Q This is an old question, but I would appreciate additional input. We have a utility main service, wye-wye pad-mounted transformer, three-phase, four-wire secondary with ground ring and four rods, one at each corner. Service-entrance conductors run to the 480/277V main switchboard, which has a ground ring around the footing of the electrical room with four rods, one at each corner. The transformer and the electrical room are 25 feet apart and ground rings are bonded to each other. If we ground the grounded conductor at the switchboard and at the transformer as AHJ recommends, does not the grounding electrode system become a parallel path with the grounded conductor? Does this not make the grounding electrode conductor a current-carrying conductor?
A You have indicated that this is an old question; however, you did not indicate how old. My answer is based on the 1999 edition of the NEC. You stated that you have a service-supplied AC system as covered by Section 250-24. Article 100 defines a service as a system supplied by a utility. Your system is utility supplied.
Section 250-24(a)(2) seems to apply in your example. This section requires the two grounding electrode systems and the electrodes you describe comply with Part C of Article 250. Section 250-28 requires the main bonding jumper requested by your inspector.
The NEC does not require that the two grounding electrode systems be bonded together. However, the bonding of the two systems is also not prohibited. You are correct that bonding of the two grounding electrode systems will establish a parallel path for neutral current and for ground-fault current.
If the transformer was customer-owned, you would be dealing with a separately derived system as covered by Section 250-30. In the 1999 NEC you will find a new exception No. 1 in Section 250-30(a)(1). This exception will prohibit the bonding since it does establish the parallel path.