How do you derive the voltage drop calculation from the NEC handbook?

Discussion in 'Math' started by boule, Dec 20, 2012.

  1. boule

    Thread Starter New Member

    Nov 13, 2012
    It says this in the book (for a 2-wire dc circuit, 2-wire ac circuit, or 3-wire ac single-phase, at 100% power factor) where reactance can be neglected:

    Vd = (2 x L x R x I)/1000

    Vd = voltage drop
    L = length (one way) of circuit
    R = resistance in ohms
    I = load current

    How do you derive that simply from V=IR or V=IZ (for ac current)?

    1. What is a power factor?
    2. How do the 2-wire and 3-wires factor in the equation?
    3. Where did the 1000 come from?
    4. Does anybody know how material type and phase affect the equation?
    Last edited: Dec 20, 2012
  2. WBahn


    Mar 31, 2012
    The R is not resistance, it is ohms per thousand feet.
    The length is in feet.

    So (L/1000ft) gives the number of thousands of feet. Multiplying that by R gives the number of ohms going one way. The 2 is to get the round trip total resistance.