There is a vociferous debate going on in the Fl IAEI BB about this. It is currently a tie. Some say the table rules, the GEC is a "conductor" and if it is buried between the electrodes or from the electrode to the building it has to be covered per 300.5 table. Others say article (text) 300.5 only applies to "cables and raceways". NFPA has rejected several proposals to specifically expand the language to include the GEC so the intent seems to be that this is not what they think it says. The other side says they rejected the proposal because it would be redundant. Why keep a fun fight like this in Florida? What say ye?
Just being devil's advocate, you can't use a table until the text directs you there and article 300.5 says table 300.5 applies to "cables, conduits and other raceways". Without that text guidance you could easily mis-apply a table.
The second question is that since you would have to excavate down 24" to connect the EGC would you need to drive an 8' rod down from that point or use a 10' rod? 250.53(G) does say the rod has to be "driven" into the soil, not buried, unless soil conditions prevent driving a rod.
The easy answer to this is, who cares! However that it not a professional answer. Low voltage cable can be buried 6" down as per table 300.5. So can't a conductor that doesn't have current (in normal conditions) be allowed to be buried 6"? As long as it is not laying on the ground where it could trip people or be damaged by power equipment. Right?
I am in Harold's camp on this one. It is a low voltage wire and 6" is plenty, particularly since this is usually right up against the building but there are several people over at Fl IAEI who want 24". When I asked about the driven rod (undisturbed soil) question they pointed out you can bring a conductor up to grade for the termination but since the whole run could only be 6' long, with a 45 degree slope on the rise, you would really only have 33% of this wire at depth.
Since our footings are also only 12" below grade, the structural inspector might have an opinion about digging a trench a foot below the footing, right next to it. If it was open for a few days of rain in the inspection process that footing might end up being "a bridge over troubled water".
Discussed this for all of two minutes years ago. As long as it is protected from damage, depth makes no difference. While waiting for the phone to ring consider this, is the wire between ground rods a Grounding conductor or a Bonding conductor ? Seems that the Code people made a big deal about telling them apart. Alan