Single Ph. 208 /120 Transformer One-Line diagram--stuck on details

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I'm very rusty on Electrical Design . I'm trying to redesign an existing electrical service.
I have a 2 wire 208 feed coming from a 3 phase breaker on 208Y/120 3 Phase 4 Wire Panel through an old conduit.
I'm stuck with what I have, which is 2 Hots and a Neutral.
For some reason, I am thinking I should be getting 2 Hots and a Ground, but what I have is 2 Hots and a Neutral.
I'm using a 3 KVA ACME T-2-79743-S Transformer.
I have some questions about the one line I developed.
1. Is it generally connect?
2. Do I place the Disconnect Switch prior to or after the Transformer? Does the NEC specifications
for a Separately Derived system govern this?
3. Do I tie the Neutral to the Ground at the Counterpoise ( see red circle) Does the fact that I have 2 H,1N vice 2H 1G make a
big difference
Thank you in Advance
DESIGN QUESTION.png
 
Ignore the source N conductor, pull a ground wire instead.
A counterpoise serves no purpose, a simple grounding electrode system is sufficient.
Is the breaker feeding the transformer capable of being locked open?
 
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That still baffles me - when you have 120/120/208V. It's different phases. Especially when it is explicitly states 208/120V 3-phase four wire.
And single phase........
 
Besoeker3 said:
That still baffles me - when you have 120/120/208V. It's different phases. Especially when it is explicitly states 208/120V 3-phase four wire.
And single phase........
Click to expand...
Basically he's showing two 120 volt to ground phase legs that are used to provide the 1Ø, 208 volts.
 
Besoeker3 said:
That still baffles me - when you have 120/120/208V. It's different phases. Especially when it is explicitly states 208/120V 3-phase four wire.
And single phase........
Click to expand...
Stop using the neutral as your reference point. Not all systems have neutrals, so your phase reference philosophy should not be dependent on one.

Context is everything. Understand some people use the term 'phase' as an adjective, like phase conductor, while others use it as a noun, like how many phases are there.
 
Besoeker3 said:
That still baffles me - when you have 120/120/208V. It's different phases.
Click to expand...
That's true, but you never connect 3 wire loads directly to it, only 2 wire single phase loads, either L-N (120V) or L-L (208V). So the naming of single phase is based on what loads it supports.

Cheers, Wayne
 
jim dungar said:
Stop using the neutral as your reference point. Not all systems have neutrals, so your phase reference philosophy should not be dependent on one.

Context is everything. Understand some people use the term 'phase' as an adjective, like phase conductor, while others use it as a noun, like how many phases are there.
Click to expand...
I'm not using the neutral. 120.120 ,and 208 are different phases.
208 isn't the same phase as either of the 120 volts.
Even the original poster calls it 3-ph 4-wire.
 
Besoeker3 said:
I'm not using the neutral. 120.120 ,and 208 are different phases.
208 isn't the same phase as either of the 120 volts.
Even the original poster calls it 3-ph 4-wire.
Click to expand...
120, 120, and 208 are not different phases they are different voltages.
208 is the only phase voltage (line to line) because the 120V is measured line to neutral.
The system has three different sets of possible line to line voltages therefore it is called 3 phase. It also has a neutral point so it said to be 4 wire.
The OP is only using one of the sets of line to line voltage, so we call it single phase. This has been the common practice in our industry for almost 100 years.
 
jim dungar said:
120, 120, and 208 are not different phases they are different voltages.
208 is the only phase voltage (line to line) because the 120V is measured line to neutral.
The system has three different sets of possible line to line voltages therefore it is called 3 phase. It also has a neutral point so it said to be 4 wire.
The OP is only using one of the sets of line to line voltage, so we call it single phase. This has been the common practice in our industry for almost 100 years.
Click to expand...
But the 120-0-120 is a different phase to the 208. So, not single phase.
 
Besoeker3 said:
But the 120-0-120 is a different phase to the 208. So, not single phase.
Click to expand...
That is immaterial to how we, in North America, define electrical power systems.
As I said, ignore the neutral and then your 120-0-120 is no longer valid.

Back in engineering school, I don't think this was even discussed (I can find no text book presentation). We used a method that allowed us to easily deal with both Delta and Wye sources. Delta voltages were shown as Vl-l and Wye as Vl-n. Eventually textbooks simply shifted to using Vp and Vl.
 
jim dungar said:
That is immaterial to how we, in North America, define electrical power systems.
As I said, ignore the neutral and then your 120-0-120 is no longer valid.
Click to expand...
That's neither near not there. 120-0-120 would be single phase. But once you introduce a 208V the angles are different. Thus are not single phase.
I know I have laboured this point and I appreciate your patience.
 
Besoeker3 said:
That's neither near not there. 120-0-120 would be single phase. But once you introduce a 208V the angles are different. Thus are not single phase.
I know I have laboured this point and I appreciate your patience.
Click to expand...
Three phases at the 4 wire, Wye panel A, B, C, phase angle is 120°. Isn't 1Ø, 208 volts either A-B, A-C, or B-C?
 
I can't change the Conductors. That's the problem. They direct bury under a paved area. I'm stuck with the the 2 Hots and 1 Neutral per the diagram.
Can I make this work? Do I tie the incoming Neutral to Ground?
 
Installer said:
I can't change the Conductors. That's the problem. They direct bury under a paved area. I'm stuck with the the 2 Hots and 1 Neutral per the diagram.
Can I make this work? Do I tie the incoming Neutral to Ground?
Click to expand...
No you do not ground the neutral at the transformer primary. You need to change the neutral to an EGC.
 
infinity said:
No you do not ground the neutral at the transformer primary. You need to change the neutral to an EGC.
Click to expand...
Thank you INFINITY. I don't know transformers that well.
What am I missing?

1. Please remember I am stuck with 2 Hots and 1 Neutral conductor because its a direct bury under a busy area.
2. The Neutral is coming from the Neutral bus that is bonded to the Ground bus at the Panel.
3. I tied the Neutral to the Ground at the Counterpoise at the Site. (red circle)
4. What am I missing?
Thanks for your help DESIGN QUESTION.png
 
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