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".
