3 phase to single phase

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Rick Christopherson said:
Jim, if memory serves me this is your area of expertise, so please forgive me, but I cannot find anything in code that makes such a statement (Edit to add: requiring the line-side neutral connection). ....

This is a single-phase transformer connected across two phases of a 3-phase system, which creates a separately derived single-phase 120/240 volt system, and as such, it would require ground neutral bonding on the load-side.

Not only would requiring the line-side neutral completely negate the possibility of creating a 120/240 volt separately derived system, but part of me thinks this could create problems with a grounded conductor on both sides of a transformer, and each of them having their own grounding electrode system.

If I am off base, can you tell me which section of code I should read?
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There is no line side N connection at the buck-boost transformer, it is at the beginning of the 208Y/120 system where the 'line side' neutral to ground bonding occurs.

At the start of the feeder circuit we have 208V L-L & 120V L-G, therefore at the 'line side' of the buck-boost transformer we must also have 208V L-L and 120 L-G. Now we must ground the neutral of the 'load side' 240V side per the NEC, and the now grounded neutral is common to both the line and load sides. So if the 'line side' is 208/120 how can the secondary also be 240/120?

An autotransformer (i.e. buck-boost) is not a separately derived system.
 
LarryFine said:
I have a problem with the Delta diagram: it shows the high leg as if the 208v was a source (a secondary.) In my opinion, they should show sources where the dotted lines are, and a dotted line where the 208v source is.
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But they are not the source reference terminals. If you were to use the 240 volt legs, then that would be true. We don't normally show everything leading up to the source terminals.

add: I'm not sure what you would do with the 208 volt source, but it is what it is.
 
jim dungar said:
If you are wiring per the NEC, you can never have a 2-wire to 3-wire center-tapped buck-boost fed from a wye system.
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Not so. Even if your 208v primary had a center tap, you should not connect it to anything.

Since each 120v side of the 120-N-120 of a 208Y/120v system is not in phase with the other, if you tried to power each side of a single center-tapped primary that way, they would buck, electromagnetically speaking.

Whenever there is a phase difference between two primary lines, the transformers they feed cannot share a single core. 3-phase transformer cores share a frame, but not actual cores.

For the sake of discussion: N= system neutral and Nt= buck-boost neutral
Lh = high side line, and Ll low side line.

If the output of the 3-wire buck-boost is 120/240 (120V Lh-Nt) then the input side must measure 104/208 (104 Ll-Nt). This means the neutral point of the buck-boost is not the same as the neutral point of the system which is 120/208V).
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That's not an issue. The primary should be merely connected line-to-line across the 208v, and no neutral connection at all.

But because NEC 250.20(B) requires Nt to be grounded, it must be, electrically, the same point as as N or current will flow over the 'grounding' connection. If they are the same point then Ll-Nt must be 120V which means Lh-Nt must be 138V.
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The secondary neutral-point must be bondes, yes, but the primary neutral-point (if there even is one, which I doubt on a 208v winding), will not connect to anything.
 
mivey said:
But they are not the source reference terminals. If you were to use the 240 volt legs, then that would be true. We don't normally show everything leading up to the source terminals.
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Added: We do use the line-to-line connections, even across the open side of an open Delta, but never the high-leg to the neutral. I'm just talking about consistency among the diagrams.

Look at the three diagrams again. The 1ph drawing is, of course, correct (with the exception that the two sources actually share a core, which locks them in phase.)

The 208v diagram is also accurate. The actual sources are drawn where the actual secondaries are, and the line-to-line 'derived' voltages are shown as dottec lines.

Now, look at the Delta. The center-tapped secdondary is a clone of the first diagram, as it should be. But, where the Delta has secondaries, the drawing has dotted lines.

To be as accurate as the other diagrams, the secondaries should be shown where the dotted lines are, connected line-to-line, and the 208v should be a dotted line.

I believe that someone who didn't know better would see that as an actual 208v secondary, T-connected, and would see the 240v as measureable, but not really there.
 
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LarryFine said:
Added: We do use the line-to-line connections, even across the open side of an open Delta, but never the high-leg to the neutral. I'm just talking about consistency among the diagrams.

Look at the three diagrams again. The 1ph drawing is, of course, correct (with the exception that the two sources actually share a core, which locks them in phase.)

The 208v diagram is also accurate. The actual sources are drawn where the actual secondaries are, and the line-to-line 'derived' voltages are shown as dottec lines.

Now, look at the Delta. The center-tapped secdondary is a clone of the first diagram, as it should be. But, where the Delta has secondaries, the drawing has dotted lines.

To be as accurate as the other diagrams, the secondaries should be shown where the dotted lines are, connected line-to-line, and the 208v should be a dotted line.

I believe that someone who didn't know better would see that as an actual 208v secondary, T-connected, and would see the 240v as measureable, but not really there.
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I see what you are saying....they are not consistent.
 
jim dungar said:
An autotransformer (i.e. buck-boost) is not a separately derived system.
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See! Didn't I say way back in an earlier post that this wasn't my neck of the woods? I wasn't aware that a buck/boost transformer is always an autotransformer.

I was looking at this from the standpoint of a 208:240 isolation transformer, such as what was installed at a local church here for stage power and I was called in to troubleshoot it. The EC and Inspector forgot to bond the load-side neutral, and the voltage to ground was wreaking havoc on the electronics.

So anyway, back to the autotransformer. I think 210.9 is what Jim was referring to, which does require the grounded conductor to be shared. The problem is that being an autotransformer, the line and load side share two voltage reference points, instead of the single reference point of a grounded isolation transformer.

EDIT: jim, Sorry, I didn't see your newest posting until now.
 
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LarryFine said:
I have a problem with the Delta diagram: it shows the high leg as if the 208v was a source (a secondary.) In my opinion, they should show sources where the dotted lines are, and a dotted line where the 208v source is.
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Larry, you say the exact same thing every time you see the diagram. Get over it. It is created for explaining voltage and phase relationships for the different systems, not for showing someone how to connect transformers.

It is electrically correct, and that you cannot argue. It may not match mechanically what you see in a transformer system, but that was never its intention. The reason why I drew it and labeled it as a center-tapped delta was to make it easier to understand the high leg. It is a teaching tool, not a schematic. The typical people that learn something from these diagrams don't need to know how the transformers are connected, but they do need to understand the resulting voltage relationships.
 
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