Grounding and Bonding Multiple Transformers

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What is the proper grounding/bonding for multiple step down transformers.

Lets sat I have a 12470V/480V DELTA/WYE transformer and my service entrance panel is 277/480 panel that has the EGC from the grounding system.

From this service entrance panel, I have a circuit breaker feeding a 480V/208V WYE/WYE transformer in the same building in the same room. This 480V/208V transformer feeds a 120/208V Panel.


Does a ground wire get carried from the the 480V service entrance panel all the way through the 120/208V transformer to the 120/208V panel?

Does the 120/208V transformer need its own grounding electrode system? If yes, does it get bonded to the ground conductor from the 277/480 panel?


Thank you for your help.

EE_Intern_2
 

don_resqcapt19

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First it would be very rare to use a wye/wye transformer for this application. It would normally be a delta/wye.
You would bring an equipment grounding conductor with the 480 volt feeder conductors to the transformer and the EGC would connect the the non-current carrying metallic parts of the transformer.
The 208/120Y side needs its own grounding electrode conductor. The system bonding jumper can be installed at the transformer or at the first overcurrent protective device. You will need a supply side bonding jumper between the transformer and the first OCPD.
 
Hi don_resqcapt19

I understand this part "First it would be very rare to use a wye/wye transformer for this application. It would normally be a delta/wye. You would bring an equipment grounding conductor with the 480 volt feeder conductors to the transformer and the EGC would connect the the non-current carrying metallic parts of the transformer."

Can you please confirm the following.

1.So what you are saying is that the 208/120Y Panel needs its own grounding electrode conductor to the existing building ground system?

2.What's the difference between the "system bonding jumper" and the "supply side bonding jumper" that you mention?

3. If the 480V panel and the 208V panel both have a grounding electrode conducter to the same ground electrode they are essentially bonded.

Thanks,
EE_Intern_2
 

don_resqcapt19

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retired electrician
Hi don_resqcapt19

I understand this part "First it would be very rare to use a wye/wye transformer for this application. It would normally be a delta/wye. You would bring an equipment grounding conductor with the 480 volt feeder conductors to the transformer and the EGC would connect the the non-current carrying metallic parts of the transformer."

Can you please confirm the following.

1.So what you are saying is that the 208/120Y Panel needs its own grounding electrode conductor to the existing building ground system?
You need a grounding electrode per 250.30(A)(4). In many cases, yes, the SDS will have a grounding electrode conductor run to the existing building grounding electrode.

2.What's the difference between the "system bonding jumper" and the "supply side bonding jumper" that you mention?
A supply side bonding jumper is required between the transformer and the first overcurrent protective device per 250.30(A)(2). The system bonding jumper is the equivalent of the main bonding jumper on a service. It is the connection between the EGCs, the GEC and the grounded conductor. This can be installed either at the transformer or at the first OCPD.

3. If the 480V panel and the 208V panel both have a grounding electrode conducter to the same ground electrode they are essentially bonded.

Thanks,
EE_Intern_2
Yes, all grounded systems in a building will be bonded together in a code compliant installation.
 

hurk27

Senior Member
Don has covered everything else well but I just want to point out that since you have a WYE primary on this transformer do not bond the X0 as this will cause all kinds of heating problems with the primary EGC and can cause a failure of the transformer as it can cause it to run hotter, leave the X0 floating on the primary side, I'm sure you already knew this but just wanted to make sure, with a common core transformer this is a big no-no.
 

hurk27

Senior Member
Hi don_resqcapt19

I understand this part "First it would be very rare to use a wye/wye transformer for this application. It would normally be a delta/wye. You would bring an equipment grounding conductor with the 480 volt feeder conductors to the transformer and the EGC would connect the the non-current carrying metallic parts of the transformer."

Can you please confirm the following.

1.So what you are saying is that the 208/120Y Panel needs its own grounding electrode conductor to the existing building ground system?
This can also be run to the nearest grounding electrode that is also use by the 480 supply such as a metal structure of the building.

2.What's the difference between the "system bonding jumper" and the "supply side bonding jumper" that you mention?

