Ballasts: 7 of 21

Two wires from the ballast go to the electrical line—120 volts or 277 volts, whatever—and two wires go to the lamp.

Usually you have different ballast models for 120 and 277 volts. However, in "Multivolt ballasts" the same ballast will run on any voltage provided you pick the right two wires—depending on what voltage you are connecting them to.

GE's latest UltraMAX™ ballast senses the voltage coming in and automatically adjusts (between 108 and 305 volts). You don't have to stock separate ballasts for different input voltages; you don't even have to worry about picking the right input wires!