How to size electric service cables

Updated March 21, 2017

There are a variety of types and sizes of electrical cables and wires from which to choose when undertaking electrical projects. Knowing some of the common uses and types of electrical cables will help to narrow down the different choices from which to choose. Single conductor wires THW and THHN are protected by a metallic or plastic sheath and are commonly used in modern homes. There are other wires that are produced for specialised applications such as those with a UV protective coating or USE (underground service entrance) wiring which is typically used for outdoor needs, such as those related to solar panel systems.

Write down a list of appliances that you intend to use on the electric circuit that you are wiring. Add the maximum amperage that will be travelling through the system for which you are building. Note that the maximum amperage is not necessarily the operating amperage of each appliance, as the start up amperage can be up to twice that of the normal operating amperage. Most appliances should have a label with the operating and maximum amperage load.

Use a tape measure to determine the length of the wires and cables that will be used in the project you are completing. The length, as well as the amperage, impacts the size of wire or cable that is most appropriate for your electrical system.

Consult with your electricity supplier in order to determine the restrictions or mandates on electrical cable sizing for certain applications. It is important to check with your local government or a licensed electrician to make sure that you are not violating laws and regulations concerning electrical cable and wiring. According to Electrical-Online, some jurisdictions require #12 AWG (American Wire Gauge) as a minimum wire size.

Consult a wire sizing chart in order to determine the appropriate wire size based on the amperage and the length of the run for the electrical wire. Sizing charts can be found at the Powerstream, Wiring Help or the 12 Volt websites (see Resources).

Check other resources such as the National Electric Code for recommendations concerning specific wiring applications. NMD 90 (non-metallic dry service cable), sized #14 AWG, is used in most modern houses. Some other commonly used cables include 2-conductor #14, which is used for plugs, switches and lights; 3-conductor #14 is typically used for 3-way switches and split receptacles; 2-conductor #12 is commonly used for kitchen outlets, heaters and air-compressors; 3-conductor #10 is used for anything that requires a maximum load of 30 amps; 3-conductor #8 should be used for any device with a maximum of 40 amps.

Things You'll Need

  • Computer with Internet access
  • Pen
  • Paper
  • Tape measure
bibliography-icon icon for annotation tool Cite this Article

About the Author

David McGuffin is a writer from Asheville, N.C. and began writing professionally in 2009. He has Bachelor of Arts degrees from the University of North Carolina, Asheville and Montreat College in history and music, and a Bachelor of Science in outdoor education. McGuffin is recognized as an Undergraduate Research Scholar for publishing original research on postmodern music theory and analysis.