All questions and answers are based on the 2008 NEC.
Q. Do I need a grounding conductor for a pole light, or is the ground rod good enough?
A. You absolutely need an equipment grounding conductor. Metal parts of electrical raceways, cables, enclosures, or equipment must be bonded together (and to the supply system) in a manner that creates a low-impedance path for ground-fault current that facilitates the operation of the circuit overcurrent device [250.4(A)(5)], as shown in the Figure. Because the earth is not suitable to serve as the required effective ground-fault current path, an equipment grounding conductor is required to be run with all circuits.
Danger: Because the contact resistance of an electrode to the earth is so high, very little fault current returns to the power supply if the earth is the only fault current return path. What’s the result? The circuit overcurrent device will not open, and all metal parts associated with the electrical installation, metal piping, and structural building steel will become and remain energized.