# 300W power supply with 110VAC input, and 12VDC output, what would be the max safe amperage you could draw?

I understand the "electrical triangle" I.E.... watts\volts=amps etc. but in this situation would you use the voltage from the ac or dc to calculate the amperage? I.E... 12x=300, Or maybe 110x=300.? (where x = amperage)

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• 1 decade ago

If the 300 watts is ac input power to the power supply then the maximum output current you could safely draw from the power supply would be (300w/12V) times the efficiency rating of the power supply. The efficiency rating is always less than one.

If the 300 watts is DC output power from the power supply then the maximum output current you could safely draw from the power supply would be the full 300w/12V=25 amps.

• 1 decade ago

If it is truly 300 watts, then the output will handle 300/12 or 25 amps.

The input will see 300/120 or 2.5 amps.

Of course losses will change those numbers, as will marketeers sometimes fanciful numbers. In other words, I wouldn't count on more than 20 amps out.

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• 1 decade ago

Watts = Amps * Voltage

Amps = Watts / Voltage

300/12=25Amps

If you count in resistance and such, you should not try to draw more than 23 or 24 amps. bear in mind that you will also need to use wires rated for that high of amperage.