Watts divided by voltage equal amps.
A 400 watt ballast plugged into ordinary house current (120v) would draw 3.33 amps.
Using a little math we can see then:
Amps = Watts / Voltage
Watts = Voltage x Amps
Voltage = Watts / Amps
Don't forget to include ballast losses!
400W ballast burns up 65 watts in core.
That's why they actually draw closer to 3.95 Amps.
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