I work at a plant that uses three phase, three wire power. None of the three phase transformers are grounded. The transformer in question is a 2400 to 480 volt AC, delta - delta. We ordered some 277 volt light fixtures that will not work. A co-worker is sure that since you can measure 277 VAC from any one leg of the three phase to ground, then you should be able to hook up a 277 volt lighting ballast, one side of the ballast to a phase, the other to ground, and the light will work. When we actually do this our 277 volts to ground drops to 159 volts. The more lights we put in the circuit, the more the voltage drops. I'm pretty sure that a transformer has to have it's own reference to ground (center tap to ground on wye, H1 on delta) before you can get a usable 277 volt power supply. If I'm wrong, why wont our lights work? If I'm right, how do I explain this to my co-worker?