Why does a 1000w light with ballast use 8 amps at different voltages?

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Zula110100100
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I have a question about the power usage of a light with a ballast. If the light is rated as a 1000w light and must be run through a ballast. I believe the ballast is like a transformer, because I saw somewhere that the bulb runs at 300 volts. So since Watts = amps x volts, does the wattage stay the same and the amps change... or does the wattage change with the voltage, and if so, which voltage is running 1000w? It said 8amps on it somewhere also, so I am confused
 
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Watts stay the same and amps change. So if your ballast is rated 1000 watts at 120 volts that is 8.3 Amps (probably this is the 8 amps you saw). Your bulb is 300volts and thus will be somewhat under 3.3 Amps (assuming the ballast takes up some power).