I am installing a 400 watt multi vapor light fixture with a photocell added to it. When viewing the wiring diagram and looking inside the fixture itself one white wire is labeled "common". This fixture has a multi tap transformer and it seems the individual voltage taps are also colored white. There are no other free wires to be utilized. Is this wire labeled "common" the comon neutral ?

The common can be the neutral, or another hot, depending on the voltage applied. It is also sometimes a wire to the lamp shell in some lights.

Where is this light going?