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Thread: Voltage Drop

  1. #1
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    Voltage Drop

    Using Vd= 1.73*K*I*L/CM

    Load= 3 phase 3 Wire.
    Feeder Length 150 FT

    Question: In regards to total length to be used in the formula, do you consider the sum of all 3 ungrounded conductors length. IE. 150 FT X 3 (250).

    Just trying to brush up.

  2. #2
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    No
    one way length
    the 1.73 accounts for the supply/rtn



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    Quote Originally Posted by Ingenieur View Post
    No
    one way length
    the 1.73 accounts for the supply/rtn
    Thanks Ingenieur , I assume that single phase length then, is the total of both of the current carrying conductors?

    Why is that, do you know?
    Do 3 PH systems harmonically cancel in some way as to only consider one conductor length for Vd calculations?

    Please Advise,

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    Quote Originally Posted by Unbridled View Post
    Thanks Ingenieur , I assume that single phase length then, is the total of both of the current carrying conductors?

    Why is that, do you know?
    Do 3 PH systems harmonically cancel in some way as to only consider one conductor length for Vd calculations?

    Please Advise,
    I think it has to do with the phases being 120 degrees out from each other.

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    With single phase you have to double length because same current is traveling in one conductor and out the other. Single phase is simple - what comes in on one has to go out on the other.

    With balanced three phase load the current returning on A is partially coming from both B and C, current returning on B is partially coming from A and C, current returning on C is partially returning on A and B. All that happening so fast it is nearly the same time from a human perspective. Square root of 3 is the key factor that results in the amount of RMS current that flows and is associated with the fact the phase angle is 120 degrees.

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    Quote Originally Posted by kwired View Post
    With single phase you have to double length because same current is traveling in one conductor and out the other. Single phase is simple - what comes in on one has to go out on the other.

    With balanced three phase load the current returning on A is partially coming from both B and C, current returning on B is partially coming from A and C, current returning on C is partially returning on A and B. All that happening so fast it is nearly the same time from a human perspective. Square root of 3 is the key factor that results in the amount of RMS current that flows and is associated with the fact the phase angle is 120 degrees.
    Yes, I understand phase 3 PH currents being 120 Degrees apart. So you are saying that this also true with voltage as used in Vd calc's?

  7. #7
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    As others have said
    it is the vector or phasor sum of 2 phase lines (if you sum 3 = 0 in a balanced system)
    ph ia = 1/0 deg = 1 + 0j
    ph ib = 1/120 deg = -0.50+ 0.866j
    sign convention is opposite, one is entering a node the other leaving
    1 + 0j -(-0.50 + 0.866j) = 1.5 - 0.866j = 1.732/-30 deg
    1.732 is sqrt3
    so magnitude of the supply/'rtn' currents is sqrt3 x 1 current



  8. #8
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    Quote Originally Posted by Unbridled View Post
    Yes, I understand phase 3 PH currents being 120 Degrees apart. So you are saying that this also true with voltage as used in Vd calc's?
    Yes, basically (but offset from i based on load type)
    but in vdrop calcs you are actually using current
    vdrop = 1.732 x i x conductor R
    the equation you used substitues Area (circ mils) and resistivity (K) for R
    R = resistivity x L/A



  9. #9
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    Quote Originally Posted by Unbridled View Post
    Yes, I understand phase 3 PH currents being 120 Degrees apart. So you are saying that this also true with voltage as used in Vd calc's?
    Yes, because the voltage drop always obeys ohm's law: E=I×R. Outgoing current on one line returns on the other two lines, varying proportionally to the respective voltage across the lines. Incoming current is just the reverse. This is true of all three lines... but 120° phasing shifts the timing of how it occurs.
    I will have achieved my life's goal if I die with a smile on my face.

  10. #10
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    We assume a balanced system
    so we only calculate drop on one line-line v, 2 lines
    and we assume the other 2 v's will be the similar



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