Exterior Switchboard Grounding



Have a project on existing building throwing a curveball. The building service consists of an exterior combination service entrance switchboard/automatic transfer switch located in the middle of the chiller unit yard on a separate concrete pad about 6 feet from the utility transformer and 15 feet from the building. The combination switchboard/transfer switch is 1600A, 277/480V, 3 phase served by the utility company transformer. The incoming utility service feeders land at the OCPD to the automatic transfer switch and the automatic transfer switch feeds a switchboard with 7 separate OCPD's and feeders to the building landing on different panelboards in two separate electrical rooms. The existing "as-builts" show the incoming utility service feeder as 4 sets of (4) 600 MCM and (1) #4/0 AWG ground in 4" PVC conduit. The actual installed conditions appear to be 4 sets of (4) 600 MCM conductors in 4" PVC conduit and (2) bare copper #4/0 AWG grounds in separate 2" conduits. The building has a lightning protection system without a counterpoise. Cannot locate any grounding electrodes at the service entrance switchboard. The building is a tilt-crete construction with structural steel roof and steel interior columns. The 7 separate feeders to different panels inside the building each have a #6 AWG ground as no feeder exceeds 225 amps. Interior step down transformers each have their own ground rod. The building is not more than 10 years old.


1) Cannot locate separate grounding electrode at the service entrance. It appears to be derived from inside the utility transformer.
2) Cannot identify a common electrode grounding system with structural steel, water piping, gas piping, or concrete encased electrode.
3) Cannot identify a bond to the lightning protection system.

The building owner is wanting a main ground bus installed for a new telecommunications system to meet TIA standards but when trying to find a common ground point inside the building ran into all of the above.


1) Does NEC Article 250.64 (D) apply to this situation? If yes, would the #6 AWG grounds need to be removed and replaced or is there a better solution?

2) For the new main ground bus, would a better solution be to derive a new main ground bar (MGB) inside the building to the exterior switchboard/ATS with a separate ground conductor and bond the existing water piping, structural steel, gas piping, lightning protection system, and step down transformers to this new MGB?

tom baker

First Chief Moderator
Staff member
"The actual installed conditions appear to be 4 sets of (4) 600 MCM conductors in 4" PVC conduit and (2) bare copper #4/0 AWG grounds in separate 2" conduits."
1. What is the function of the 4/0 in the separate conduit
2. When you state ground, that is not a code term - what is it?
3. 250.64, need to know which section 1, 2, or 3?


1. The function of the 4/0 AWG in separate conduit appears to be grounding electrode conductors from the switchboard ground bus to ground rods but cannot confirm without digging up the yard area.
2. The #6 grounds of the 7 separate feeders appear to be equipment grounding conductors as they are not sized per Table 250.66.
3. Article 250.64 (D)(2).