Buyer Guide: Wiring gage and voltage loss?

Wiring gage and voltage loss for access control installations


Wiring in many cases is a slightly lower priority subject when discussed with electric locks and door access control installation. Many installers and do-it-yourself people use a wire gage that they are familiar with, usually 18 or 22 gage. However, it is important to account for potential voltage loss over longer wire runs. At Maglocks, our advisors are always available to assist with wire selection and gage requirements. In addition, the following formula and example, provide a simple approach for figuring out the general voltage drop (or loss) in a given current.

Derive the resistance per 1,000 feet of wire, using the table below, and multiply it by the current. This equals the voltage loss.

Gage Wire Resistance / 1,000 Feet in Ohms @ 77 degrees Fehrenheit (25 degrees Celsius)

  • 12 Gage wire = 1.62 Ohms
  • 14 Gage wire = 2.58 Ohms
  • 16 Gage wire = 4.09 Ohms
  • 18 Gage wire = 6.51 Ohms
  • 20 Gage wire = 10.4 Ohms
  • 22 Gage wire = 16.5 Ohms

Note: The Ohms figures are sourced from the National Institute of Standards and Technology. The National Institute of Standards and Technology (NIST) is a measurement standards laboratory, and a non-regulatory agency of the United States Department of Commerce.

Voltage Loss example

Proposed Installation:

1,500 ft. of 18 gage wire with an electric strike drawing .3 amps.

Voltage loss Formula:

Ohms x (Wire Length / 1,000) x Current = Voltage Drop

Voltage loss Equation:

6.51 x (1,500 / 1,000) x .3 Amps =

.51 x 1.5 x .3 = 2.91 Voltage Drop

If you have an electric strike that takes .3 amps at 24 volts, by the time you send the power out through 1,500 feet of 18 gage wire you only have 21.09 volts at the strike [24 volts less voltage loss of 2.91 volts = 21.09 volts or 87.9%]. This would usually fall within the tolerances of the electric strike and not be a problem. But, if you had a 12 volt electric strike at the same current and wire length, you would only have 9.09 volts at the strike [12 volts less voltage loss of 2.91 volts = 9.09 volts or 75.75%] which would not fall within the nominal 10% to 15% variance given by most manufacturers. In this case you would only have circa 75% of the rated voltage needed to power the strike. The result would be marginal operation of the strike.

As a rule of thumb, if you increase your wire length you must increase your wire gage. Likewise, the lower your voltage, the larger your wire gage must be. In any case you should calculate the voltage loss and make your wire selection based on your findings.



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