Service Grounding Creating Possible Parallel Path

[Andy Sherrill]

I have a baseball field lighting application where I have an existing pad mount transformer supplying single phase 120/240 V. The project I have is to add a DP near the transformer to supply lighting to 3 fields. There is already one dp at one of the fields and another service disconnect on the concession building. As I understand, code requires a ground at the new service disconnect (DP), and the ground must connect to the grounded conductor. The new DP will be located close to the transformer and it's ground rod(s) or possible grid. If I drive a ground rod at the new DP, this ground rod could end up very close to the existing grounds for the transformer. With the neutral connected to the ground rod at the new DP and connected to the transformer grounds, it seems to me that I could be creating a parallel path for neutral current under certain soil conditions. I understand the earth is not a great conductor, but in close proximity it seems I could have neutral current flow on the "ground" system. Does anyone have any experience with this? Is it allowed to treat this like a feeder and install a four wire service with a full size equipment ground? I am concerned for safety, but also because the lighting control system is electronic, so any objectionable ground currents will cause problems. Thanks in advance for anyone who has thoughts or experience that might help!

 

[iowa]

If I understand what you are saying correctly I would say of course it is allowed. More than that it would be the best way to do it. Of course when you run the neutral and ground wire you would not bond in that panel.

 

[tryinghard]

...As I understand, code requires a ground at the new service disconnect (DP), and the ground must connect to the grounded conductor...

Yes this is as per 250-24 & 250-50 also the neutral grounded conductor must bond to the enclosure as well 250-28. These are all common at the service disconnect and it really does create a parallel path with earth and grounded neutral conductor returning current will flow all paths according to their resistance that have an earth connection.

 

[dnem]

We have what we have and what we have is a multipoint grounded system in the US. . Line side of the service main is a landmine of grounding points and parallel paths. . That's how it is, like it or not. . The NEC itself requires this multipath in 250.24(A)(2).

I am concerned for safety, but also because the lighting control system is electronic, so any objectionable ground currents will cause problems.

If you think it's causing a problem, you have some relief in 250.6(B).

David