How to Size Wire for Low Voltage


The voltage that a wire carries affects whether the electrical-driven device performs efficiently or is "starved" for proper voltage: for example, whether a light is fully illuminated or dimmer than normal. Sizing a wire for low voltage in a conventional home electrical system requires following a formula that takes into account the resistance or the wire, the distance from the electrical source to the device that will consume the electricity and the acceptable amount of voltage loss --- nominally taken as three percent. You can size a wire using this formula to ensure that your low voltage system is providing the needed amount of electricity to those devices that are connected to it.

Things You'll Need

  • Tape measure
  • Measure the distance --- in feet --- between the electrical junction box and where you will be wiring an electric socket for use by low-voltage devices. Write this measurement down on a piece of paper: for instance 6 feet 6 inches would be written 6.5 feet.

  • Determine the acceptable voltage drop that you will have to deal with on a low voltage system --- this number is typically three percent; write down the number "3."

  • Find the "amperage" capacity of the electric wiring that is to be used --- this information is provided with the wiring.

  • Determine the voltage drop index, or "VDI," with the following formula: (A D)/(Vd V)=VDI. "A" is amperage, "D" is distance, "Vd" is acceptable voltage drop (normally 3) and "V" is voltage (120).

  • Compare the results written down with a VDI chart --- the chart can be found online or found next to wiring for sale at hardware stores. The chart will show you the nearest wire size that matches your results.

Tips & Warnings

  • Fire safety is a vital concern when dealing with low voltage systems.
  • Handling electrical wiring must be done wearing the proper protective gear and should not be done by an amateur.

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