Going from delta to wye?

[VernB]

Greetings all!
So here I am, plenty familiar with 120/208Y and what do they hand me to connect to? 208D *sigh*.
I see that most of the 208D in the building have been outfitted with 1P 208(240) to 120/240 transformers. This seems a bit inefficient to me (at the very least it looks like it would unbalance the heck out of the 3P).
Would someone gut check me that it would be more correct to use a 3P 208D to 120/208Y transformer in this app?

Vern

 

[George Stolz]

Vern, what is a 208 Delta transformer?

Delta transformers are generally 120/240....?

 

[VernB]

Vern, what is a 208 Delta transformer?

Delta transformers are generally 120/240....?

The incoming power is 208D. The current transformers are 1P 240 (primary, apparently tapped down to 208) to 1P 120/240 (secondary). What I'm talking about is using a transformer of 3P 240D (primary, tapped down to 208) to 120/208Y (secondary). I don't see Square D listing any transformers of 240D to 1P 120/240.

What I'm questioing here is the efficiency of using a single phase transformer from the three phase power vs a three phase transformer from the three phase power.

Vern

 

[e57]

:-? :-? huh? :-? :-?

 

[LarryFine]

The only thing I can envision as 208-D would be 208-Y without the neutral. Just out of curiousity, test your line-to-ground voltage.

 

[al hildenbrand]

Vern,

I'm just guessing here, . . .

But,

It sounds like you have a space in an existing structure with an existing feeder or branch circuit in it that used to supply something. It sounds like that "something" was a 208 Volt 3-wire 3 ? load.

Now, that "something" is gone, the 3-wire feeder (or branch circuit) is still there, and you are being asked to put it to use for another purpose.

Still guessing here, but, are the new loads to be simple 120 Volt single phase loads like lights and receptacles in an office?

 

[winnie]

It would certainly be possible to wind a transformer with a 208V delta secondary. There are some weird wired systems out there. But I expect that this really is a 120/208V wye service where for whatever reason the neutral is not brought out. Though I believe that such would be a code violation, and that at this voltage a wye system is required to be grounded.

In any case, with a three wire three phase feed, a single phase transformer would load only 2 of the 3 lines. But if you are talking multiple transformers distributed around the building, with loads balanced amongst the 'distribution' phases, then I see no significant efficiency difference between using single phase versus three phase transformers. Remember that a three phase transformer is simply 3 single phase transformers sharing a yoke.

Are you considering changing out the single phase transformers and replacing them with three phase? Or are you considering additions to this system?

On the secondary side, the 120/240V single phase panels would have the _benefit_ of being insensitive to harmonics (which are likely not a problem anyway). If you were to change to 120/208V panels, all of the neutrals in any multi-wire-branch-circuits would become current carrying conductors, and you would reduce the supply voltage to any two pole loads.

I would not consider changing any of the transformers to three phase transformers.

I might try to hunt down the service neutral to see if transformers could be eliminated.

-Jon

 

[kingpb]

Would you guys quit using the wrong voltage designation..........geez

It is 208Y/120V, unless of course you really do mean a 120/208Y (single phase open wye derived from a 3 phase 4 wire system)

I feel better now, carry on!

 

[kingpb]

Greetings all!
So here I am, plenty familiar with 120/208Y and what do they hand me to connect to? 208D *sigh*.
I see that most of the 208D in the building have been outfitted with 1P 208(240) to 120/240 transformers. This seems a bit inefficient to me (at the very least it looks like it would unbalance the heck out of the 3P).
Would someone gut check me that it would be more correct to use a 3P 208D to 120/208Y transformer in this app?

Vern

Whay not put a zig-zag transformer on the 208V Delta connected side to create a neutral, then you don't need any of the single phase transformers at all (assuming they are for nothing other then deriving a neutral)

 

[winnie]

It is 208Y/120V

You are absolutely correct that this is the designation that is supposed to be used for three phase wye service with 120V line to neutral and 208V line to line.

Unfortunately the designation is totally arbitrary; some standards body decided 'wouldn't it be neat if 208Y/120V meant something different from 120/208Y'. As such, I can never remember which it is, and don't have a logical chain that I can follow to tell me which it is. So I keep getting this thing wrong. *sigh*

-Jon

 

[cpal]

Whay not put a zig-zag transformer on the 208V Delta connected side to create a neutral, then you don't need any of the single phase transformers at all (assuming they are for nothing other then deriving a neutral)

If this delta ??? supply is actually a 3 phase feeder or BC with out the grounded circuit conductor the zig zag may be an issue.

