Greg, I am not sure that capacitive coupling is what you mean. In the case of extremely long runs of THHN there will be current leakage through the insulation of the wire to the ground which could add up to trip a GFCI. This can be mitigatted by going to a better insulation type like XHHW. But given enough length even it won't work. This really dosen't matter to much though because we have exceeded even the usual operation lengths of the circuit (volatage drop and it is better to place another panel to shorten the lengths of runs). I have not bothered to calculate what those lengths may be, if I had time I would. I just all stems from electricity takes all paths and even insulation is a path, just a mighty poor one.
