Would it be ok to run a 250 watt PSMH bulb on a 250 watt HPS ballast?
You will actually overdrive a 250w pulse start metal halide lamp on a 250w high pressure sodium ballast unless the pulse start metal halide lamp is a retrofit lamp for high pressure sodium ballasts because most North American 250w pulse start metal halide lamps run at 135v 2.13a while 250w high pressure sodium lamps run at 100v 3.0a. Interestingly, if you use a European superimposed ignitor, you can actually run a 250w pulse start metal halide lamp on a 250w mercury vapor ballast without much problems.