How to Calculate Electrical Line Loss

A voltage meter on an electrical box.
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The amount of power that can be transmitted through a material is limited by several factors, including the material itself and the temperature of the environment. This limitation is observed as a decrease in power from the source to the destination, called line loss. Line loss can be a relatively difficult value to obtain, depending on the circuit's physical makeup and environment. However, the basic calculation is fairly simple and, once understood, can be used to extrapolate line loss for any environment or variables.

Step 1

Record the amount of power ("P") required by your destination, found by multiplying the current ("I") by the voltage ("V") of your circuit. This will give you at least two-thirds of the following equation: P=I*V This equation is the foundation of calculating electrical line loss.

Step 2

Determine the overall resistance ("R") of your circuit using the following equation: R = ρ L / A ρ is your coefficient of resistance, measured in ohms, and determined by use of a resistivity chart, like the one provided in the Resources section. L is the length of cable or wire used in the circuit in meters. A is the area of a cross-section of the cable or wire in meters squared.

Step 3

Calculate line loss as the square of your current multiplied by your calculated resistance: P(loss)=I²R Using Step 1, you may substitute for I, leaving an equation of: P(loss)=P²R/V² This presents a fundamental axiom of electrical theory: line loss is greatly decreased by an increase in voltage.

Step 4

Plug your determined values into a calculator to present your line loss in decimal value, then multiply by 100 to determine your percentage.


Line loss should not be more than 5 percent for alternating current systems. Due to the complexity of environments and temperatures, you may want to use a line loss calculator, like the two in the Resources section, for more accurate results.


Miscalculation of line loss can cause failure in critical systems during power surges.

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