In my time it needed a clearance of 6 metres above, 3 metres lateral and 2 metres below, to prevent arcing from HT cables passing through the aircraft .
Many years ago, as a young thruster of an SH pilot, I did it once as per the SOP, at the slow hover taxy. Then, on the way back, I admit to my shame, I flew under the wires the opposite way at 140 kts. Scared the crewman a bit. In fact he threatened to deck me if I ever did that again.
