How to calculate transmission line losses

Written by doug leenhouts
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How to calculate transmission line losses
Electricity is transmitted at high voltage to reduce power loss. (transmission line image by Vladimir Kirienko from

As electrical transmission lines span hundreds of miles, there will be some load loss incurred. The longer the line, the more electricity will be lost to the resistance inherent in the line material. Joule's Law states that energy losses are proportional to the square of the current; therefore, if the voltage is kept high, the power lost will be comparatively low. The equation for line losses is defined as P(loss) = (I^2)(R) where I is the current and R is the resistance in the wire.

Skill level:

Things you need

  • Power demand (P)
  • Resistance (R)
  • Line Voltage (V)

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  1. 1

    Calculate the resistance of the wire by multiplying the cross sectional area by the length and multiplying by the resistivity of the material. The resistivity of copper is typically 0.000999 ohms per foot, so the resistance of a 14 gauge copper line (a cross section of 2.081 square millimetres) 1 mile long would be 13.6 ohms, according to Jeff Lucius's resistance calculator.

  2. 2

    Determine the voltage produced by the source. This is the output voltage of the transformer; long distance transmission is often stepped to 400 kV, according to Enmax, a Canadian electricity provider.

  3. 3

    Determine the power being produced by the power station. Nine Mile Point, for example, supplies 1,758MW of electricity.

  4. 4

    Determine the current by dividing the voltage by the resistance. Square this result and multiply it by the resistance in the line, and the result will be the load loss, measured in watts.

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