Transformer Grounding (Separate Derived System)

[phil_edwards123]

According to NEC, each separately derived system must be system bonded and also grounded (earthed) to a suitable grounding electrode (such as water pipe or structural beam, etc).

In the example below in which the Delta-Wye transformer is installed in a tenant space. There is already a ground wire coming from the Main Service - a 277/480V 3-ph Power Panel located in electrical room. This ground wire is already bonded and grounded (earthed) to a grounding electrode inside the electrical room.

Do we still need to ground (earth) the transformer secondary again to an acceptable grounding electrode?

 

[augie47]

Yes, you still need a grounding electrode as noted in the sketch
250.121 prohibits the equipment ground from being used as a grounding electrode conductor

 

[kwired]

250.50 says: "All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system."

So when you connect a SDS to an electrode - it needs to be a part of the GES as well not just an isolated electrode for the SDS.

Your water pipe and building steel electrodes should already be a part of the GES.

 

[infinity]

According to NEC, each separately derived system must be system bonded and also grounded (earthed) to a suitable grounding electrode (such as water pipe or structural beam, etc).

In the example below in which the Delta-Wye transformer is installed in a tenant space. There is already a ground wire coming from the Main Service - a 277/480V 3-ph Power Panel located in electrical room. This ground wire is already bonded and grounded (earthed) to a grounding electrode inside the electrical room.

Do we still need to ground (earth) the transformer secondary again to an acceptable grounding electrode?

View attachment 16846

The "ground wire", really the EGC, would typically be smaller than the required GEC so it could not serve both purposes.

 

[packersparky]

The "green wire" from the transformer to the 208/120 panel is a supply side bonding jumper. It is required to be installed by 250.30(A)(2)and is required to be sized in accordance with 250.102(C). In the picture there it shows the secondary conductors terminating in a MLO panel. It may just be the way the picture was drawn, but overcurrent protection for secondary conductors should comply with 240.21(C).

 

[phil_edwards123]

The upstream 277/480V source panel is located in the electrical room. All equipment in the electrical room are bonded together and then connect to a grounding electrode.

Now this transformer is located in a tenant space, which is about 300 ft from the electrical room. If there is no water pipe or structural steel within the tenant space (no grounding electrode nearby), then do we need to run a GEC (Grounding Electrode Conductors) all the way back to the electrical room for this SDS (Separately Derived System)??

 

[ActionDave]

Now this transformer is located in a tenant space, which is about 300 ft from the electrical room. If there is no water pipe or structural steel within the tenant space (no grounding electrode nearby), then do we need to run a GEC (Grounding Electrode Conductors) all the way back to the electrical room for this SDS (Separately Derived System)??

If you are working under NEC then the answer is yes.

 

[augie47]

Not to make the original post more confusing but two questions in regard to the SDS grounding electrode:
Assuming building steel is not available and a common grounding electrode conductor as allowed in 250.30(A)(4) is not installed....
(a) If ground rods are provided, must they be connected to the building grounding electrode system to comply with 250.50 ?
(b) If a water pipe is used, must that connection be made within 5 ft of the entrance to the main building if the exception to 250.52(A)(1) can not be used ?

 

[GoldDigger]

a. The electrodes must be connected to the rest of the GES.
I think you may call that connection a GEC or a very long bonding jumper depending on how you categorize the rest of the conductors.
b. I think you do have to meet that requirement for at least one GEC in the system. Not necessarily for the one between the SDS and the water pipe.
Remember that only one wire going to the water pipe is a GEC. The rest (if any) are bonding jumpers and do not have to be continuous. They do have to meet jumper as well as GEC size requirements, again depending on the configuration. For some geometries you may not be able to take advantage of the maximum size rule for ground rods.

mobile

 

[Ponchik]

I removed my post.

 

[Smart $]

The upstream 277/480V source panel is located in the electrical room. All equipment in the electrical room are bonded together and then connect to a grounding electrode.

Now this transformer is located in a tenant space, which is about 300 ft from the electrical room. If there is no water pipe or structural steel within the tenant space (no grounding electrode nearby), then do we need to run a GEC (Grounding Electrode Conductors) all the way back to the electrical room for this SDS (Separately Derived System)??

Being from BC, are you are under NEC purview? Assuming yes, the answer is dependent on the Code edition. Under 2014 you can run a combo GEC/EGC from the 480/277V 3Ø 4W service equipment per the exception as follows...

250.121 Use of Equipment Grounding Conductors. An
equipment grounding conductor shall not be used as a
grounding electrode conductor.

Exception: A wire-type equipment grounding conductor installed
in compliance with 250.6(A) and the applicable requirements
for both the equipment grounding conductor
and the grounding electrode conductor in Parts II, III, and
VI of this article shall be permitted to serve as both an
equipment grounding conductor and a grounding electrode
conductor.

250.121 was added in 2011 edition without the exception. The exception was added in 2014. Most AHJ should look at the added exception as a clarification, for any installation still under 2011. However, many considered a combo EGC/GEC prohibited under 2011. Prior to 2011 it may have been questioned, perhaps even rejected by interpretation, but there was no direct prohibition.