Improper wire can disrupt a message sent from a controller to an electric valve in the field. Even though all the other components of an irrigation system are functioning properly, wire can destroy the operation of the overall system. Properly selected irrigation wire insures clear, crisp transmission between irrigation components. Improperly selected wire will result in faulty communication or will cost more than necessary.
Preserving the Design
Irrigation wire, whether a single strand or multi-strand, must be insulated to prevent damage to the wire during installation and settling and loss of energy during operation. There are differences in the performance of polyethylene (PE) and polyvinyl chloride (PVC) insulation.
Although the insulation thickness of PE is less than that of PVC, its mechanical and insulation characteristics are equal. PE has been reported to cope better with cold weather. Because of its higher resistance characteristics, PE wire is also less likely to be damaged by lightning, chemicals (fertilizers and pesticides), cultural activity (shoveling, trenching, etc.) and stretching.
Two types, UF and TF, are commonly used for valve circuit wiring. Both are PVC insulated and provide adequate moisture and chemical protection. UF is more heavily insulated and offers greater protection and therefore is recommended the most.
Factors in sizing wire include length of run, static water pressure, the primary input voltage to the controller and the number of valves on the system that will run concurrently.
Most residential and light commercial irrigation designs specify 14-gauge wire for electric control valves and 12- or 14-gauge wire for the controller. Smaller gauge wire might be appropriate for small, tight projects.
The new generation of valves requires less voltage to carry the signal while installation techniques have improved. Such smaller projects can be fitted with 18 or 16-gauge wire, while the larger projects might require 14- to 10-gauge wire. However, don't deviate from the recommendation of the manufacturer. If the opportunity to use smaller gauge wire is limited on an extensive project, it might be prudent to forego mixing wire sizes.
When selecting wire size, start with the requirement that most remote control valves need 0.25 amperes at 24 volts AC, 60 Hz to operate properly. Energy is lost as a signal travels over wire because of the electrical resistance of the wire. The longer a wire is, the more energy will be lost to resistance. Larger wire is used to compensate for resistance as wire runs become longer.
If the wire is too small, the valve won't receive enough current to open or close against the pressure of the water. Static water pressure, or the pressure measured when no water is flowing, is an element that needs to be considered when determining wire size. The higher static water pressure will be, the greater the in-rush of current must be to actuate the solenoid, and therefore the larger the wire.
Identify the maximum distance of wire required from the controller to the furthest valve on site. The distance should include the "hot" and "common" wires. In other words, if the distance from the controller to the furthest valve is 750 feet, the combined distance of the hot and common is 1,500 feet. At 85 psi static pressure, 18-gauge wire might be used for runs up to 2,000 feet, unless otherwise specified by the manufacturer.
Applying the Design
Proper installation techniques can add considerable life to the effectiveness of the control system. Locating a faulty wire splice or damaged wire can be frustrating and time consuming. Paying attention to installation details can prevent both short- and long-term problems in the field.
Wiring should be laid in the same trench as any PE or PVC pipe -- preferably beneath the pipe for added protection. Do not bury wire in the same trench as metal pipe. During installation, provide loops or wire at the valve sites, the elbows and about every 100 feet in a straight trench. The earth expands and contracts from moisture or temperature extremes, and wire can stretch or break.
Inspect your backfill for large rocks or other material that can score or damage insulation. Don't walk or drive equipment over wire on the ground or in the trench. Damage to the insulation might not affect performance immediately but can create problems later.
Finally, never splice your wires with electrical tape, glue or any other short-cut method. Use time-tested and approved splice kits that will shield the exposed wire. All valve splices should be put in a valve box for future inspection. Shoddy splices can result in open- or short-circuits. They can prevent valves from closing because of current loss.
Neglecting design and installation details, such as wire sizing and splicing, can punch a black eye on an otherwise immaculate project. All of the sophisticated technology, conservative head spacing, matched precipitation rates, hydrozoning and hydraulics won't compensate for a misfiring electrical system. Sweat the details and your reputation will pay dividends down the road.