120/240V, 3-Phase Closed Delta Questions

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Volt-Amps

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Regards,
To my knowledge, we will always have a high leg with open or closed delta 3-phase 120/240V distribution. The designer at ComEd, the power utility company in Chicago, said they are not the same, and that we can use A, B and C legs just like 120/208, 3-phase. I'm worried if I populate the schedule this way and issue to the contractor, someone would be calling me in the middle of the night about a fire somewhere :unsure:. With open delta, we can't use the B phase for 120V, or 240V, 1-PH loads (please correct me if I'm wrong. How does the closed delta differ? there is a 208V in a closed delta between C and the neutral (or B and the neutral, not sure). is it usable? it's like a 1-pole breaker with hot and neutral connections that is a 208V breaker. is this correct? can I use it? and if I can,208V equipment usually have 2 points of connections to connect 2 hots and a ground, how do I feed it from a 1-pole breaker rated at 208V?
I guess there are multiple questions in this post, I hope I'm not confusing anybody, but based on the above, this is what it comes down to:
A-B phase: Can I connect 120V and 240V, 1-Phase here?
B-C phase: Can I connect 120V and 240V, 1-Phase here?
A-C phase: Can I connect 120V and 240V, 1-Phase here?

And the second question is, is there such an thing as a 1-pole, 208V breaker that can power 1-Phase, 208V piece of equipment (hot, neutral and ground).
Finally, what can I not do circuiting-wise on a 3-Phase, 4-Wire closed delta system with high leg?

Screenshot 2022-12-28 144727.JPG

Thanks
 
The designer is wrong or you misunderstood them. A high-leg system by definition (and obvious meaning) has one hot leg that is more than 120V to ground, as in your diagram. Whether open or closed.

The possibilities I can think of:
- designer is actually proposing 208/120V wye as an option
- you had a miscommunication with them
- they are unqualified for their job since they don't understand a high-leg
 
I’m a utility guy. I would never tell you that.
That “designer” is unqualified for his job title if he really told you that malarkey.

Open delta is for smaller three phase loads, along with single phase loads.
The delta is closed when the three phase loads get to be the bulk of the load.

NEVER use the B high leg for ANYTHING but 240V three phase loads.
The high leg goes to the far right in the meter base.
 
jaggedben said:
The designer is wrong or you misunderstood them. A high-leg system by definition (and obvious meaning) has one hot leg that is more than 120V to ground, as in your diagram. Whether open or closed.

The possibilities I can think of:
- designer is actually proposing 208/120V wye as an option
- you had a miscommunication with them
- they are unqualified for their job since they don't understand a high-leg
Click to expand...
The proposed system is definitly 120/240V, 3-Phase, 4-Wire, closed delta.
The high leg is 208V, which is higher than 120V, don't see a conflict with what you wrote unless I misunderstood.
 
Rock86 said:
You asked this question nearly 4 years ago.. https://forums.mikeholt.com/threads/120-240v-3-phase-3-transformers-on-pole-closed-delta.147256/
Still no resolve huh? I would design it as a normal 240 delta high leg.
Click to expand...
I sure did or a similar one, I do recall, but when the utility designer told me the closed delta is different, I got confused. I even told the architect I'm not aware of being able to utilize the B leg on any high leg system, open or closed, but I will check...hence this post :)
 
The utility engineer/ designer is creating all this confusion for me again, he is the second guy after my 2019 post who says the same thing.
To conclude, and I will follow your word vs. his:

A-B phase: Can I connect 120V and 240V, 1-Phase here? No I can't
B-C phase: Can I connect 120V and 240V, 1-Phase here? No I can't
A-C phase: Can I connect 120V and 240V, 1-Phase here? Yes, I can

Is this correct?
 
Volt-Amps said:
The utility engineer/ designer is creating all this confusion for me again, he is the second guy after my 2019 post who says the same thing.
To conclude, and I will follow your word vs. his:

A-B phase: Can I connect 120V and 240V, 1-Phase here? No I can't
B-C phase: Can I connect 120V and 240V, 1-Phase here? No I can't
A-C phase: Can I connect 120V and 240V, 1-Phase here? Yes, I can

Is this correct?
Click to expand...
If B is the high leg, then your red text is correct.
 
There is no difference between a closed delta and an open delta as far as the useable and unusable voltages go. The closed delta is better when there is a predominant amount of line-line connected loads (240V), like with an industrial facility, while an open delta is cheaper to install and thus is typical for predominant line to neutral loading (120V), like with a commercial building.
 
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I can only speculate why this utility office has misled you twice. But no one ever stopped me from speculating here.

One possibility is that someone in that office thinks that 'closed delta' is a synonym for a wye. That's just completely wrong.

Another possibility is they are trying to tell you something about using 240V and it's getting garbled and coming out as something about 120V.

On either an open or closed 240/120 high leg, you cannot get 120V to the high-leg, only between the neutral and two 'low' legs. Full stop.

However if you have 240V single phase (no neutral) loads, there's likely a difference between open and closed. Open is usually intended for a smaller 3-phase load that is actually three-phase 240 delta. With an open delta they probably do not want you to use the high leg for 240V single-phase (no neutral) loads. Or at any rate, not an unbalanced load that's larger than they sized the second transformer for. As Jim said, the voltage will be there. But you might get unbalanced voltages with too much load. A closed delta is probably sized the same on each phase so its not a problem to put additional 240V load on the high leg. So possibly they are trying to communicate something about 'don't load up the high leg with single phase 240V loads' on an open delta and it's just not getting communicated clearly.
 
One more…

Utility “designers” go by a few names in the utility world. Staking specialist, design specialist, job planner, etc…
They are older linemen that moved into a lower paying job, or could be someone that has never been in linework.
I have never met a “designer” that truly understood electrical theory.
 
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