480 Delta or maybe not?

[physis]

One difference is how they behave if the system is bonded to ground.

Another difference is how zero sequence currents are handled.

I'll give you that one, I've never even heard of "zero sequence current". I love terminology that seems to be industry fabricated.

As far as wiring of equipment goes, 3-phase 3-wire is almost never dependent on whether the source is a delta or a wye.

First, I was never talking about how the equipment is wired. Second, you're getting close when you say:

3-phase 3-wire is almost never dependent on whether the source is a delta or a wye.

Edit: You still haven't described any differences.

 

[jim dungar]

I'll give you that one, I've never even heard of "zero sequence current". I love terminology that seems to be industry fabricated.

First, I was never talking about how the equipment is wired. Second, you're getting close when you say:

Edit: You still haven't described any differences.

If you have a 480Y/277V wye isolation transformer with the center point grounded (but not used as a neutral), its 3-wire output will behave differently, during a single line to ground fault, than will the output of a 480V delta with one corner grounded. The equipment install on these two systems require different ratings.

 

[winnie]

Given two transformers, both with the same KVA and the same impedance, both corner grounded, and both with only the three 'phase' conductors brought out. Both supplied from the same primary source.

Could you tell which transformer was which solely by making measurements on the secondary side?

Both of these would commonly be called 'corner grounded delta'. I agree that it would be incorrect to refer to the wye transformer itself as a delta, but I tend to agree with the description of the service. This is akin to calling a 120/208 service to an apartment, consisting of 2 ungrounded conductors and 1 grounded conductor, a 'single phase' service.

-Jon

 

[charlie]

I saw a case were the utility set a 480 Y transformer, ran all phases and neutral to a CT cabinet but only the the phase wires were from to the service disconnect which is a 2000 amp MCC that was setup as a delta. . .

This is scary. Both the NESC and NEC prohibit this practice. If the neutral is taken as a circuit conductor, I must go all the way to each service disconnecting means and I has to be grounded to the MGN by the serving electric utility and the grounding electrode system in the service equipment. 250.20(B)(2) in the NEC and rule 092B1 in the NESC.

. . . it's also really good to see Charlie Eldridge back again.

Thanks, and you also. :smile:

 

[Besoeker]

If you don't use a wye's center although it's coils in the transformer are still there they have no effect on the transformer's behaviour as a delta.

A different tack.
Each transformer configuration has three windings.

For delta, the windings are connected to form a closed triangle. It resembles a delta, hence the name. The three phases are taken from the corners of the triangle.

For wye, one end of each of the three windings is connected to a common point so it resembles the letter Y. The common point is the neutral. Even if it isn't used, the configuration is still wye.

 

[glene77is]

... but if you leave out the center of a wye you have a delta. ...

The Delta provides NO Inductively generated Neutral.
The Wye provides an Inductively generated Neutral.

The Ground lines are not Neutral lines. Different subject there.

Choosing not to use the Wye Neutral does not make it a Delta.
You could choose to run your equipment "As If" it were,
but that is an installation/usage issue.

I am glad the chosen method works for your motors.
Probably saves a lot of money and installation time.

 

[e57]

If you don't use a wye's center although it's coils in the transformer are still there they have no effect on the transformer's behaviour as a delta.

Items that you connect to it may know the difference..... (i.e. Motors)

Here they are with no grounds - they are still different.

 

[physis]

You guys are right, I'm sort a making a bit of a mess of the subject.

I'm sorry I misspelled your name Charles E., I notice you corrected that for me. :grin: It is good to see you back. The last I heard you're doing pretty well, that's good to hear.

 

[physis]

Let me add this though.

