Grounding and bonding for high leg center tapped delta transformer

M

muddyboots

Member
Location
Florida
Occupation
Electrician
I have a 3 phase 45 kva, step down, 480 delta to 240/120 high leg delta with a center tap nuetral, and I'm a little confused with regard to grounding and bonding.
Xfmr Primary: 70A 3ph (Cu)3-#4 + #8 EGC.
Xfmr Secondary: 150A 3ph (Al)4-2/0 + #2 SSBJ
GEC: (Cu) #4
This is one of many step down xfmrs at a new construction, production facility, with GECs being bonded to the grounding system through building steel.
One thing that's throwing me off is the riser diagram is calling for 3ph 4 wire, but the panel schedule is calling for 3ph 3 wire, which, that may be neither here nor there.
I have my primary EGC bonded to transformer case, and my secondary SSBJ bonded to the transformer case. Does the GEC bond to the transformer case as well? Or does it have to terminate to x-4? Similar to how you would terminate your GEC to x-0, in a typical step down 480 delta to 208/120 wye xfmr, with a bonding jumper bonded to xfmr case?
The buss on the x-4 is sized to only accommodate one lug, which seems to indicate there is no bonding jumper. Is there even a nuetral ground bond required on this type of system, being that it's separately derived? Is that taken care of internally? Is the nuetral even a "grounded conductor" in a center tap high leg delta system? If this were to be a corner grounded high leg delta transformer, what would be different? Am I even in the ballpark?
Regardless I appreciate any advice and insight you guys may be able to provide me, and I appreciate you taking the time out your day. I hope this is an appropriate thread for this forum and doesn't veer too far off topic. Thanks again. P.S. I Tried to upload a picture to this post not sure if it made it through though.

~muddyboots
 
muddyboots said:
I have a 3 phase 45 kva, step down, 480 delta to 240/120 high leg delta with a center tap nuetral, and I'm a little confused with regard to grounding and bonding.
...
This is one of many step down xfmrs at a new construction, production facility, with GECs being bonded to the grounding system through building steel.
Click to expand...
I'm a bit surprised to see high-leg delta being used in new construction, but I'll take your word for it as far as answering the questions below.
muddyboots said:
I have my primary EGC bonded to transformer case, and my secondary SSBJ bonded to the transformer case. Does the GEC bond to the transformer case as well? Or does it have to terminate to x-4? Similar to how you would terminate your GEC to x-0, in a typical step down 480 delta to 208/120 wye xfmr, with a bonding jumper bonded to xfmr case?
Click to expand...
I can't answer what's most typical, but the NEC gives you the option to land the GEC at the transformer or at the first means of disconnect for the secondary, provided you put your system bonding jumper at the same location.

muddyboots said:
The buss on the x-4 is sized to only accommodate one lug, which seems to indicate there is no bonding jumper. Is there even a nuetral ground bond required on this type of system, being that it's separately derived? Is that taken care of internally? Is the nuetral even a "grounded conductor" in a center tap high leg delta system?
Click to expand...
Assuming you truly have high-leg delta, the NEC requires the neutral to be grounded and bonded. So, yes, it is a grounded conductor. A "neutral-to-ground bond" is required for any grounded system under the NEC ; technically it's the System Bonding Jumper, see 250.30(A)(1). Not bonding the neutral in a separately derived system would be bad.

muddyboots said:
If this were to be a corner grounded high leg delta transformer, what would be different? Am I even in the ballpark?
Click to expand...
In a corner grounded system there would be no neutral, just three phase conductors, one of which would be grounded. Not totally unlike a residential 120/240 system, except the voltages and phase angles would be totally difference. It's a different animal from a high-leg delta.

One thing that's throwing me off is the riser diagram is calling for 3ph 4 wire, but the panel schedule is calling for 3ph 3 wire, which, that may be neither here nor there.
Click to expand...
Possibly some circuits don't need the neutral, or possibly you've misunderstood the system design and it isn't high-leg-delta. Either way I'd consult the designer and transformer nameplate and make sure the equipment matches the designer's intent and you know how it's supposed to be wired.
 
