# when calculating apparent power, how do you know if its leading or lagging?

### 3 Answers

- 8 years agoFavorite Answer
Well, if the pf is given and you know nothing else, then you cannot know.

If YOU are determining the pf in a circuit, it just depends on if it is ultimately inductive or capacitive reactance that dominates:

Recall the equation for average power: ½ VmIm cos (θ)

The Power Factor is simply cos(θ): pf = cos (θ)

Therefore we can rewrite average power as: ½ VmIm (pf)

Recall that θ is the angle of the impedance Z

In a purely resistive load, θ=0, therefore the pf=1.

In a purely reactive load (all capacitance or all inductance), θ=±90°, therefore the pf=0.

Note that the angle of Z must be between +90° and -90° because if there is any resistance,

resistance can not be negative. Only reactance can be negative, as is the case with a capacitor.

Since pf = cos(θ), then the pf will always be some number between 0 and 1.

The power factor is never negative because resistance is never negative.

If the pf = ½, how do you know if θ= +60° or -60°? You don't unless

there is some way of specifying that the reactance is either inductive or capacitive.

Source(s): 30 years engineering- Login to reply the answers

- Mr. Un-couthLv 78 years ago
Apparent power = (Volts)*(Amps) with complete disregard to the phase angle between the two. As such it is not normally said to lead or lag any thing. Apparent power is the hypotenuse of the power right triangle whose adjacent side is Watts and whose opposite side is VAR (Volt-Amps reactive). As such it is always of a positive polarity regardless of whether it's in the 1st quad or 4th quad. Watts and VAs (apparent power) are always positive but VAR can be positive or negative. The polarity of the load's VAR is what determines whether the load's power factor is leading or lagging. Nothing regarding a leading or lagging power factor can be determined from the magnitude of the apparent power alone. If the phase angle between load Voltage and load current was included with the magnitude of the apparent power, which it never is,then you might stretch things a little bit and use Watts as reference 0 degrees and say that the apparent power is leading or lagging the Watts.

If an electrical load's net reactance is inductive, that is XL > XC, then the load current lags the load Voltage and the load is said to have a lagging power factor. If XL < XC then the load's net reactance is capacitive and the load current leads the load Voltage and now the load is said to have a leading power factor.

- Login to reply the answers

- monophotoLv 78 years ago
In order to calculate apparent power, you need to know voltage current, and phase angle (or power factor).

Now, the situation may be that you don't know phase angle (power factor) numercially, but in the vast majority of practical applications, power factor is lagging (current lags voltage).

- Login to reply the answers