 

[templdl]

Jim dungar is correct.
A motor control center is a very good example.
The majority of MCCs that I have written proposals for are 3ph3w w/grd because all of the loads are 480v L-L-L. As such there are no L-N 277v loads and no neutral is required.
The MCCs are most often feed with a transformer with a 480Y/277 secondary where only the line conductors and Grd conductor are brought out to the MCC.
Any time that one has only 3ph loads does not mean that they have to be feed with a delta,
I always remind those who a specifying transformers about how they plan to ground the system.
Yes, there are some good reasons such as dealing with phase shift for example where D-D transformers are needed which have zero phase shift. Likewise, Y-Y transformers are often specified for the same reason. Where D-D costs the same as a D-Y transformer the Y-Y can needlessly increase the cost of the transformer 25% of more.
Then there are those who specify a 480Y/277-208Y/120 transformer because they have a 480Y/277 source which is not a good reason. I have had to explain this countless time as to why this is wrong.
The majority of 3ph transformers that I sell are D-Y and I do sell more Y-Y transformers than I do D-D transformers with the majority of Y-Y transformers being supplied to UPS OEMS.

 

[glene77is]

WINNIE,
THANKS,
You reminded me of the Kuphardt IBilio site,
and the text book I forgot to read.
BTW, your response is clearly worded.

 

[maloo]

Sorry I have been away and couldn't access a computer. I beleive there are some things I need to add but I am still hazy if this is an acceptable method. There is a grounding conductor that is run from the MCC to the CT cabinet in all phase pipes and in the CT cabinet there is a bonding jumper between the grounded (netrual) and the grounding wires.
Also there is no GFCI main. I am guessing someone is trying to loop hole around 230.95.

Thank you everyone for your imput.

BTW, is there anywere that states a ground detector is required other then say healthcare.

 

[augie47]

In direct amnswer to your question.....250.21(B) requires ground fault detection on ungrounded systems
however...
I have reread the postings and may have missed something, but I am of the opinion if the neutral is present at the utility, it must be brought to the service per 250.24, so "ungrounded" detection would not be relevant here.

 

[jim dungar]

I am guessing someone is trying to loop hole around 230.95.

The only loophole to 230.95 is to keep the OCPD size below 1000A. If the utility bonds the X0 to ground it must be brought to the service equipment, not just the meter.

If the utility does not bond the X0 then 230.95 is not in play and ground detectors are now required (they used to be an FPN).

 

[maloo]

My thought was if it had truly been feed by a 480 delta transformer and not a solidly grounded wye then there would be no need for a GFCI main.
I do agree that the netrual should have be present at the main and it should be a GCFI in its current state. If the transformer wasn't bonded why would the utility bring the neutral to the CT cabinet.

 

[charlie]

The post about 14/3 and double pole afci for new CH panels was moved to its own thread.

 

[glene77is]

...The classic non-argument argument...

Charlie,

I think this is called a logical fallacy of "Non Sequitur",
and, if I read it right,
it verges on a "Non Distributed Middle Term".

I hope the weather breaks,
so you all can go back to work!

 

[petersonra]

I have seen a fair number of 480V wye services where the neutral leg ends at the service point and is never actually used anywhere.

You can even add a kit to many MCCs to allow them to be a service panel for wye services. It looks to me like a one pole distribution block that is added next to the main disconnect. The innards of the MCC are otherwise identical.

I am not sure why but I am told Comed started requiring many small 480V/3 phase services to be wye instead of delta. I am not sure it is a consistent policy, as I don't deal with it all that much.

 

[charlie]

Take a grounded neutral or don't ground it

Take a grounded neutral or don't ground it

. . . 250.20(B)(2) in the NEC and rule 092B1 in the NESC. . .

It seems strange to quote myself.

Rule 092B1 in the NESC states in part, "The grounding connections shall be made at the source, and at the line side of all service equipment."

250.20(B)(2) in the NEC states, "Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor"

The bottom line is that the neutral is the current return path. If the neutral is grounded at any point and it is not taken to the service equipment, a bad accident may happen at some time in the future. In other words, take a grounded neutral or don't ground it!

 

[LarryFine]

Doesn't the POCO run the system neutral/EGC to the service equipment for grounding even if the secondary system itself is not bonded?