I answered this question on another post recently.
I had a customer who kept replacing 400W HPS lamps and then asked me why they only last a day or a week at most. I found that he was putting the 400W lamps in a 1000W fixture.
But what do I know. I'm an electrician and I've never tried some of these "experiments" myself....<G>

Shov
Shovelhandle Reviewed by Shovelhandle on . 250watt bulb to a 400watt ballast What happens when you use a 250 watt hps bulb with a 400watt hps ballast i have a self ballasted reflector thats 400watt and i put a 250wat bulb in it and it lit up for 20 minutes until i shut it off. Is this safe? Rating: 5