1. What is the definition of apparent power?
  2. How to calculate apparent power for three-phase circuit? Is it S = S1 + S2 + S3 or S = sqrt(P^2 + Q^2)


    P is total active power P = P1 + P2 + P3


    Q is total reactive power in system Q = Q1 + Q2 + Q3?


1 Answer 1


Apparent power is the vector sum of real power and reactive power. In effect, apparent power is V\$_{RMS} \times I_{RMS}\$ without any consideration of phase angle. Because real and reactive power are at right angles, the vector sum is: -

\$\sqrt{Real^2 + Reactive^2}\$

Once you have it for one phase then it you can, just like in real power) multiply by 3 to get the value for a balanced supply/load 3-phase system.

  • \$\begingroup\$ In fact some measurement devices calculate aggregate as arithmetic sum, some as vectorial sum (like for example meters from "Meter Test Equipment" www.mte.ch company). Is somewhere authoritative source for this like for example IEEE commitee or ISO norm? \$\endgroup\$ Feb 28, 2014 at 11:41
  • \$\begingroup\$ @truthseeker when it comes to powers (real, apparent or reactive), in a 3-phase circuit they should be simply additions of quantities - they are not meaningful vector quantities when comparing between phases. Anyway, I don't know of any authoritative source other than what I was force-fed at college many years ago!! \$\endgroup\$
    – Andy aka
    Feb 28, 2014 at 11:54

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