Thanks for the reply and all the info. Definitely answered all my questions, and will use this as a reference in explaining to others.
Yes I thought it surprising to see the high leg as well. There will be 4 240v welding machines, couple single phase 'hand welders', receps and 120v motorized roll up door fed from this panel so I suppose I understand their logic from that perspective but still i know these can be a rarity these days. The panel has some of the B phase breakers blanked off with a 'warning: do not use' sign. My boss was thinking it was a corner grounded system but based on submittals for the panel and transformer, they make it pretty clear they want the center tapped high leg delta system. The nameplate on the transformer seems to reflect that as well.
Thanks again for the advice, and code references. If I may ask, as far as voltage readings what can I expect if wired properly?
My guess:
X1-g -> 120v
X2-g -> 208v (b phase high leg)
X3- g -> 120v
X1-x3 -> 240v
X1-x4 -> 120v
X2-x4 -> 208v
X3-x4 -> 120v
Or
?
X1-x4 -> 240 v
x3-x4 -> 240v
Logic telling me but I'm overthinking all of this
?
Also, if hypothetically it was their intent to have a corner grounded system can this kind of transformer be reconfigured to work as one? Maybe by omitting any wiring from x4 on the transformer? Not that I intend to do that, just trying to better understand this kind of system. Thanks again for your time.
 
The high-leg delta is a 120/240v 1ph source and a 240v 3ph source, superimposed on each other.

You may wonder why anyone would want a high-leg service. It may help to understand that the (open) delta began as a 3ph modification to existing 1ph services, where a single piece of 3ph equipment was needed. Thus, the center-tap neutral was pre-existing.
 
muddyboots said:
Also, if hypothetically it was their intent to have a corner grounded system can this kind of transformer be reconfigured to work as one? Maybe by omitting any wiring from x4 on the transformer
Click to expand...
You are correct about skipping an X4 connection.

But they definitely do not want a 3 wire corner-grounded system, as they have 120V loads.
 
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Thank you both for the reply, that does make a lot more sense to me thinking about it like that, Larry. And good to know I am on the right track with the system and on the same page with the owner.
 
muddyboots said:
Thanks again for the advice, and code references. If I may ask, as far as voltage readings what can I expect if wired properly?
My guess:
X1-g -> 120v
X2-g -> 208v (b phase high leg)
X3- g -> 120v
Click to expand...
Yes,
FYI watch your breaker type,slash rated 120/240. Shall be rated for the highest voltage to ground,208.
See it done wrong a lot lately on 3 sites. Worse thing inspector did not red tag.

Use panel and breakers rated for the system voltage.
muddyboots said:
X1-x3 -> 240v
Click to expand...
Yes; Add
X 2 -X 3, 240v
X 1 - X 2, 240v
muddyboots said:
X1-x4 -> 120v (A)
X2-x4 -> 208v (B) as noted *
X3-x4 -> 120v (C)
Click to expand...
Yes,
120 x 1.732= 207.84
208 / 1.732 = 120.09

muddyboots said:
Or
?
X1-x4 -> 240 v
x3-x4 -> 240v
Click to expand...
NO
muddyboots said:
Yes I thought it surprising to see the high leg as well. There will be 4 240v welding machines, couple single phase 'hand welders', receps and 120v motorized roll up door fed from this panel so I suppose I understand their logic from that perspective but still i know these can be a rarity these days. The panel has some of the B phase* breakers blanked off with a 'warning: do not use' sign.
Click to expand...
Add, for 120v
 
