In my lab I have three-phase electric power according to the following diagram:

enter image description here

I'm using a three-phase power supply TRIO-PS-2G/3AC/24DC/40 which delivers 24V and max current 40A. Using a Current Clamp I measured the input current on each phase to be 0.6A.
My question is, how to calculate the output current according to this three-phase input current.
From its manual the power supply efficiency is typ. 93 % (400 V AC).

  • \$\begingroup\$ Are the loads connected in star or delta? \$\endgroup\$
    – Rens
    Nov 25, 2021 at 14:09
  • \$\begingroup\$ The load is the three phase power supply, don't know how its connected inside. I just want to know the current going out from the 24V output. \$\endgroup\$
    – Pedro NF
    Nov 25, 2021 at 14:22
  • \$\begingroup\$ Seems like walking across the bridge to fetch water, but you may have your reasons. I do however need to ask, why not measure the 24 V DC current directly with a clamp? \$\endgroup\$
    – winny
    Nov 25, 2021 at 14:29
  • \$\begingroup\$ Its a 40A power supply, I don't have a 40A max current multi-meter. For now the current consumption is low but it can be much higher, I need a simple strategy to evaluate the 24V output current from its input three-phase AC current. \$\endgroup\$
    – Pedro NF
    Nov 25, 2021 at 14:32
  • 2
    \$\begingroup\$ This is an XY Question -- your real question is "How do I measure 40A with a meter that doesn't have that range?" If the power factor of the power supply were 1, if the efficiency was exactly what the label said (and I could use up a comment explaining why it's not), if the incoming voltage were exactly as specified, and if the output were exactly 24V, then you could compute the supply's input power, divide by 24V, divide by the efficiency, and have the output current. There's too many "ifs" for that to work. \$\endgroup\$
    – TimWescott
    Nov 25, 2021 at 16:09

2 Answers 2


To calculate the input power you need the power factor from the data sheet which is 0.77.

\$ Input\,Power = 3 \times 230\;V \times 0.6\;A \times 0.77 = \sqrt 3 \times 400\;V \times 0.6\;A \times 0.77 = 320\;W \\[2ex] Output\,Power = Input\,Power \times Efficiency = 320\;W \times 0.93 = 297.6\;W \\[2ex] Output\,Current = \frac{Output\,Power}{Output\,Voltage} = \frac{297.6\;W}{24\;V} = 12.4\; A \$

But I wouldn't calculate the DC current this way. The power factor and efficiency will change with output load so this calculation can only give a rough guide. Your supply voltage is also likely to vary somewhat.

You would be much better off getting a Hall effect AC/DC clamp meter which will let you read the DC current directly with more accuracy. Clamp meters tend to have high current ranges so will read 40 A directly. Make sure you get one that is DC. Many clamp meters are transformer-operated rather than using a Hall effect sensor and are therefore AC only.

As a bonus you won't need to cut the DC power leads to insert the ammeter in circuit. If you have multiple leads running from the power supply you just need to put all the positive leads (usually) or all the negative leads through the jaws of the ammeter (but not both sets of leads which should give a zero reading). Check the jaws are large enough to accept all the wires you are using.

It is also much safer to be making measurements on a 24 V circuit than on a 400 V three-phase circuit which typically have very high fault current levels if you make a mistake.


You are measuring the line-currents. Assuming your measurements are right and you have a balanced load. The total current can be simply calculated using Itot = 3 * Iline = 3 * 0.6 = 1.8A

  • \$\begingroup\$ These are the input currents, I want to know what is the actual output current from the 24V output. \$\endgroup\$
    – Pedro NF
    Nov 25, 2021 at 14:23

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