HPS Ballast come in various styles and sizes and wattages. The most common style is the core & coil variety, this type is a standard open framed transformer. High Pressure Sodium systems must have the standard components operating in harmony for the system to work properly. The system is comprised of the ballast or transformer the ignitor and the capacitor. In F-Can style or encapsulated style HPS ballast the components are all inside of the unit. High Pressure Sodium lamps will run only on a HPS ballast. Typically on each ballast there will be a listing for the ANSI code that the ballast will operate. For example a Philips C400S51 which is a 400 Watt high pressure sodium lamp carries an S51 ANSI code. The replacement ballast that would be need to operate this lamp would need to carry an S51 Code as well or the lamp and ballast combination will not work. For help selecting a ballast please call 1-888-988-2852.