muddyboots said:
The buss on the x-4 is sized to only accommodate one lug, which seems to indicate there is no bonding jumper.
Click to expand...
Yes use a system bonding jumper to the terminal bar that the GEC ,SSBJ,primary EGC lands on inside the transformer so you have a grounded system.
muddyboots said:
Is there even a nuetral ground bond required on this type of system, being that it's separately derived?
Click to expand...
Yes, for 4 wire. Once you install the SBJ noted above.
muddyboots said:
Is that taken care of internally?
Click to expand...
Internally?
Within the core No, it's simply a tap between X-1 and X-3 to get X -4
Within the transformer enclosure it is as noted above via the SBJ.
muddyboots said:
Is the nuetral even a "grounded conductor" in a center tap high leg delta system?
Click to expand...
Yes, once the SBJ is installed to the transformer thermal bar where the GEC,SSBJ and primary EGC is located since your doing your grounding in the transformer.
muddyboots said:
If this were to be a corner grounded high leg delta transformer, what would be different?
Click to expand...
You would have to choose one can not have both, high leg andcorner grounded.
Then you SBJ is connected accordingly.
High leg 4 wire:
As noted above SBJ to X-4.

Corner grounded 3 wire.
SBJ to one of the three X-1,2 or 3
Usually X-2. Then nothing on X4.
I like to place a piece.of shrink wrap on X-4 so some one will ask why.

One reason to take GEC, SSBJ and primary EGC to a single bar then run SBJ to the correct tap X- 4 or X-2 depending on system required. NEC also requires you to terminate on a single bar. I find this the best way to handle all the bonding and grounding requirements.

Be sure to size all conductors correctly using the correct tables.
250.122 ( primary EGC)
250.102 (SBJ and SSBJ and neutral if reduced in size)
250.66 (GEC)

I like to use a double barbell lug for the SBJ and X-4. This way I don't worry about a loose lug cause voltage balance issues on the 120 side.
 
Wow, very impressed with all the information from everyone. This website exceeds my expectation everytime I turn to it! I think I am finally starting to wrap my head around all this, which is helping with my understanding of transformers as a whole.
I have it wired as described in post #10, with grounding and bonding done within the xfmr. With paying close attention to sizing requirements for EGC, GEC, and bonding jumpers respectfully. Neutral is sized at the size of ungrounded conductors. I did notice the breakers are dual rated as well. I have also utilized the double barbell lugs, and agree with the logic behind it.
I'm glad I was leaning in the right direction as far as voltage readings. Definitely going to share this info with my boss and peers.
As far as the 2 holes on the bus bar, and me only saying I have provisions for only one lug:
I have worked in a jurisdiction where the ahj required the transformers termination lugs to match the whole pattern on bus bar. So two holes vertically spaced meant 1-2 hole lug, with hardware matching hole size and not smaller. I am assuming this was just an overly particular inspector and that there is not anything in the code for this rule, maybe something in the UL? But it was a rule agreed upon during bidding process so it was hammered into my brain.
I do have one more question as far as primary OCPD for transformers, which I hope is not out of pocket for this post. I know from table 450.3(b) that the Maximum primary OCPD for transformers rated 9A or more is 125% of current rating of the transformer. What if any would be the minimum OCPD? Does that mean (hypothetically) you can use a breaker between 0 and not more than 125% of FLA of the transformer, with the exceptions noted factored in as well? Obviously practically speaking the inrush current of the transformer will be a large factor in the size required. But is there an actual code section that addresses minimum primary ocpd, or does it naturally come about due to inrush tripping?
For example: 30kva 480d -> 208/120y with a FLA of 36A. The Primary OCPD is sized at 30A with #10 (Cu)wire, secondary OCPD sized at 100A with #1 (Al). Is there anything in the code that prohibits this install? I understand load calcs would come into play, and inrush. Seems a little wonky to use a breaker lower than FLA but would it be 'illegal' according to the NEC? I have begun to look up inrush current for different sizes of transformers but not their respective breakers.
Anyways, thanks again for all the advice and insight. Starting to feel more comfortable with my understanding of transformers in general, especially in regard to phase angles and resulting voltages.
 
