Transformer grounding electrode

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Rick Johnston

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I am replacing a 45 kva 480 to 120/208 xfmr. With out taking the cover off I didn't notice a grounding electrode conductor leaving going back to the grounding electrode. If I find out there is not one do I need to install one or is the equipment grounding conductor run with the primary feed wires sufficiant as long as I bond all the grounds together.
 
What if

What if

benaround said:
Rick,

The EGC would be too small for the SDS's GEC.
Click to expand...

What if: The EGC on the primary had been sized for the secondary derived phase conductors? [i.e. Intentionally sized to be able to be used as the GEC].

I saw such an installation and passed it. [yes, they did provide the 250.104(D) bond]
 
dana1028 said:
What if: The EGC on the primary had been sized for the secondary derived phase conductors? [i.e. Intentionally sized to be able to be used as the GEC].

I saw such an installation and passed it. [yes, they did provide the 250.104(D) bond]
Click to expand...

In this case it would actually be the GEC and it must comply with 250.30(A)(3) & (7).

Notice 250.30(A)(7), ?The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system.?

The grounding (earth) purpose is to ?limit voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines? and the grounding electrode conductor is the route to this. It is important to keep the GEC short or it may fail when used.

As example notice 800.100(4), 820.100(4), & 830.100(4) these are ?primary protectors? for nearly the same purpose and all require not more than 20? length.
 
dana1028 said:
What if: The EGC on the primary had been sized for the secondary derived phase conductors? [i.e. Intentionally sized to be able to be used as the GEC].

I saw such an installation and passed it. [yes, they did provide the 250.104(D) bond]
Click to expand...

There would be no need for 250.30(A)(3)excp.#2 , if that was compliant.

NEC 2005
 
benaround said:
There would be no need for 250.30(A)(3)excp.#2 , if that was compliant.

NEC 2005
Click to expand...

It seems like 250.30(A)(3), x-2 is exactly what he did do [yes, his primary originated in the service equipment].

However, your response seems to indicate his installation was not compliant??
 
tryinghard said:
In this case it would actually be the GEC and it must comply with 250.30(A)(3) & (7).

Notice 250.30(A)(7), ?The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system.?

The grounding (earth) purpose is to ?limit voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines? and the grounding electrode conductor is the route to this. It is important to keep the GEC short or it may fail when used.

As example notice 800.100(4), 820.100(4), & 830.100(4) these are ?primary protectors? for nearly the same purpose and all require not more than 20? length.
Click to expand...

The 2008 NEC has added a FPN to 250.4(A)(1) to address the issue of the length and routing of the GEC. Here is what the new FPN says:

"FPN: An important consideration for limiting the imposed voltage is the routing of bonding and grounding conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided."

Chris
 
dana1028 said:
It seems like 250.30(A)(3), x-2 is exactly what he did do [yes, his primary originated in the service equipment].

However, your response seems to indicate his installation was not compliant??
Click to expand...

Dana, What that says is if the SDS is part of the service equipment, as listed

by the manafacture, you are allowed to use the service GE as the connection.
 
benaround said:
Dana, What that says is if the SDS is part of the service equipment, as listed

by the manafacture, you are allowed to use the service GE as the connection.
Click to expand...

I'm not near my code book, but is that the exception that allows the SDS GEC to connect to a bus that is in "service equipment"?
 
raider1 said:
The 2008 NEC has added a FPN to 250.4(A)(1) to address the issue of the length and routing of the GEC. Here is what the new FPN says:

"FPN: An important consideration for limiting the imposed voltage is the routing of bonding and grounding conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided."

Chris
Click to expand...

Glad to see this
 
If the transformer is within the same building as the service disconnect that supplies the primary of the transformer, there is no technical reason why the primary EGC is not suitable as the transformer GEC. There is only a code rule.
There is very very limited exposure to higher voltages, other than the primary voltage and the primary EGC conductor is suitable for that, and also very limited exposure to lightning within the building.
 
Don,

Keeping with the subject, EGC for GEC , I could see the following :

Not be a continuous conductor, ie, disco for primary @ transformer.

The primary EGC being EMT or other non conductor.

The GE not being the nearest water pipe or structual metal GE.

Not being in listed service equipment.

Primary feeder being parallel runs each with an EGC.

as some reasons an EGC as GEC could add confussion to grounding a SDS.

Don, May I ask why so adament on this subject ? I have never seen you

miss a chance to share your opinion when this comes up ? An opinion I might

add, that I respect.
 
We had a similar discussion this week amongst my inspectors.

Wood Frame Addition to an existing wood frame building.
Non metallic water piping system.
New 480v panel.
New 480v-120/208 SDS xfmr.
New 120/208 Panel

One inspector said that he allowed the use of the EMT feeding the 480v panel to carry the GEC back to the main service. The GEC between the 480v panel and the SDS xfmr was sized based upon the secondary conductor size.

Not being the biggest code geek in the office I was struggling to counter his arguments.

Any suggestions to help him understand the correct interpretations?

Thanks,
Mark
 
Sorry about the re-post but need to add more information.

MarkWhite said:
We had a similar discussion this week amongst my inspectors.

Wood Frame Addition to an existing wood frame building.
Non metallic water piping system.
New 480v panel.
New 480v-120/208 SDS xfmr.
New 120/208 Panel

One inspector said that he allowed the use of the EMT feeding the 480v panel to carry the GEC back to the main service. The GEC between the 480v panel and the SDS xfmr was sized based upon the secondary conductor size.

Not being the biggest code geek in the office I was struggling to counter his arguments.

Any suggestions to help him understand the correct interpretations?

Thanks,
Mark
Click to expand...

The ecxeption to 250.30 A (3) Ex. #2 states:
Exception No. 2: Where a separately derived system originates in listed equipment
suitable as service equipment, the grounding electrode conductor from the service or
feeder equipment to the grounding electrode shall be permitted as the grounding
electrode conductor for the separately derived system, provided the grounding electrode
conductor is of sufficient size for the separately derived system. Where the equipment
ground bus internal to the equipment is not smaller than the required grounding
electrode conductor for the separately derived system, the grounding electrode
connection for the separately derived system shall be permitted to be made to the bus.

Is this being interpreted correctly to allow the EMT as the GEC to the service GEC?
 
MarkWhite said:
...Is this being interpreted correctly to allow the EMT as the GEC to the service GEC?
Click to expand...
There is no provision in the code that permits you to use the EMT as a GEC...you can use it as an EGC, but not as a GEC. 250.62
 
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