Just finished a job like that. (see the job comes together thread of a few weeks ago.)Make sure that you are running the other side of your sensor to the neutral line. If it's not your putting 480 across the light cell. Sorry if I'm being elementry but you always double check yourself when the troubleshooting starts.
Next question, where in the scheme of things is the light sensor? Is it in a building where you are pulling the power, or is it mounted to one of the light poles. If you have room for it I think I would recommend a transformer to take your control voltage down to 110 or less. That way the guy behind you changing the sensor doesn't get a rude suprise.
Good luck, troubleshooting is always the fun part.