# How to calculate the AC current consumption of an external power supply if I only know its DC volts and amps output

I'm working in an AV project and need to calculate the total AC current consumption of an AV equipment rack for the electrician to calculate breakers and the electrical installation, but some of the equipment use external power supply and other use AC (IEC) connections, They have internal power supply.

The equipment with internal power supply gives you the AC voltage and current needed directly in it rear panel. IE: 120V / 5A, there is no problem.

I see the DC Amps & Volts needed in the dc jack on the equipment, or the DC output in the external power supply but it doesn't indicate the AC input current that the equipment needs to work properly.

How may I calculate this AC current?

I have been done this assuming that the efficiency of the power supply is 75% and assuming that Power (watts) is the same in DC and AC.

I just use the calculated (or indicated) output power of the power supply as the input power that this psu needs to work, knowing that the output voltage (DC) will decrease and the output current (DC) will increase, and my input (AC) current will be lower than it.

I don't know if this procedure is really the correct. I just want to hear your advice.

• A power supply should have rated AC input current printed on it just like any other device. If it does not, then I don't think you should use them, as they likely are markings required by any local electrical code. Sep 10 at 23:12
• Input power max will be, usually, slightly more than output power max. This may be wrong if eg a linear regulator is used on the output with large DC voltage drop in the regulator, as happens. Sep 23 at 3:59

$$\ I_{rss} = \sqrt{I_{DC}^{2}+I_{ACrms}^{2}} \rightarrow I_{ACrms} = \sqrt{I_{rss}^2-I_{DC}^2} \$$ where $$\I_{rss}\$$ is the current measured by the ampmeter