Don has covered it already but to add some more info:

This is some wording changes to the 2005-2011 in 250.28 and .30 the wording "system bonding jumper" replaced the "main bonding jumper" between the grounded (neutral) and case of the transformer, or between the grounded conductor and the grounding bar at the first disconnect which can also be the bonding screw we install in a service panel or disconnect.

2002 just use the term "bonding jumper" the 2005 used both the term "main bonding jumper" and "system bonding jumper" and in the 2008 it was changed to just "system bonding jumper"

Supply side bonding jumper is any conductor used as an EGC ahead of any overcurrent protection but on the secondary side of the transformer, it is also the conductor that is used to bond the conduits to the cases, this conductor can also be the raceway that encloses the secondary conductors to the first disconnect, it is sized from table 250.66 same as a grounding electrode conductor except a GEC stops at a 3/0 these conductor must use the 12.5% rule after this point as found in 250.66

As above for the system bonding jumper, the 2002 used the term "equipment bonding jumper" this was used all the way up to the 2011 which changed it to todays "supply side bonding jumper" which some confuse with it meaning that this is some kind of bonding jumper on the primary side, but as said above it is any bonding jumpers that are on the secondary side of the transformer but at or ahead of the first disconnect.

3. If the 480V panel and the 208V panel both have a grounding electrode conductor to the same ground electrode they are essentially bonded.

Thanks,
EE_Intern_2

Yes the grounding of the secondary is required to be bonded to the grounding of the primary, otherwise if you have circuits that are off the secondary side that fault to something grounded to the primary side it might not cause a breaker to trip and you could have a voltage potential between the two systems that someone could come into contact with, so with the primary EGC bonded to the transformer case, and the secondary X0 is also bonded to the transformer case of grounding bar, you will have an effective fault path between the two systems, that doesn't include the GEC ran to the primary systems electrode.

Also just a heads up keep in mind that when you have two different voltage systems the wires must be identified to distinguish them to each system ( 210.5(C) ) as well as the grounded conductors ( 200.6(D) )
 
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Don has covered everything else well but I just want to point out that since you have a WYE primary on this transformer do not bond the X0 as this will cause all kinds of heating problems with the primary EGC and can cause a failure of the transformer as it can cause it to run hotter, leave the X0 floating on the primary side, I'm sure you already knew this but just wanted to make sure, with a common core transformer this is a big no-no.

Thanks Hurk27,

I didn't know about bonding the x0 terminal on WYE common core transformer. First time I hear about it.
 
Thanks Hurk27,

I didn't know about bonding the x0 terminal on WYE common core transformer. First time I hear about it.

Hurk27,

do you know of any other source (website, book, etc..) that explains not bonding the x0 terminal for a WYE common core transformer? I tried a google search but it didn't turn up any good info.

Thank you for your help.

EE_Intern_2
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
A simpler and perhaps sufficient explanation is this:

In a wye/wye transformer, the voltages on each phase, on both primary and secondary, are not locked together.
If one primary Y L to N voltage drops, the corresponding Y L to N secondary voltage drops also, and equipment connected to it may be unhappy but the transformer itself will not be.

But in a wye/delta, each primary L to N voltage corresponds directly to a secondary L to L voltage.
And since the secondary windings are physically connected into a closed delta, the sum of the vector voltages around the delta MUST be zero or else potentially large circulating currents will flow.
If one leg of the input wye goes low, then circulating current will flow in the secondary and neutral current will flow in the primary neutral until balance is restored. That could be a lot of current if the imbalance is in a stiff primary supply.
And don't even think of using different taps on different phases on the primary side to "correct" the imbalance.

If you leave the X0 unconnected, the neutral point will immediately float to a voltage such that the primary wye voltages are properly balanced, since no current is allowed to flow from the X0 point to the primary supply.
 

iwire

Moderator
Staff member
Location
Massachusetts
Don has covered everything else well but I just want to point out that since you have a WYE primary on this transformer do not bond the X0 as this will cause all kinds of heating problems with the primary EGC and can cause a failure of the transformer as it can cause it to run hotter, leave the X0 floating on the primary side, I'm sure you already knew this but just wanted to make sure, with a common core transformer this is a big no-no.

Do you mean HO or XO?

I suspect I would leave HO unbonded but still bond XO.

This is not a reverse fed transformer.
 
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