Hi it's been a while since I have been around to ask another question. I haven't been able to figure this one out. Will anybody please tell me the difference between a HX ballast and a reactor ballast for a high pressure sodium lamp? I think they called the fixture a vapor tight. It said they were both 120 volts. I wanted to wire it outside above an attic window we worry about getting broken into. Thanks if you can. I would really appreciate it.

HX ballast has two coils...a primary and a secondary, while a reacter ballast has only one coil. They both do essentially the same thing electrically, and the difference usually lies in the line voltage vs. the lamp voltage requirements. If the line voltage needs to be stepped up or down for the lamp, the HX is used. If the line voltage is the same as required for the lamp, then reacter is used.

In your case, it seems both ballast types are available for 120v operation. In this case, the reacter ballast will be a tad more efficient since it drives one coil instead of two.

  That makes a lot of sense! I guess it needs two coils so that you can switch it back and forth. Thank you so much!