(These same assumptions are used for the Cable Size Calculator)
The resistivity used in the voltage drop calculation was for copper at 30 degrees C.
The resistivity rho_0 of copper at t_0 = 20 degrees C was taken to be 1.68e-8 ohm meters.
The temperature coefficient alpha_20 of copper at 20 degrees C was taken to be 0.003862 K^-1
The resistivity of copper at t = 30 degrees C was calculated using the following linear approximation:
rho(t) = rho_0 * [1 + alpha_20 * (t – t_0)]
which gives, for t = 30 deg C,
rho(30) = 1.68e-8 * (1 + 0.003862 * 10) = 1.74e-8 ohm meters
The cross sectional area of the wire was determined by the AWG standard and used with the resistivity rho to calculate the resistance R in a wire of length L which is given by R = rho * L / A.
The voltage drop was calculated using Ohm’s law; Vd = IR where Vd is the voltage drop, I is the current, and R is the resistance of the wire.
Finally, the percent voltage drop was calculated using Vdp = Vd / V * 100, where Vdp is the percent voltage drop, Vd is the voltage drop, and V is the input voltage.
Ampacity was calculated using the 2002 National Electric Code table which assumes an ambient temperature of 30 degrees C. It was assumed that the wire being used has an insulation of type TW/UF rated at 60 degrees C.