I was working on a 480v hps lighting circuit and went to turn the circuit off to change the fixture but still measured 66 volts to ground on each leg. Does. Any one know why this would be?

What type of meter did you measure the voltage with? If it is an electronic meter with the high input impeadence, it will measure "ghost" voltages. These "ghost" voltages are due to the wires traveling next to energized wires. Both set of wires are acting as a transformer.

Measure the voltage with a Wiggy or equivalent and the voltage will PROBABLY disappear. If they do not disappear, you have a problem. If the voltage does disappear then you are measuring ghost voltages.

Measured it with a fluke multimeter.

Fluke multimeters ARE high impedance testers.

You need to use an old-style analog meter to load down these induced voltages. Since there is no current flow, it's really a capacitive effect.

Cinner:

You need a 'Wiggy', or any other brand solenoid type tester, or an analog multi-meter.

WIGGGGGGGY!!! I carry one in my pouch just for that

This happens in a cogen I work in alot.. 480V wiring from VFD loads seem to induce voltage on other conductors more than anything.. HID loaded circuits have the same tendency