Transformer Grounding Detail

[philly]

I'm looking at an installation where two panelboards and dry type transformer are being replaced by two newer panels and a dry type and wanted to make sure that I thoroughly understood the attached transformer grounding detail. In this particular application a 480V panelboard is feeding a 45kVA transformer 480V-120/208V with a 50A breaker and the #8AWG secondary of the transformer feeds a 120/208V panel with #2 AWG and a 110A main breaker in the panel. Both panels and the transformer are currently mounted on a wall with a 1/4" x 2" ground bus located on the wall as well with a connection from the ground bus to the grounding electrode system (GES) using a 4/0 bare copper wire.

Currently the existing panels and transformers have a ground wire (I believe #6) running from the ground bar on the wall to the respective ground locations in the existing panels and transformer. I know that because the transformer represents a SDS a GEC is required to the transformer (assuming bonding jumper is at transformer) but is there a need to have the GEC's between the ground bar and the ground buses in the existing panels? When replacing this equipment with new equipment we are trying to decide weather or not we need to ground grounds from the ground bus in the panels to the ground bar on the wall. My understanding is that it is not required since we have the GEC to the transformer which is all that is required in this case. Is it a violation if we were to install these grounds between the panels and ground bus in addition to the GEC between the ground bus and transformer? Is this a good idea/practice or should we omit these when we install the new equipment?

I also noticed that the existing ground wires between the ground bus and panels/transformer, are run in rigid conduit (even though the distance is only a few feet on the wall). Isn't there a requirement in 250.64(E) to have these ground wires bonded to the conduit (with lay-in lug or similar) on both ends of the conduit? I noticed that the existing installation did not have this bond.

Looking at the attached detail I had a few questions I wanted to make sure I understood:

1) Is it necessary to bond the conduits in the panel and transformer to the equipment ground and ground bus as shown in the detail? Is there an NEC reference for this requirement?

2) Is 250.94 used to size the bonding jumper in the transformer as shown on the detail between the transformer neutral and ground? I do not see a table in this section so how is this size determined? Is 250.102 to be used?

3) In this case the GEC between the ground bar and transformer ground lug would be sized per 250.66. Since my feeder conductors to the panel are #2 AWG then it looks like I can use #8AWG as my GEC. Can this be run in conduit as discussed above or better off leaving this outside of conduit? Can I use any of the provisions of 250.66 A through C here?

4) I'm assuming that the Equpiment Ground Conductor (EGC) between the transformer and panel is sized per 250.122? In this case however there is no overcurrent device directly ahead of the panel so would I use the transformer primary breaker rating of the panel main breaker rating as the basis for choosing EGC from 250.122?

Thanks

 

[don_resqcapt19]

1) Is it necessary to bond the conduits in the panel and transformer to the equipment ground and ground bus as shown in the detail? Is there an NEC reference for this requirement?

Assuming metallic enclosures and two locknuts, the NEC does not require grounding bushings or bonding jumpers for the conduit.

2) Is 250.94 used to size the bonding jumper in the transformer as shown on the detail between the transformer neutral and ground? I do not see a table in this section so how is this size determined? Is 250.102 to be used?

First 250-94 and 250-95 were last used in the 1996 NEC. 250-94 was replaced by 250-66 and 250-95 was replaced by 250-66 in the 99 code. The "-" was replaced by "." in the 2002 code.

The conductor between the transformer and the panel is a supply side bonding jumper and sized per 250.102 in the 2014 code. Note that the conduit is permitted to be the supply side bonding jumper and the wire bonding jumper is not required by the code. If the wire bonding jumper is installed, even where the conduit is permitted to be used as the supply side bonding jumper, the supply side bonding jumper of the wire type must still be sized by 250.102.

3) In this case the GEC between the ground bar and transformer ground lug would be sized per 250.66. Since my feeder conductors to the panel are #2 AWG then it looks like I can use #8AWG as my GEC. Can this be run in conduit as discussed above or better off leaving this outside of conduit? Can I use any of the provisions of 250.66 A through C here?

That conductor is not a GEC. One end of a GEC always connects to a grounding electrode. I think you are asking about the system bonding jumper, which is sized per 250.102. The provisions of 250.66(A)-(C) only apply to Grounding Electrode Conductors. If a GEC is run in a ferrous raceway, there must be a full size bonding jumper between the raceway and the GEC at both ends of the raceway.

4) I'm assuming that the Equpiment Ground Conductor (EGC) between the transformer and panel is sized per 250.122? In this case however there is no overcurrent device directly ahead of the panel so would I use the transformer primary breaker rating of the panel main breaker rating as the basis for choosing EGC from 250.122?

That conductor is not an EGC, it is a supply side bonding jumper and is sized per 250.102.

 

[kingpb]

I surmise this is an old utility "typical" detail; that needs updating.

 

[philly]

Assuming metallic enclosures and two locknuts, the NEC does not require grounding bushings or bonding jumpers for the conduit.

Enclosures are metallic. Are the locknuts you are referring to part of the conduit connection means to the enclosure? Lets say hypothetically there are no locknuts, then is there a requirement to bond the conduit to the enclosure using a grounding bushing or bonding jumpers. Even if grounding bushings are used, is there a need to install bonding jumpers? Is there a section in the NEC that requires the bonding of RMC to the enclosure either by locknut, ground bushing, bonding jumper, etc....?

