Grounding Separately Derived Systems, NEC250.30(A)(7)

[ccarr7]

Hi Guys,

I have a couple questions regarding the grounding of a stepdown transformer via 250.30(A)(7). The code states that the GEC for the separately derived system shall be from the water pipe or building steel. I have a case where the main panel is closer to the step-down transformer than the water pipe. Can I install the GEC for the transformer back to the ground bus at the main panel where a GEC from the water pipe and ground rod is connected? Also, if I run the GEC for the transformer to the water pipe, is a supplemental ground rod required per 250.53(D)(2). I only ask because it's not referred to in 250.30(A)(7).

Thanks,
Creig

 

[raider1]

Hi Guys,

I have a couple questions regarding the grounding of a stepdown transformer via 250.30(A)(7). The code states that the GEC for the separately derived system shall be from the water pipe or building steel. I have a case where the main panel is closer to the step-down transformer than the water pipe. Can I install the GEC for the transformer back to the ground bus at the main panel where a GEC from the water pipe and ground rod is connected?

Electrically it would work fine to connect the GEC to the main panel where the water pipe GEC is connected. As far as if the code permits it is a rather grey area.

Also, if I run the GEC for the transformer to the water pipe, is a supplemental ground rod required per 250.53(D)(2). I only ask because it's not referred to in 250.30(A)(7).

Thanks,
Creig

If the water pipe is already being used as an electrode as required by 250.50 then it should already be supplemented in accordance with 250.53(D)(2)

Chris

 

[tom baker]

Keep in mind a SDS needs to be grounded to a grounding electrode system, and it needs to be bonded to any metallic piping in the area served. Sometimes the grounding and bonding can be done with one wire to a water pipe or to structual steel, which has a bonding jumper to the water piping.

 

[tryinghard]

Hi Guys,

I have a couple questions regarding the grounding of a stepdown transformer via 250.30(A)(7). The code states that the GEC for the separately derived system shall be from the water pipe or building steel.

I?m thinking you know this but the ground (earth) connection must be re-established because of the SDS this is done with an electrode. The water pipe or metal frame of a building are only truly electrodes if they comply with the criteria of 250.52(A)(1) & (2) if neither of these types are available ?Any of the other electrodes identified in 250.52(A) shall be used.?

 

[tryinghard]

...the main panel is closer to the step-down transformer than the water pipe. Can I install the GEC for the transformer back to the ground bus at the main panel where a GEC from the water pipe and ground rod is connected?

Creig, I personally feel there?d be no problem but technically I don?t know if you?re in compliance with 250.64(C) one continuous length GEC. If the water pipe complies as an electrode and is supplemented you really can refer to this as ?the electrode?