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

[Installer]

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

 

[jim dungar]

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?

 

[David Castor]

You need a ground wire that connects to the 208/120 V panel ground. The neutral isn't needed.

 

[Besoeker3]

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........

 

[infinity]

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........

Basically he's showing two 120 volt to ground phase legs that are used to provide the 1Ø, 208 volts.

 

[jim dungar]

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........

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.

 

[wwhitney]

That still baffles me - when you have 120/120/208V. It's different phases.

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

 

[Besoeker3]

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.

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.

 

[petersonra]

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.

it works just like 400/230 V systems do in Europe, except it is 208/120 V.

 

[Besoeker3]

it works just like 400/230 V systems do in Europe, except it is 208/120 V.

It is just single phase for 230V. Not single phase.

 

[jim dungar]

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.

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.

 

[Besoeker3]

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.

But the 120-0-120 is a different phase to the 208. So, not single phase.

 

[jim dungar]

But the 120-0-120 is a different phase to the 208. So, not single phase.

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 V_l-l and Wye as V_l-n. Eventually textbooks simply shifted to using V_p and V_l.

 

[Besoeker3]

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.

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.

 

[infinity]

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.

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?

 

[Installer]

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]

If I tie the Neutral to Ground will this work?

 

[jim dungar]

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?

Why can't you change the neutral to be a ground back at the source?

 

[infinity]

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?

No you do not ground the neutral at the transformer primary. You need to change the neutral to an EGC.

 

[Installer]

No you do not ground the neutral at the transformer primary. You need to change the neutral to an EGC.

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