Possibility of 120/208 3 phase open delta?

[BTC]

Hi there. I am an electrical engineer who has seem 120/240V, 3 phase, 4 wire open delta (high leg) systems in the past. Question: is 120/208V 3 phase open delta a possibility as well? don't think so, but looking for feedback. Thanks in advance.

 

[roger]

No, however the high leg to grounded conductor will measure 208.

Roger

 

[jim dungar]

No, it is not a possibility. the relationship between 208V L-L and 120V L-N does not occur in a delta configuration.

However, it is possible to have an 'open wye' 120/208V connection, although this would not be a common name for it. Two legs (208V L-L) and one neutral (120V L-N) are not uncommon (e.g. in apartment complexes).

 

[electrofelon]

Hi there. I am an electrical engineer who has seem 120/240V, 3 phase, 4 wire open delta (high leg) systems in the past. Question: is 120/208V 3 phase open delta a possibility as well? don't think so, but looking for feedback. Thanks in advance.

Just an additional minor note, you can have a 120/240 high leg system with either an open or closed delta. In some parts of the country it is very uncommon to see the closed variety. In others, it's the opposite.

 

[grich]

Just an additional minor note, you can have a 120/240 high leg system with either an open or closed delta. In some parts of the country it is very uncommon to see the closed variety. In others, it's the opposite.

I have a northern MO client with a 120/240 high-leg closed delta, and another client in southern IA with an open delta...only two primary phases on the pole at this site, the third phase got left behind a couple miles upstream.

 

[jim dungar]

I have a northern MO client with a 120/240 high-leg closed delta, and another client in southern IA with an open delta...only two primary phases on the pole at this site, the third phase got left behind a couple miles upstream.

An open delta still require all three phase conductors, it is simply missing one set of transformer windings and core.
An open wye requires two phase conductors plus a neutral.
You may have a 'ground return' neutral. I have never heard of a ground return phase conductor.

 

[electrofelon]

An open delta still require all three phase conductors, it is simply missing one set of transformer windings and core.
An open wye requires two phase conductors plus a neutral.
You may have a 'ground return' neutral. I have never heard of a ground return phase conductor.

That is how Seattle City Light does their open delta banks, even when all three phases are right there. Here is the construction plans:

http://www.seattle.gov/light/engstd/docs2/0125.02.pdf

 

[BTC]

That is how Seattle City Light does their open delta banks, even when all three phases are right there. Here is the construction plans:

http://www.seattle.gov/light/engstd/docs2/0125.02.pdf

Thanks, that looks like a good reference document.

 

[Hv&Lv]

How about a two pot 120/208 three phase from two primary phases?

 

[jim dungar]

It looks like Seattle is using an open-wye primary to open-delta secondary transformation.

 

[jim dungar]

??

A delta connected primary requires all three phases, even if it is 'open' (only using two sets of transformer windings).
If it uses a neutral point, then it is an open wye primary.

 

[Hv&Lv]

A delta connected primary requires all three phases, even if it is 'open' (only using two sets of transformer windings).
If it uses a neutral point, then it is an open wye primary.

Yes, I know...
i would go so far as to guess 90 percent of what people call an open delta is really an open wye, open delta bank.
if it is two phases on the primary it is...

my iPad keeps asking me to accept cookies to post, Im having a hard time posting...

maybe time to restart.

 

[jim dungar]

i would go so far as to guess 90 percent of what people call an open delta is really an open wye, open delta bank.

 

[grich]

At least I was kinda half right. Open wye open delta explains this site perfectly...never thought much about the primary side. There are two pots (50 and 100) and the high leg is on the C phase of the panel. It's the only site I care for with an open delta secondary.

 

[jim dungar]

...never thought much about the primary side...

Yeah, I can appreciate that. We often think of our side of a transformer without considering the other side which is usually independent.