How to Determine the Wire Size for an Electrical Run

For safety's sake, determine the correct wire size before installing electrical outlets.

Planning the work for an electrical project must be done carefully, and in steps considering all the correct information. One of the key processes in installing a new outlet and/or wiring, is knowing how to determine the wire size for an electrical run. In order to properly calculate the right size wire, you need to know: total length of the run, size of the circuit breaker in amps and how much current the device will be pulling from the outlet (for example, a home appliance). Improper wiring size can result in a fire, so be sure to consult a licensed electrician before beginning any work.

Things You'll Need

• Calculator
• Measuring tape
• Wiring chart (see Resources link)

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Instructions

1. Turn off all power to the electrical panel before beginning any work or measurements. Any accidental contact with live electrical lines can result in injury or electrocution.

2. Measure the overall distance from the main circuit panel to the location of the outlet. This measurement will give you the total distance and amount of cable you will need to make the run. Be sure to add a few extra feet for unexpected problems.

3. Check the specifications chart on the back of the appliance that will be plugged into the outlet to determine the voltage required to power the appliance.

   Example: Refrigerator would require 110 volts.

4. Locate the circuit breaker, in the main circuit panel, to be used for this wiring. Stamped on the front of the breaker you will see the amp rating for the breaker.

   Example: 20 amp, or 40 amp.

   It is important to have the right size (amp) circuit breaker for the appliance you are running. Example: A refrigerator needs 110 volts so a 20 amp breaker is needed. A stove or electric dryer needs 220 volts so a 40 amp breaker is needed.

5. Take the total length of wire needed, the amps of the circuit breaker and the voltage needed by the appliance and insert this information into the standard formula below to determine the size wiring needed for the project. (Note: Percent Voltage Drop is already given as a national average; you may need to adjust this per your project of acceptable voltage loss.)

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

   VDI = Voltage Drop Indicator

   AMPS = AMP size of the circuit breaker

   FEET = Total length of the wire needed

   PERCENT VOLTAGE DROP = 3% (This is the national standard average)

   VOLTAGE = Amount of voltage needed by the appliance/device)

6. Use the formula in Step 5 to calculate the size of wire needed using a 20 amp circuit breaker to connect a 110V refrigerator that is plugged in 50 feet away from the breaker.

   VDI = (20 x 50) / (3 x 110)

   VDI = 1000 / 330

   VDI = 3.03

   Using the chart in the Resource link, locate the VDI column and find 3 VDI, which shows we need a wire size of 12 AWG (American Wire Gauge).

Tips & Warnings

• Never attempt to do any electrical work without first consulting a licensed electrician.

• Be sure that any and all electrical work meets the building codes for your area.

References

• Alt E University: How to Size Wiring and Cabling for Your System

Resources

• Read this Article in Spanish
• CSGNetwork.com: Wire Size Calculator