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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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Justme
    Sep 10 at 23:12
  • \$\begingroup\$ 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. \$\endgroup\$
    – Russell McMahon
    Sep 23 at 3:59

3 Answers 3

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You can estimate AC Amps with the information you have. Watts out is known. Mains voltage is known. AC amps is more that one might expect because of efficiency never being 100% and power factor not 100%. Some cheap equipment has poor power factor like 70% and lousey efficiency like 78%. Check your equipment specifications. I have seen power factor of 99% and 95% efficiency. If the manufacturer does not say then assume the worse like the two earlier numbers. A crude rule of thumb is to double the expected AC current to avoid problems.

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  • \$\begingroup\$ ”I have seen power factor of 99% and 95% efficiency.” Power factor and efficiency are unrelated. You mean power factor. \$\endgroup\$
    – winny
    Sep 11 at 9:49
  • \$\begingroup\$ @winny they are unrelated to each other but pretty much related to the current estimate. \$\endgroup\$
    – fraxinus
    Sep 11 at 10:46
  • \$\begingroup\$ @fraxinus By multiplying them. Still does not make power factor = efficiency. \$\endgroup\$
    – winny
    Sep 11 at 12:21
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if you know how many watts come out can can figure out how much input current is needed to make that many watts, but then you have to divide by its efficiency and then multiply by its power factor.

If you don't have those statistics you can only guess or measure.

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\$ 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

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