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Thread: NEC Voltage Drop Calcs & Power Factor

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    NEC Voltage Drop Calcs & Power Factor

    Been putting together a voltage drop calculation spreadsheet based on the NEC tables and noticed something surprising. I though that the voltage drop in a circuit would always increase with decreasing power factor. That tends to be the case with larger wire sizes (3/0 and above) but is definitely not the case with smaller wires. These calculations were based on Chapter 9 Table 9, I could show my work if you want but taking the following parameters, the voltage drop is worst with unity pF:

    A=50A
    wire size=8awg
    length=50
    voltage=120
    phase=1

    Here are the percent voltage drops I got starting with pf=1, then .9, .8, .7, .6 ,......,.2

    pf=1 - 3.24%
    3.04%
    2.76%
    2.47%
    2.17%
    1.86%
    1.55%
    1.23%
    pf=0.2 - 0.92%

    This happens for all the wires up to 2/0. What gives? Is it just due to the impedances of smaller wires?

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    Quote Originally Posted by shespuzzling View Post
    ..I though that the voltage drop in a circuit would always increase with decreasing power factor..
    My Tbl-9 calculator followed your results also - for purely resistive loads only. Very interesting, perhaps the engineers can explain this?

    However, for inductive-motor loads adjusted per NEC 220.18(A), both Current and Voltage Drop increase with decreasing Power Factor, inversely proportional, as expected.
    Roger Ramjet NoFixNoPay

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    any thoughts?

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    Ask Ingenieur to join this discussion.
    Roger Ramjet NoFixNoPay

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