The power drop or line loss in a wire depends on the wire length, size, and the current running through the wire. Larger wires have less resistance and can transmit more power without large losses. Losses in smaller wire remain low if the amount of power transmitted is small, or if the wire is not very long.
Electric wires have a resistance per foot; the longer the wire, the greater the resistance. When an electric current flows through the wire, the current flowing through the wire's resistance results in a voltage drop according to Ohm's law:
Voltage = Current x Resistance
Watts = Voltage x Current
Electrical current and wire resistance define the applicable voltage drop. For example, if it is 10 volts for a current of 10 amps, the power lost in the cable is 100 watts. (Larger wire has less resistance per foot than smaller wire.)
To put it in terms of an automobile, the voltage can be compared to the octane of the fuel that is put in a car, amperage would be the equivalent to the car's horsepower.
The following formula and example provide a simple approach for figuring out the general voltage/line loss for a given current. The table below gives the resistance per 1,000 feet of wire; multiplying it by the current equals the line loss.
Gage Wire Resistance / 1,000 Feet in ohms at 77 degrees Fahrenheit (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
Line loss example installation: 1,500 ft. of 18 gauge wire with an electric strike drawing .3 amps.
Voltage loss Formula - Ohms x (Wire Length / 1,000) x Current = Voltage drop (line loss)
Voltage loss Equation - 6.51 x (1,500 / 1,000) x .3 Amps = .51 x 1.5 x .3 = 2.91 Voltage Drop
As an example, if you have an electric strike that takes .3 amps at 24 volts, by the time power is sent out through 1,500 feet of 18 gauge wire, there is only 21.09 volts at the strike (24 volts minus a voltage loss of 2.91 volts = 21.09 volts or 87.9%). This would generally fall within the tolerances of the electric strike and wouldn't be a problem. However, for a 12-volt electric strike at the same current and wire length, there would only be 9.09 volts at the strike (12 volts minus a voltage loss of 2.91 volts = 9.09 volts or 75.75%). This would not fall within the nominal 10% to 15% variance given by most manufacturers, providing only about 75% of the rated voltage needed to power the strike.
As a rule of thumb, when the wire length is increased, the wire gauge must be increased. Likewise, the lower your voltage, the larger your wire gauge must be. In any case, calculate the voltage loss and make your wire selection based on your findings.
Determining Wire Gauge Requirements Based on Distance.
When installing electronic access control systems, it is necessary to determine the correct gauge of wire to prevent line loss over long wire runs. These charts show approximate distance/wire gauge recommendations based on voltage/amperage requirements.
Determine the total amperage required by the equipment you are using before determining wire size. The vertical column on the left indicates the amperage. Follow the line which matches your amperage requirement to the maximum distance your wire will run and then go up to the recommended wire size.