How to Calculate the Voltage Drop on 12-Volt Systems
- 1). Gather all the necessary data. You will need to know the amount of amps in the system, the length of wire in meters, the specific resistance for the wire's metal in ohms*mm*mm/m, and the cross-section area in square mm. Both the specific resistance and the cross-section area can be referenced (see Resources). But the cross section can also be calculated from the AWG rating using the following formula:
Cross Section Area (in mm^2) = (((0.127 * 92 ^ ((36 - AWG) / 39)) / 2) ^ 2) * pi - 2). Input the data into the following formula:
Voltage Drop = Amps * (2 * Wire Length * Specific Resistance / Cross Section Area)
The "2" in this equation doubles the length, accounting for the electricity's return trip. As an example, you have a 16 AWG copper wire running 20 meters with one amp. You reference the specific resistance of this particular kind of copper at 0.01785 ohm*mm*mm/m, with a cross-section diameter of 1.3 mm*mm:
Voltage Drop = 1 * (2 * 20 * 0.01785 / 1.3)
Voltage Drop = 1 * (0.714 / 1.3)
Voltage Drop = 0.549 - 3). Compare this to the original voltage (12 volt) to see the percentage of drop:
Percent Voltage Drop = Voltage Drop / Original Voltage * 100
Percent Voltage Drop = 0.549 / 12 * 100
Percent Voltage Drop = 4.58%