The following sections are the ANSI / IEEE Std 141 formulae used by Volts for voltage drop computations.
AC Circuits
For AC circuits, where AC resistance and inductive reactance are considered, the following is the IEEE Std 141 exact voltage drop formula.
where: 
Vd = Voltage drop (Line to Neutral) 
V = Voltage (source) 
I = Current in amperes (A) 
R = AC Resistance from NEC® Chapter 9 Table 9 (Ohms to Neutral) 
X = AC Reactance from NEC® Chapter 9 Table 9 (Ohms to Neutral) 
distance (L) is considered from the Resistance & Reactance Tables 
where Ohms per unit / 1000 * L in same unit = R or X 
θ = Angle of Phase Offset = Arc Cosine (device or circuit Power Factor)
Line to Line is computed by Line to Neutral VD / Sqrt(3).

DC Circuits
where: 
Vd = Voltage drop 
R = DC Resistance from NEC® Chapter 9 Table 8 
L = Distance 
I = Current in amperes (A) 

K = Material Resistivity constant  12.9 for Cu & 21.2 for Al 
CM = Circular mils of conductor 
Ambient Temperature
Additionally, ambient temperature is considered with the following ratio of temperatures formula. This formula is used to adjust
the Chapter 9 Table 9 values from 75ºC to the installation ambient temperature in Celsius (C).
R2 = New Conductor Resistance 
R1 = Original Conductor Resistance 
K = Material Resistivity 
T2 = Ambient Temperature in Celsius (TA) 
Eddy Currents
Eddy currents are induced currents in surrounding magnetic or nonmagnetic metal. These currents create heating in the metal and therefore act as an
energy loss that translates into an increase in resistance of the circuit. NEC® Chapter 9, Table 9 segregates
conduits into three groups, PVC, Aluminum and Steel to account for the added resistance with each of the three conduit group types. PVC, being nonmetallic, does not produce any eddy currents and therefore has the least
resistance value of the three. Steel and Aluminum conduit, being metallic, do produce eddy currents and their respective resistance values are reflective of this.
Volts utilizes these factors in determining the correct conductor size and voltage drop computations.