How far can you run 18 gauge irrigation wire and still have the valve operate properly?
The answer to this question requires some basic understanding of the concept of "voltage drop". When electricity is flowing through a wire, some voltage is lost due to resistance in the wire. The amount of voltage loss is dependent upon the length of the circuit, the number and quality of splices, the size/resistance of the particular wire being used and the amount of current flowing through the wire (the amp draw of the solenoid).
Electrical resistance is comparable to friction loss in piping. As water flows down a pipe, it loses pressure (PSI). As current flows down a wire, it loses voltage.
Let's consider a "normal" installation: a single irrigation solenoid that needs 0.25 amps at 24 volts AC to operate properly that is located 750 feet from the controller. Let's assume voltage output at the controller is at least 24 volts. The total length of the circuit we need to consider is 1,500 feet (750 feet out to the solenoid on the "hot" wire and 750 feet back on the "common" wire.)
If the wire is too small or is run too far, the valve won't receive enough current to open or close against the pressure of the water. The higher the water pressure is, the greater the current must be to actuate the solenoid. With 85 psi static pressure at the valve and a standard solenoid, 18-gauge wire might be used for total circuit runs up to 2,000 feet, unless otherwise specified by the manufacturer. So it would be safe to use for a single solenoid circuit up to 750 feet away.
If you're sizing long wire runs, please ask us to help you make the right wire size choices. You don't want to install the wrong size!