450.5 Grounding Autotransformers.
Grounding autotransformers covered in this section are zigzag or T-connected transformers connected to 3-phase, 3-wire ungrounded systems for the purpose of creating a 3-phase, 4-wire distribution system or providing a neutral reference for grounding purposes. Such transformers shall have a continuous per-phase current rating and a continuous neutral current rating. Zig-zag connected transformers shall not be installed on the load side of any system grounding connection, including those made in accordance with 250.24(B), 250.30(A)(1), or 250.32(B)(2).

 

[al hildenbrand]

I think we need more info from Vern, first.

If all he's connecting is single phase, and he has load that is 120 V and/or will accept 208 V (as opposed to 240 V), then he might just be able to reassign one hot supply conductor to use as a neutral. . .assuming he can get into the supply side of his source feeder or branch circuit and actually make the alteration.

 

[jim dungar]

Unfortunately the designation is totally arbitrary; some standards body decided 'wouldn't it be neat if 208Y/120V meant something different from 120/208Y'.

Jon,

I am sure that IEEE will be glad to hear that it made an arbitrary decision to use ANSI-C84.1-1989.

 

[winnie]

I am sure that IEEE will be glad to hear that it made an arbitrary decision to use ANSI-C84.1-1989.

Standards are necessary.

Most standards are arbitrary, at least in part. For most standards, you have some arbitrary components, and then logic and physics dictate the rest.

I accept this standard, and will try to remember it and use it.

But the numbers could just as easily been the other way around.

-Jon

 

[jim dungar]

But the numbers could just as easily been the other way around.

Jon,

How often have you ever heard anyone say 277/480? My guess is most people learned the bad habit of saying 120/208 just because it sounds like 120/240.

Yes standards are necessary. Just in this forum, there have been many tangential answers to questions because the two parties did not understand what each was describing.

ANSI C84.1 contains the US nominal voltages and how to describe them. I believe this standard has been in existence at least since the 1920's with the only major recent change being the nominal voltages in the 50-60's. The order of the voltages, the seperator (/ or X), and the location of the letter Y all mean something.

 

[VernB]

The only thing I can envision as 208-D would be 208-Y without the neutral. Just out of curiousity, test your line-to-ground voltage.

The power entrances are all aerial so it's easy to see what's coming in. There's a 208Y entrance with 4 conductors in the drop and one side of the secondaries of the three transformers is tied to a common conductor (the neutral). On the entrance in question, there's only 3 conductors in the drop and there is no common connection among the 3 transformers, so I'm sure it's a delta configuration.

Also the main entrance breaker is also marked "208 Main Supply".

Vern

 

[George Stolz]

Jim, I ask these questions as a student, not as an adversary:

The order of the voltages, the seperator (/ or X), and the location of the letter Y all mean something.

Can you elaborate? I've never seen an "X" as a seperator. What does it mean?

What is the significance of the location of the "Y"?

What is a zig-zag transformer?

It is 208Y/120V...

My apologies for offending your delicate sensibilities, I have miswritten nominal voltages in the past as well.

Incidentally, I hope this post is up to your quality standards. I'd hate to waste anyone's time. :roll:

 

[VernB]

Vern,

It sounds like you have a space in an existing structure with an existing feeder or branch circuit in it that used to supply something. It sounds like that "something" was a 208 Volt 3-wire 3 ? load.

Now, that "something" is gone, the 3-wire feeder (or branch circuit) is still there, and you are being asked to put it to use for another purpose.

Still guessing here, but, are the new loads to be simple 120 Volt single phase loads like lights and receptacles in an office?

Yup, this is an old mill, all the original equipment was 208 3P and the lighting was all fluorescent with 220V ballasts so they never had any provision for anything else except 208V in the older areas of the building. Loads will be standard 120V.

Vern

 

[VernB]

Are you considering changing out the single phase transformers and replacing them with three phase? Or are you considering additions to this system?

I would not consider changing any of the transformers to three phase transformers.

I might try to hunt down the service neutral to see if transformers could be eliminated.

-Jon

I've confirmed from visual inspection of the POCO transformers, this is definitely delta, not wye. There's only 3 phase conductors in the drop from the pole, no neutral.

Vern

 

[VernB]

I think we need more info from Vern, first.

If all he's connecting is single phase, and he has load that is 120 V and/or will accept 208 V (as opposed to 240 V), then he might just be able to reassign one hot supply conductor to use as a neutral. . .assuming he can get into the supply side of his source feeder or branch circuit and actually make the alteration.

Al, that's a confirm, 120V for most of it and anything that needs 240 will be just fine with 208.

Vern