Electric & Safety - How do I convert watts to amps?

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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