How to Determine Wire Gauge for Distance

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Employ best practices to ensure proper electrical cable selection and safety.

Fires and electrical damage can occur if electrical cables used in your home or office are not of the proper wire gauge for the amperage in use. Conversely, electrical devices may fail to work properly if the wire gauge is not large enough for the distance of the circuit. Use a standard "Voltage Drop" formula to determine the correct wire gauge for a given distance so electrical devices plugged into the circuit work properly.


Step 1

Find the Voltage Drop Index (VDI) for the circuit by using the following formula: VDI = amps x distance in feet / % voltage drop x voltage. "Amps" refers to the total number of amps required for all devices that will be connected to the circuit. "Distance in feet" is the total distance for the power circuit. Substitute "3" for the "% voltage drop." Replace "Voltage" with the voltage required by devices that will be using the circuit. All devices must use the same voltage if they will be plugged into the circuit directly.

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Step 2

Calculate the "Voltage Drop Index" using the values entered into the formula in the previous step.

Step 3

Look up the "Voltage Drop Index" result in the VDI column of the Universal Wire Sizing Chart found at the AltE University website (see Resources).


Step 4

Confirm that the number in the "Ampacity" column of the Universal Wire Sizing Chart meets or exceeds the total amperage planned for the circuit. If not, move down the "Ampacity" column to the next value that meets or exceeds the planned amperage for the circuit.

Step 5

Reference the "Wire Size" column in the same row of the Universal Wire Sizing Chart as the "Ampacity" value that meets or exceeds the planned amperage for the circuit to find the appropriate wire gauge for the circuit distance.


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