Calculating the Voltage Drop on 12-Volt Systems

The greater wire resistance that you encounter, the greater voltage drop will incur. Besides that, the metal in the wire also plays its part in contributing to the voltage degradation. In order to allow lower voltages than the one supplied to the circuit for specific purposes within the device, voltage drop series circuits need to be used in direct and alternating current (DC and AC) devices. For instance, you would be having a device that comes with a 24-volt DC battery, an 18-volt motor, 3.4-volt lights and a 5-volt control logic chip, which all need a steady supply of voltages. All this calculation follows the principle of Kirchhoff’s Voltage Law, which mentioned that the summation of all voltages contains in a series of circuit, should be equal to zero.

What you need to do:

  1. Calculator

Voltage Drop


  1. Firstly, you need to gather all the necessary data in order for you to know certain information such as 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.
  2. You can use both specific resistance and the cross-section area as reference. However, the cross section can be calculated from the AWG rating by using a formula as described below:

Cross Section Area (in mm^2) = (((0.127 * 92 ^ ((36 – AWG) / 39)) / 2) ^ 2) * pi

  1. You can also input your data by using the following formula:

Voltage Drop = Amps * (2 * Length of Wire * Specific Resistance / Cross Section Area)

In the formula given above, the number “2″ is representing the accounting for the electricity’s return trip meaning that it doubles the length of the wire. For instance, you are having a 16 AWG copper wire that runs up to 20 meters along with one amp. You need to reference a specific resistance for this kind of copper at 0.01785 ohm*mm*mm/m with a cross-section diameter of at least 1.3 mm*mm. The sample calculation is as follow:

Voltage Drop = 1 * (2 * 20 * 0.01785 / 1.3)

Voltage Drop = 1 * (0.714 / 1.3)

Voltage Drop = 0.549

  1. Once you have done the calculation, you would be able to compare the above figure against the original voltage of 12 volt in order for you to see the percentage drop. The comparison calculation for the percentage drop is as follows:

Percentage of Voltage Drop = Voltage Drop / Original Voltage * 100

Percentage of Voltage Drop = 0.549 / 12 * 100

Percentage of Voltage Drop = 4.58%

  1. Whenever you are using high impedance resistor voltage drop circuits you would be able to conserve power. However, if you install resistors, which are too low of its overall circuit ohm value, will result to more flowing of your current usage thus creating the possibility of smoking or burning of your electronic components.

Image Credit:

Flickr CC

Additional Reading:

Ohm’s Law, Electrical Math and Voltage Drop Calculations

Wiring Simplified: Based on the 2008 National Electrical Code