Grounding & Bonding of Transformers

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Billy B

Member
Location
Northern Va
We are having and interoffice disagreement/discussion regarding the code requirements for the subject issue.

My position is that a grounding conductor is required as per articole 250-30 and bonding is required to the NEAREST interior metal water piping as per 250-104(a) 4 with both conductors connected at the SAME point in the transformer to the XO terminal.

Rather than describe the other opinion which I am not sure I even understand, can someone please tell me if the above codes and interpretation is correct? Or advise otherwise.

trans is a 277/480 pri 120/208 sec

Thanks,
Bill
 
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RayS

Senior Member
Location
Cincinnati
I'd go with the structural steel as per 250.104 (D) 2.
Reading it now, looks like bonding nearby water piping to the steel would also be required, but this seems exceedingly redundant as the pipe is usually hung with metal hangers from the steel...
 
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cpal

Senior Member
Location
MA
I'm surprised that there is only a few responses to this question!!

I would say that 250.30 will require you to connect the Xo to a electrode recognized by 250.52 that conductor should be sized IAW 250.66 and the bonding jumper between the GEC and EGC on the secondary of the SDS should be made IAW 250.30 (A) (1).

If there are multiple SDS look at 250.30 (A) (4).

Regarding 250.104 (D) I believe that the local water piping system is required to be connected to grounded conductor or the SDS The two exceptions to (D) (1) may provide convience but appears in fact to require the inter bonding (so to say) of the structual metal (assuming you have metal ) and the water pipe
 
B

Billy B

Member
Location
Northern Va
Thanks Charlie and Ray. I guess I should have noted that the discussion came as result to some revisions made to our drawing by way of "Field Clarification" after the bid and we are trying to determine if their revised detail meets the NEC. Based on your responses it sounds like we all agree the pri and sec are grounded per 250 30 and sized by 250 66.

The "FC" revised drawing detail depicts a bond from the XO to the nearest cold water pipe and the main disagreement in our office is that only the ground should terminate on the XO, otherwise it would lead to the short taking the path of least resistance or the shortest path, be it the ground conductor or the bond. But the way I read 104 4a1 it's clear that they should both be terminated at the same point on XO which is consistent with the FC. Please advise.

Thanks again for you input.

Bill
 
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cpal

Senior Member
Location
MA
Billy B said:
Based on your responses it sounds like we all agree the pri and sec are grounded per 250 30 and sized by 250 66.


Bill
Click to expand...


My post ONLY referenced the secondary of the transformer!!
 
J

jtester

Senior Member
Location
Las Cruces N.M.
Billy B said:
Thanks Charlie and Ray. I guess I should have noted that the discussion came as result to some revisions made to our drawing by way of "Field Clarification" after the bid and we are trying to determine if their revised detail meets the NEC. Based on your responses it sounds like we all agree the pri and sec are grounded per 250 30 and sized by 250 66.

The "FC" revised drawing detail depicts a bond from the XO to the nearest cold water pipe and the main disagreement in our office is that only the ground should terminate on the XO, otherwise it would lead to the short taking the path of least resistance or the shortest path, be it the ground conductor or the bond. But the way I read 104 4a1 it's clear that they should both be terminated at the same point on XO which is consistent with the FC. Please advise.

Thanks again for you input.

Bill
Click to expand...

Bill

250.30(A)(7) requires you to use the closest listed electrode as the grounding electrode. 250.30(A)(3) and(A)(1) allow you to make this connection at the transformer or any point between there and the first OCPD. YOu don't necessarily have to ground at the transformer.

250.104(D) provides requirements for bonding. That bond must be made at the same point as the grounding connection.

It sounds like the FC is correct.

Jim T
 
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