The conductor between the transformer and the panel is a supply side bonding jumper and sized per 250.102 in the 2014 code. Note that the conduit is permitted to be the supply side bonding jumper and the wire bonding jumper is not required by the code. If the wire bonding jumper is installed, even where the conduit is permitted to be used as the supply side bonding jumper, the supply side bonding jumper of the wire type must still be sized by 250.102.

OK I see now that this cable between transformer and panel is the supply side jumper sized per 250.102. I assume this is based on size of ungrounded conductors between transformer secondary and panel as shown in the table.

What about the bond between the transformer neutral point and ground in the transformer. I believe this is the system bonding jumper? Is there a particular section that dictates the size requirement for this system bonding jumper? (After reading your response below it looks like this is sized per 250.102 as well based on size of secondary conductors.

That conductor is not a GEC. One end of a GEC always connects to a grounding electrode. I think you are asking about the system bonding jumper, which is sized per 250.102. The provisions of 250.66(A)-(C) only apply to Grounding Electrode Conductors. If a GEC is run in a ferrous raceway, there must be a full size bonding jumper between the raceway and the GEC at both ends of the raceway.

Here I was referring to the GEC between the grounding electrode (via ground bus on wall) and the transformer ground. I believe this is sized per 250.66 as mentioned and will be a #2AWG. Since I am talking about grounding GEC here can provisions of 250.66 apply? If run in ferrous raceway what section dictates the full size bonding jumper between raceway and GEC at both ends? I assume that this bond can be done with a Lay-In lug and route the GEC through lay-in lug as it enters and exits conduit?

That conductor is not an EGC, it is a supply side bonding jumper and is sized per 250.102.

Yes I see now that this is the supply side bonding jumper sized as you mentioned.

Thanks for your help! Starting to become clear.

 

[mwm1752]

There are many ways to do this install and all depends on the specifics of the wiring method used. I basically see how the isolated grounded conductor connection is achieved.

 

[philly]

Is there any issue with having a GEC electrode conductor connected to the panelboards as well even though it is not required (since GEC is connected to transformer). I am thinking that these would just be considered Auxiliary Grounding Electrodes per 250.54?

 

[philly]

Enclosures are metallic. Are the locknuts you are referring to part of the conduit connection means to the enclosure? Lets say hypothetically there are no locknuts, then is there a requirement to bond the conduit to the enclosure using a grounding bushing or bonding jumpers. Even if grounding bushings are used, is there a need to install bonding jumpers? Is there a section in the NEC that requires the bonding of RMC to the enclosure either by locknut, ground bushing, bonding jumper, etc....?

From what I can tell it looks like 250.96(A) states the requirement to bond raceway to enclosure even if a supplementary ECG (such as in this case) is being used. But as stated previously the locknut should take care of this bonding requirement.

 

[don_resqcapt19]

Is there any issue with having a GEC electrode conductor connected to the panelboards as well even though it is not required (since GEC is connected to transformer). I am thinking that these would just be considered Auxiliary Grounding Electrodes per 250.54?

The grounded conductor can only be connected to the grounding electrode at a single point. For a Separately Derived System (a transformer is the most common SDS), you can make the connection between the earth and the grounded conductor either at the transformer or at the first disconnect, but not at both.

Auxiliary Grounding Electrodes can only be connected to the EGC, they cannot be connected to the grounded conductor.

 

[don_resqcapt19]

Enclosures are metallic. Are the locknuts you are referring to part of the conduit connection means to the enclosure? Lets say hypothetically there are no locknuts, then is there a requirement to bond the conduit to the enclosure using a grounding bushing or bonding jumpers. Even if grounding bushings are used, is there a need to install bonding jumpers? Is there a section in the NEC that requires the bonding of RMC to the enclosure either by locknut, ground bushing, bonding jumper, etc....? ...

The locknut or locknuts are part of the connection to the enclosure, and in most cases these are suitable as the bonding connection to the enclosure.
Why would you go to the expense of using a grounding bushing without a bonding jumper. The grounding bushing itself does not provide any better connection to the enclosure than does the standard conduit connection.

 

[philly]

The grounded conductor can only be connected to the grounding electrode at a single point. For a Separately Derived System (a transformer is the most common SDS), you can make the connection between the earth and the grounded conductor either at the transformer or at the first disconnect, but not at both.

Auxiliary Grounding Electrodes can only be connected to the EGC, they cannot be connected to the grounded conductor.

Yes this I understand. But the existing installation has grounds run between the grounding electrode and the panelbaord grounds in addition to where it is connected to the grounded conductor in the transformer. So in the oriongal detail I posted this would be a ground wire from the grounding electrode (attached to busbar on wall) to eh ground bus shown in the panel. From what I can tell this isn't a violation b/c its not connected to the grounded conductor in the panel, and the GEC is only connected to the grounded conductor in the transformer.

These additional grounds between the panel ground bus and the grounding electrode would simply become auxiliary ground's. Maybe its better off not to include these grounds between the new panel ground buses and the ground bar when the panels are replaced?