2008 Article 250.58 clarification please

[Brad D]

Hello all,

Longtime lurker, first time poster here. I have a grounding/bonding question that I "think" I know the answer to, but wanted to get a consensus on. I have installed two switchgear with two separate services/meters fed by two separate utility XFMRs in a very large parking garage. Switchgear A is a 2500A 480/277v gear fed by seven sets of 500MCM conductors with a generator back up and four, 4 pole 400A ATSs, one of which is supplying a UPS. Switchgear B is a 2000A 480/277v gear fed by six sets of 400MCM conductors with it's own generator back up with a 800A and a 400A ATS (both 4 pole). Both are Schneider Square D and the Main Bonding Jumpers are located in the second cabinet next to the main switch.

I have a ground bar on the wall at Switchgear A that connects the Grounding Electrode Conductor from the gear, the Ufer ground, a tri-ground on the west side of the building, the north side water main, the south side water main, the riser ground for the east and west electrical closets and five XFMRs.

I have a ground bar on the wall near Switchgear B that connects the Grounding Electrode Conductor from the gear, the Ufer ground and four XFMRs.

The electric rooms are on different sides of the building and separated by an expansion joint (no building steel connection).

Currently there is a 1/0 copper conductor (which was specified on the prints) bonding the ground bars between Swichgears A and B as per Art. 250.58 which states:

250.58 Common Grounding Electrode. Where an ac system
is connected to a grounding electrode in or at a building
or structure, the same electrode shall be used to ground
conductor enclosures and equipment in or on that building
or structure. Where separate services, feeders, or branch
circuits supply a building and are required to be connected
to a grounding electrode(s), the same grounding electrode(
s) shall be used.
Two or more grounding electrodes that are bonded together
shall be considered as a single grounding electrode
system in this sense.

I believe that the drawings are not correct. 250.58 does not specifically say how to size the conductors bonding the services, but I believe that the conductor connecting switchgear A and B should be sized as per Table 250.66. Based on the area of the service conductors, that would make the required conductor to be 3/0 not 1/0.

Does anyone disagree with that? Any insight or clarification would be greatly appreciated.

Thanks in advance for taking the time to read and consider my post. I've found this forum extremely helpful in the past, and I'm happy that I've finally signed up. I look forward to interacting with you guys.

Brad D

 

[Dennis Alwon]

Sounds to me that the 1/0 is based on one set 500KCM and should it should be based on 7 * 500 kcm which means a 3/0 grounding electrode conductor for the service. So Yes I agree that 3/0 should be used but the 1/0 is not adequate, IMO

 

[copper chopper]

according to 250.102c if its bonding, then it has to be 12.5 percent of the largest conductor size times the 7 for A and 6 for B

I come up with A= 500 kcmill and B= 300 kcmill now this is only for bonding. a 3/0 would be fine for equipment grounding.

 

[Brad D]

according to 250.102c if its bonding, then it has to be 12.5 percent of the largest conductor size times the 7 for A and 6 for B

I come up with A= 500 kcmill and B= 300 kcmill now this is only for bonding. a 3/0 would be fine for equipment grounding.

I'd agree that we'd have to use the equipment bonding jumper rules if the last part of 250.58 did not say "Two or more grounding electrodes that are bonded together shall be considered as a single grounding electrode system in this sense.". This leads me to believe that Table 250.66 applies. Do you see what I mean? It's ambiguous. I wish it stated clearly how exactly it should be sized.

Edit: Thank you for the reply!

 

[Brad D]

Sounds to me that the 1/0 is based on one set 500KCM and should it should be based on 7 * 500 kcm which means a 3/0 grounding electrode conductor for the service. So Yes I agree that 3/0 should be used but the 1/0 is not adequate, IMO

Thank you for the reply. I don't believe that 1/0 is adequate either. The thing is that it's more than a 500' pull... So before I start talking about spending the time and money to change it, I wanted to hear a few opinions.