Q Factor in Voltage Drop Calcs

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jimingram

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I have a question about the "Q" factor when doing a voltage drop calculation.
I am familiar with the standard voltage drop calculations. Mike Holt has some good examples in his book; however, he only mentions the "Q" factor without giving any examples. I find the definition confusing. Eg. divide Table 9 by Table 8.

I need to calculate the voltage drop for a rooftop A/C. It is rated FLA 145 amps at 3 phase, 208 volts. I will be running aluminum conductors in PVC conduit 400' from the source of power.

The A/C is rated to work at 187 volts so I have a lot of breathing room regarding voltage drop.

My question would be how to properly apply the "Q" factor adjustment in this situation.
 
I'm not sure but I'm guessing he is referring to the Q from the formula:
S^2 = P^2 +Q^2
which is the same as
(Apparant Power)^2 = (Real Power)^2 + (Reactive Power)^2
which is the same as
VA^2 = W^2 + VAR^2

and you use the power factor which is
power factor = P/S = W/VA = W/[sqrt(P^2 + Q^2)]


There is another Q that is used in metering that used to be used to measure VAR where
VAR = (2Q-W)/sqrt(3)
but I doubt this is what he is talking about.
 
jimingram said:
I have a question about the "Q" factor when doing a voltage drop calculation.
I am familiar with the standard voltage drop calculations. Mike Holt has some good examples in his book; however, he only mentions the "Q" factor without giving any examples. I find the definition confusing. Eg. divide Table 9 by Table 8.

I need to calculate the voltage drop for a rooftop A/C. It is rated FLA 145 amps at 3 phase, 208 volts. I will be running aluminum conductors in PVC conduit 400' from the source of power.

The A/C is rated to work at 187 volts so I have a lot of breathing room regarding voltage drop.

My question would be how to properly apply the "Q" factor adjustment in this situation.
Click to expand...
Can you give us an example of the caculations? I'm not familar with the Q factor.
 
The term 'Q', which stands for 'quality', is usually used to describe certain electronic circuits, such as the sharpness of the slopes of a band-pass filter. It refers to the ratio of the height of the 'bell-curve' shaped frequency response relative to the width of the slopes.

You may have heard the term "x db per octave" in reference to the sharpness of a high-pass or low-pass filter. A band-pass filter is merely a combination of the two, passing a narrow band of frequencies. The narrower the pass band is, the higher the 'Q' of the filter.

I'm not sure how that would relate to the question, but maybe this information will help.
 
LarryFine said:
...I'm not sure how that would relate to the question, but maybe this information will help.
Click to expand...
A ways removed from voltdrop. That's why I kind of stuck with stuff related to power factor.

Looking through some of the stuff Mike has written, he appears to be referring to the difference in DC and AC resistance. Mike calls the Q adjustment factor the value obtained by dividing the ac ohms-to-neutral impedance in Table 9 by the DC resistance in Table 8. One place he uses this is in the formula:
VD = (1.732 x K x Q x I x D)/cmil

He says it is only an issue for ac circuits with conductors 2/0 or larger. He says the K resistance per 1,000 cm for a 1000 ft long conductor (12.9 for copper & 21.2 for al) must be adjusted for the effects of self-induction (eddy currents).
 
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