Over the years I've seen where a duplex dwelling will have a single service drop supply 100 ampere Service Entrance conductors then split to two meters and the two meters (one for each half of the duplex) are feeding two service panels (one for each half of the duplex) grouped in one location. Let's say each panel is rated for 100a and the GEC is required to be minimum #8 cu. Since no one uses #8 for obvious reasons, the electrician will run a #6 out of each service panel to the water pipe electrode sometimes 50 feet away. Would we see this a paralleling of a #6 conductor? And if so, is there anything in the Code that prohibits paralleling of the GEC?
From your discription it appears (to me) that this is one service possiblly supplying two buildings(possiblly not). Article 250.24 requires that a grounded service supplied AC System shall have a grouning electrode conductor connected to the grounded conductor (neutral). The section further states that this connection may be made at any accessible point from the load end of the service drop up to the grounded conductor termination point at the service disconnect. The hand book displays these locations at the weather head , meter enclosure or the service equipment enclosure(exhibit 250.8).
In the example mentioned I would attempt to bring the GEC to the meter enclosure (assuming a two gang enclosure) and use the grounded conductor to ground the down stream equipment (250.142)(1,2)
If this is one building and the service equipment for each unit is installed and grouped per section 230.72 Then article 250.64 (D) allows the GEC to be tapped (so to say) in that case even if the individual loadcenters are located in two different buildings (attached) the GEC from one could go directly to the grounding electrode (or system) pass by the second loadcenter allowing the second GEC to tap to the first. There is another illistration in the 2002 handbook labled (exhibit 250.26) that may give light to this. Any way thats how I might see it Im not sure there would be a problem with the two individual runs to the electrode either.
paralleling is joining of two conductors at BOTH ends.