muddyboots said:
Wow, very impressed with all the information from everyone. This website exceeds my expectation everytime I turn to it! I think I am finally starting to wrap my head around all this, which is helping with my understanding of transformers as a whole.
I have it wired as described in post #10, with grounding and bonding done within the xfmr. With paying close attention to sizing requirements for EGC, GEC, and bonding jumpers respectfully. Neutral is sized at the size of ungrounded conductors. I did notice the breakers are dual rated as well. I have also utilized the double barbell lugs, and agree with the logic behind it.
I'm glad I was leaning in the right direction as far as voltage readings. Definitely going to share this info with my boss and peers.
As far as the 2 holes on the bus bar, and me only saying I have provisions for only one lug:
I have worked in a jurisdiction where the ahj required the transformers termination lugs to match the whole pattern on bus bar. So two holes vertically spaced meant 1-2 hole lug, with hardware matching hole size and not smaller. I am assuming this was just an overly particular inspector and that there is not anything in the code for this rule, maybe something in the UL? But it was a rule agreed upon during bidding process so it was hammered into my brain.
I do have one more question as far as primary OCPD for transformers, which I hope is not out of pocket for this post. I know from table 450.3(b) that the Maximum primary OCPD for transformers rated 9A or more is 125% of current rating of the transformer. What if any would be the minimum OCPD? Does that mean (hypothetically) you can use a breaker between 0 and not more than 125% of FLA of the transformer, with the exceptions noted factored in as well? Obviously practically speaking the inrush current of the transformer will be a large factor in the size required. But is there an actual code section that addresses minimum primary ocpd, or does it naturally come about due to inrush tripping?
For example: 30kva 480d -> 208/120y with a FLA of 36A. The Primary OCPD is sized at 30A with #10 (Cu)wire, secondary OCPD sized at 100A with #1 (Al). Is there anything in the code that prohibits this install? I understand load calcs would come into play, and inrush. Seems a little wonky to use a breaker lower than FLA but would it be 'illegal' according to the NEC? I have begun to look up inrush current for different sizes of transformers but not their respective breakers.
Anyways, thanks again for all the advice and insight. Starting to feel more comfortable with my understanding of transformers in general, especially in regard to phase angles and resulting voltages.
Click to expand...
As you said, it is pretty much based on inrush, but the code doesn't care if the primary breaker trips every time you try to energize the transformer, so there is no minimum rule in the code.
 
muddyboots said:
Thanks again for the advice, and code references. If I may ask, as far as voltage readings what can I expect if wired properly?
My guess:
X1-g -> 120v
X2-g -> 208v (b phase high leg)
X3- g -> 120v
Click to expand...
I need to revise my yes answer.
As others mentioned and looking at the picture.
X-1 (A) and X-2 (C) to G/N (X-4) will be 120v
X-3 (B) is the high leg 208 to G/N(X-4)
So B phase will go to X-3 since you have labels for your panel say do not use.

If you have a pic of the cover with please post. It will show wiring information etc.

Sorry for the yes answer.
Thanks to others for pointing it out.
 

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Tulsa Electrician said:
I need to revise my yes answer.
As others mentioned and looking at the picture.
X-1 (A) and X-2 (C) to G/N (X-4) will be 120v
X-3 (B) is the high leg 208 to G/N(X-4)
So B phase will go to X-3 since you have labels for your panel say do not use.

If you have a pic of the cover with please post. It will show wiring information etc.

Sorry for the yes answer.
Thanks to others for pointing it out.
Click to expand...
Yes, I'm glad you pointed this out as well. I did realize part way through reading this thread and some square d documentation, that my high leg is x-3 not x-2, as I noted. I should have corrected myself, but I understood your answer and because it was based on the (incorrect)assumptions I wrote I did not want to confuse the situation further, if that makes sense haha. I very much appreciate the special attention every one takes to make sure we are holding each other accountable and most importantly executing our work safely!
Somehow I seem to have a picture of everything but the faceplate. But I will post on Monday when I'm back on site.
 
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