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How to Determine Electrical Wire Gauge

Updated February 21, 2017

When planning for an electrical project, it is important to make the wiring runs with the proper gauge (physical size) wire. If the wire is too small it can easily become overheated and potentially cause an electrical short or fire. It's important to remember that the larger the number for the gauge of wire, the smaller the physical size. Therefore 14 gauge wire is smaller than 10 gauge wire. Calculating with a fixed formula will help determine the proper gauge wire for your project.

Measure the distance between the two points for the length of wire needed. Use the measuring tape and record the measurement.

Calculate using the constant formula below and plugging in the numbers.

VDI = (AMPS x FEET)/(%VOLT DROP x VOLTAGE)

VDI = Voltage Drop Index Amps = Current needed for the device Feet = Measured distance in one direction between the two points % Voltage Drop = Amount of acceptable voltage drop per manufacturer Voltage = Power needed for the device

Example: 30 Amp load at 24V over a distance of 100 feet with 5% maximum voltage drop. VDI = (20 x 100) / (24 x 5) VDI = 16.6 (rounded up to 17)

Refer to the capacity chart in the Resources and locate the VDI column. The proper size gauge wire for the closest VDI to 17 is a VDI of 20, indicating the use of 4 gauge wire.

Repeat steps as needed to determine the wire gauge for any additional electrical cable runs.

Tip

Using a scientific calculator or one with the ability to store numbers in memory may help make the calculations easier.

Warning

Never attempt to use the wrong-sized wire for an electrical run, as this could result in damage to the device or service panel, or worse, cause an electrical fire.

Things You'll Need

  • Measuring tape
  • Pen
  • Paper
  • Calculator
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About the Author

Laurie Brown has worked as a high school English teacher for the last several years and loves writing. She enjoys helping her students develop a love and appreciation for writing, reading, and literature. Laurie has a degree in education with a major in English. Currently she is a writer for eHow.