?

https://www.hubbell-acmeelectric.com/p-132-240-120240

Thanks, Wayne

 

[Smart $]

Whoops, I missed that. So now I have to ask--why are 210.9 and 215.11 in the NEC? What is the safety reason?

Edit: also, can 240V - 120V/240V autotransformers, such the ones below, be used in an NEC compliant fashion?

https://www.hubbell-acmeelectric.com/p-132-240-120240

Thanks, Wayne

Sorry, I can't answer the why and the what of it... but I can tell you the transformers you linked to are typically not connected as autotransformers. They are typical isolation transformers.

 

[wwhitney]

But I can tell you the transformers you linked to are typically not connected as autotransformers. They are typical isolation transformers.

The title of the chart I linked to is "Autotransformer - 240 Primary Volts - 120/240 Secondary Volts - 1Ø". Their isolation transformers are on a different page, and they weigh about twice as much for a given kva:

https://www.hubbell-acmeelectric.com/p-133-240x480-120240

Cheers, Wayne

 

[ciupol]

The title of the chart I linked to is "Autotransformer - 240 Primary Volts - 120/240 Secondary Volts - 1Ø".

Yep, the link you gave is definitely for autotransformers. If you dig further through their website you can find schematic showing that they have only one winding.
http://aspdotnetstorefront.hubbell-acmeelectric.com/Images/drawings/T253062.pdf

Edit: also, can 240V - 120V/240V autotransformers, such the ones below, be used in an NEC compliant fashion?

Also would like to know answer to this question.