In AC circuits, true power is which of the following relative to the apparent power?

• A. less than the apparent power in a reactive circuit and more than the apparent power in an inductive circuit.
• B. more than the apparent power.
• C. equal to the apparent power.
• D. less than the apparent power.

Answer: (D)

In AC circuits, true power is the actual energy converted to work or heat, while apparent power is the product of RMS voltage and RMS current. Because voltage and current can be out of phase due to reactance, some of the power oscillates back and forth as reactive power and doesn’t do real work. The relationship is P = S cos(phi), where phi is the phase angle between voltage and current. Since cos(phi) is always less than or equal to 1, true power is always less than or equal to apparent power, and it is strictly less when there is any reactance. Only in a purely resistive circuit (phi = 0) do they equal.