Ohms law can be used to derive the computation of DC resistance mathematically. This resistance calculation is fairly easy and removes many of the difficulties, but is only perfect in a laboratory environment. This mathematical equation is only accurate when the current density across the wire or conductor is totally uniform. However, this doesn't really happen. For practical reasons, almost any connection to a real electrical power supply will not show perfect current density, or not totally uniform electrical current. To accomodate more factors, the resistance equation can obviously be enhanced, but let's keep it simple for explanation's sake. In addition, this formula still provides a good enough approximation for many Code Inspectors, and is the standard for long thin conductors such as wires.

Where:
l is the length of the conductor, measured in meters
A is the cross-sectional area, measured in square meters
p(Greek: rho) is the electrical resitivity (also called specific electrical resistance)
Copper Electrical resistivity (microohm/cm) 1.678

Ohms of resistance in a material can be measured by a multimeter (electrical meter) over a wire. Resistance is a measure of the material's ability to oppose the flow of electric current.

Calculate Voltage Drop in Wire:
To calculate voltage drop, plug in the values: V = DIR/1000
I= amperage,
R = ohms of resistance per 10 feet of wire,
D = the total distance the current travels