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How-To: Measure Automotive Wire Sizes

Careless thinking leads some to compare wire sizes by measuring the outside diameter of the insulation with the assumption that all types of wire have the same insulation thickness. As demonstrated below, this is not so! To measure multi-stranded wire, you must add together the cross sectional measurements of the conductor copper strands. Let’s walk through the measurement of a sample wire that has been damaged.

With the wire cut and insulation stripped to reveal the conductor strands, we count 7 total conductor strands. These strands are small and can easily break when stripping away insulation, so careful inspection is important. We will use this number 7 when multiplying the total cross section.

Diameter is the distance from one side of the conductor strand to the other. Using a vernier caliper, measure the diameter of each conductor strand. Ideally, you will want to record the averaged measured diameter in the measurement unit millimeters (mm). Here we use the averaged diameter of 0.25 mm. To later calculate area, we will need to use the radius value of the measured diameter. Divide diameter by 2 to record the radius 0.125 mm.

Cross section is an area measurement. The formula for area is A = π r². We know the radius was measured to be 0.125 mm and that pi is approximately 3.14, so we can determine the area with the following multiplication: 0.125 x 0.125 x 3.14. The resulting area is approximately 0.05 mm2.

We’re not done quite yet. We’ve only determined the cross section area of each conductor strand to be 0.05 mm2. Upon initial inspection of the wire, we counted 7 total conductor strands. We must now multiply the recorded cross sectional area of one conductor strand by the total number of conductor strands. Our final math, 0.05 x 7, gives us a total cross sectional area of approximately 0.35 mm2.

Looking at our AWG to mm2 chart, we see that the wire we need to use to replace this damaged wire is 22 AWG. While it is true that you can safely substitute a larger wire size, be sure to consider termination, wire splice technique, and tooling; using a terminal and tooling for 22 AWG wire on a larger 20 AWG wire will result in a crushed crimp that may cause the terminal or conductor strands to break.

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