2 single phase transformer as 1 three phase Open delta transformer

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Etanuye said:
This is how they wired it
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The choice to ground L2 rather than what you call L1' is a bit unusual. 250.26 is a bit ambiguous as to whether it is allowed, arguably it is if all the 120V loads are connected L1'- L2, and none are connect L1' - L3.

Cheers, Wayne
 
Etanuye said:
Open delta connection , 4 wire output
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Then you definitely need a secondary overcurrent protective device to comply with 240.21(C), because you have line to neutral loads.

Proper terminology is important. Over the years I have found quite a few people that use local slang which is inaccurate when it comes to delta connected secondaries. I should know to ask for a wiring diagram.
 
wwhitney said:
The choice to ground L2 rather than what you call L1' is a bit unusual. 250.26 is a bit ambiguous as to whether it is allowed, arguably it is if all the 120V loads are connected L1'- L2, and none are connect L1' - L3.

Cheers, Wayne
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I agree. Very unusual but that is how the manufacturer installation.
 
jim dungar said:
Then you definitely need a secondary overcurrent protective device to comply with 240.21(C), because you have line to neutral loads.

Proper terminology is important. Over the years I have found quite a few people that use local slang which is inaccurate when it comes to delta connected secondaries. I should know to ask for a wiring diagram.
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Yes. That’s was my arguable article. Contractor is refusing to put secondary protection.
 
It is very simple, if you have more wires coming out of your transformer then going in you must provide secondary protection per 240.21(C).
Yes, I know it wasn't always this way, but it has been now for some 40 years.
 
jim dungar said:
It is very simple, if you have more wires coming out of your transformer then going in you must provide secondary protection per 240.21(C).
Yes, I know it wasn't always this way, but it has been now for some 40 years.
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I totally agree. So we have 3 wires in 4 wires out as you can see on the wiring diagram, the loads are (1) 3P, 240V and (1) 1P, 120V.

Does it make sense (L1,L2,L3), 3P disconnect and L1’,L2/N. 1P disconnect.

GEC bonding with L2/N maybe at the 3P disconnect or inside the secondary compartment
 
Yes, having 2 disconnects is allowed by 240.21(C).
I usually like the idea of keeping the three phase separate from the single phase, but warning labels become necessary when multiple disconnects feed the same machine.
 
Etanuye said:
I totally agree. So we have 3 wires in 4 wires out as you can see on the wiring diagram, the loads are (1) 3P, 240V and (1) 1P, 120V.

Does it make sense (L1,L2,L3), 3P disconnect and L1’,L2/N. 1P disconnect.

GEC bonding with L2/N maybe at the 3P disconnect or inside the secondary compartment
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Identifying one conductor as L2/N threw me off a little earlier, and made me question some things. IMO it should just be identified as N.

Otherwise the way you showed connections in earlier drawings is the correct way to connect it.
 
kwired said:
Identifying one conductor as L2/N threw me off a little earlier, and made me question some things. IMO it should just be identified as N.

Otherwise the way you showed connections in earlier drawings is the correct way to connect it.
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That’s how the transformer manufacturer label it, corner grounded phase can also be neutral and phase conductor right? Or Atleast that’s what I understood
 
Etanuye said:
That’s how the transformer manufacturer label it, corner grounded phase can also be neutral and phase conductor right? Or Atleast that’s what I understood
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In three phase three wire that is corner grounded, there may be some justification of calling that grounded conductor a neutral.

But in three phase four wire delta systems, they didn't always call that mid point of the one phase a neutral, simply because it was not equal voltage to all the other conductors of the system. It was just simply the grounded conductor. It was neutral in relationship to two of the ungrounded conductors but not neutral to the entire system.

But now NEC does call it a neutral, can't remember when they started doing that, maybe somewhere between 2005 and 2014 editions?
 
