WAZIPOINT WAZIPOINT Engineering Science & Technology: Calculating the resistance of a wire

Thursday, February 16, 2023

Calculating the resistance of a wire


In many cases, we will know the length of a wire l and the AWG (American Wire Gauge) or 
SWG (British Standard Wire Gauge) or IEC standard size of the wire, but not the resistance. It's easy to calculate the resistance though.

Wikipedia has a list of AWG specifications available here, which includes the resistance per meter in Ohms per kilometer or milliOhms per meter. They also have it per kilofeet or feet.

The formula for calculating resistance

R = V / I
Knowing the total current and the voltage across the whole circuit, we can calculate the total resistance using 
Ohm's Law: R = V / I.

Wire Resistance Calculation Procedure 

  1. Identify the resistivity (ρ), length (L), and radius (r) of the wire;
  2. Calculate the cross-sectional area of the wire using the formula for the area of a circle: A=πr2;
  3. Calculate the resistance (R) of the wire using the formula for resistance: R=ρLA R = ρ L A .

We can calculate the resistance of the wire Rwire by multiplying the length of the wire by the resistance per meter.

An example
We have 500m of a 20AWG wire. What will be the total resistance?

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