I have a 3 phase switching power supply that I am trying to calculate the maximum input power.

It is hooked up to a 3-phase, 277/480 VAC power source.

The sticker on the power supply says:

"INPUT : AC INPUT: 440V 3~; 3W+PE 10A 50/60Hz"

So I know since this is a switching power supply, I would expect the input power to be higher than the output, but maybe just by 10% or so.

Going off the sticker values, and according to some old notes I had, the 3-phase power would be calculated like this:

S = sqrt(3) * Vll * Il * cos(theta)

Where Vll is the line-to-line voltage (440V)
Il is the "Current in one line". Not sure if this should be 10, or 10/3?
theta is the power factor. I'll assume 1.0 for now.

This gives me 7621 V-A if I use 10A, or 2540 V-A if I use 10A/3. Neither of these seem right.

  • 1
    \$\begingroup\$ Is this a Delta/Triangle or Star/Wye connected circuit? \$\endgroup\$
    – user103380
    Commented Mar 12, 2018 at 15:22

2 Answers 2


It makes no difference whether the supply is wye or delta. You use the line-to-line voltage of the input. The input VA = V X A X SqRt(3) 440 X 10 X 1.732 = 7621. You must multiply the VA by efficiency and power factor. Because of the harmonic content of the input current to an electronic circuit, the power factor can not be known without throughly analyzing or simulating the circuit. The same is true of the efficiency. The current may actually be based on an input voltage a little below 440 volts (perhaps 5 or 10 percent) to allow for normal voltage tolerance.

The maximum input power will be output power divided by efficiency regardless of the input power factor, harmonics or input voltage fluctuation. The best you can do is guess at the efficiency and calculate input power based on output power.

  • \$\begingroup\$ Assuming a not too old device and a regulated country, one wouldexpect a pf of near 1 \$\endgroup\$
    – PlasmaHH
    Commented Mar 12, 2018 at 16:08
  • \$\begingroup\$ The required pf may vary by power level and country (or continent). Tony Stewart also has a good point re marking the input higher because of short term current requirements. \$\endgroup\$
    – user80875
    Commented Mar 12, 2018 at 17:13

The Nameplate gives the breaker rating required not the actual consumption. I would expect internal protection but for sharing higher circuit capacity, this is the allocated amount of current in the circuit.

E.g. the soft start of the Artesyn PSU is up to 40A which is partly why a 10A 3ph breaker is required.

  • \$\begingroup\$ I agree, the 10A on the sticker is more likely to be the circuit size, also known as a MOCP (Maximum Over Current Protection) rating, or maybe the MCA (Minimum Circuit Ampacity) but not the input current. Manufacturers generally don't put the input current on the nameplate because people will use that value for the MOCP or MCA and cause themselves no end of headaches. \$\endgroup\$
    – JRaef
    Commented Mar 15, 2019 at 0:35
  • \$\begingroup\$ Often device ratings are just added up to the circuit /breaker rating size. I might expect the PSU to protect itself with internal fuses or triple breaker. \$\endgroup\$ Commented Mar 15, 2019 at 1:12

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