ECN Forum Forums General Topics of the Trade General Discussion Area Questions on Hid lighting

I recently installed a 150watt HPS flood. The light draws 4 amps on startup and 3 there after at 120 volts. I was wondering why so high, dosent conform to Ohm's law? Could the ballast be going?Also does anyone know what a 400 watt HPS light draws at 120v? If I wanted to change the fixture to a 400 watt MH would I have to gut the whole thing or could I just change the ballast?
Thanks