Etanuye said:
That’s how the transformer manufacturer label it, corner grounded phase can also be neutral and phase conductor right? Or Atleast that’s what I understood
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No.
You have corner grounded or you have one winding center tapped (like a neutral) and grounded. These are electrically and physically different configurations.
 
jim dungar said:
You have corner grounded or you have one winding center tapped (like a neutral) and grounded. These are electrically and physically different configurations.
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But the OP has one corner grounded and one winding center tapped. That seems to be allowed by 250.26(4), which is a bit self-referential.

Cheers, Wayne
 
Etanuye said:
Ok. Let me ask you this. Where would you bond the GEC based on the wiring diagram?
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The wiring diagram already shows the choice of where to bond the GEC, i.e. which circuit conductor is grounded. Who made that choice? Is this a field installed pair of transformers, or internal to a manufactured and listed piece of equipment?

The more typical choice would be to ground the conductor that is the center-tapped, which you call L1'.

Note also you have labeled a conductor L2/N, but it should not be labeled N. The grounded conductor is not always a neutral, on a 3-phase 3-wire corner grounded delta you have a grounded conductor which is not neutral. The center-tapped conductor, that you call L1', is in fact neutral to L2 and L3.

Cheers, Wayne
 
wwhitney said:
The wiring diagram already shows the choice of where to bond the GEC, i.e. which circuit conductor is grounded. Who made that choice? Is this a field installed pair of transformers, or internal to a manufactured and listed piece of equipment?

The more typical choice would be to ground the conductor that is the center-tapped, which you call L1'.

Note also you have labeled a conductor L2/N, but it should not be labeled N. The grounded conductor is not always a neutral, on a 3-phase 3-wire corner grounded delta you have a grounded conductor which is not neutral. The center-tapped conductor, that you call L1', is in fact neutral to L2 and L3.

Cheers, Wayne
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The wiring and labeled are From the transformer manufacturer.

I agree L1’ neutral makes sense but for some reason they went to ground L2 which is unusual. And they labeled as L2/N
 
kwired said:
Identifying one conductor as L2/N threw me off a little earlier, and made me question some things. IMO it should just be identified as N.

Otherwise the way you showed connections in earlier drawings is the correct way to connect it.
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I partially take that back. It is connected properly but has the wrong lead grounded.
wwhitney said:
The wiring diagram already shows the choice of where to bond the GEC, i.e. which circuit conductor is grounded. Who made that choice? Is this a field installed pair of transformers, or internal to a manufactured and listed piece of equipment?

The more typical choice would be to ground the conductor that is the center-tapped, which you call L1'.

Note also you have labeled a conductor L2/N, but it should not be labeled N. The grounded conductor is not always a neutral, on a 3-phase 3-wire corner grounded delta you have a grounded conductor which is not neutral. The center-tapped conductor, that you call L1', is in fact neutral to L2 and L3.

Cheers, Wayne
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I though it was mostly clear early on that this is two single phase transformers configured to have open delta secondary. Didn't become as clear as to whether secondary was corner grounded or high leg connected until later in the thread though.
 
kwired said:
I partially take that back. It is connected properly but has the wrong lead grounded.

I though it was mostly clear early on that this is two single phase transformers configured to have open delta secondary. Didn't become as clear as to whether secondary was corner grounded or high leg connected until later in the thread though.
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Correct. It is two single phase transformers configured to have 3P open delta secondary.

yea as it shows on the drawing, it looks to me that L2 is phase conductor but shows its grounded. That means corner grounded open delta. I might be wrong
 
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Etanuye said:
Correct. It is two single phase transformers configured to have 3P open delta secondary.

yea as it shows on the drawing, it looks to me that L2 is phase conductor but shows its grounded. That means corner grounded open delta
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Well it would be corner grounded simply because of the grounding point, but that also means the mid point of the one phase (which is truly neutral to that phase) is now ungrounded, and should have overcurrent protection on it if utilizing it. All ungrounded conductors need overcurrent protection.

Also if you read through 250.26 that midpoint of that phase (when extended for use as a system conductor anyway) is the conductor of such a system that NEC requires to be grounded.

Outside that rule in theory you can ground any one point of any system.
 
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