Irrigation & Lighting Division

Irrigation System Valve Wires

Wire and cables, especially when buried, require mechanical protection against the effects of adverse environments as well as abusive handling. On the surface, all wires appear to be the same, but in reality some vast differences exist.

Wires used in landscape and turf irrigation are usually copper and are insulated with a variety of materials. The conductor could be a single strand of solid metal or a number of strands twisted together. The multi-strand conductors are more flexible and can be bent back and forth many more times than the same size solid conductor. Electrical insulation must perform the dual function of providing the electrical resistance for protecting the conductor from the environment and providing the mechanical strength required to keep its integrity during installation and trench settling.

In the smaller gauge sizes (16 & 18), the availability of UL listed wire and the multi-conductor cables has greatly improved the reliability of the electrical part of an irrigation system in the "Res-Com" market sector. You hardly see anyone using telephone or bell wire anymore.

In larger systems where 14 gauge to 10 gauge single copper conductors are used for valve wires, there are now two choices for the insulation and both are UL listed. Some twenty years ago, there was a shortage of PVC (polyvinyl chloride) material which resulted in allocations of pipe and wire for irrigation systems. At that time, Paige Electric introduced PE (polyethylene) insulated wire to the industry and it was received with some skepticism. Those who tried it had excellent results and have continued to use it over the years. It now appears that the eastern states of the USA market are using mostly PE insulated wire while the west coast has stuck to PVC, each satisfied with the performance. Here are some of the basic differences between these two products:

FEATURE Insulation
PE PVC
Temperature Range (°C) -55 to +600 -10 to +60
Cold Bend Excellent Good
Weight (pounds per 2500 feet of 14AWG) 50 70
Conductor Copper Copper
Insulation Thickness (mils) 45 60
Moisture Resistance Excellent Good
Insulation Resistance (Meg-Ohms/1000') 50,000 200
Chemical Resistance Excellent Good
Abrasion Resistance Excellent Good
Cut-Through Resistance Excellent Good
Elongation Resistance Excellent Good
Rodent Resistance Poor Poor

A short discussion of the above features follows where appropriate:

TEMPERATURE RANGE/COLD BEND
Both products exhibit an excellent performance range of temperature with PE being the better product for cold weather applications.

WEIGHT
PE wire is approximately 28% lighter than PVC wire. This allows for easier handling and reduces the risk of injury.

INSULATION THICKNESS
Even though the insulation thickness of PE is less than PVC, PE wire has better mechanical and insulating characteristics as further discussed below. Even when the insulation is partially removed in a PE wire, as in the case of installation damage, it has better resistance to the environment than PVC. When PE wire was developed, UL allowed a thinner insulation (45 mils) than PVC wire (60 mils) because of its excellent electrical and environmental resistance properties. When wire insulation is damaged, to the degree that only 15 mils remains to protect the conductor, PVC will fail UL electrical tests while PE easily passes.

MOISTURE RESISTANCE
This measurement gives some indication of how well the insulation protects the copper conductor from the humid irrigation environment. The higher the moisture resistance, the better suited the product is for installation in wet conditions.

INSULATION RESISTANCE
The insulation resistance is an electrical measurement in which the higher the value, the better the electrical protection to the conductor. The obvious punishment that could cause electrical failure are mechanical in nature. The less obvious damage is often induced by lightning strikes, which cause microscopic pin holes in the insulation and eventual failure. This latter problem was really brought to light in the early seventies when two-wire systems utilizing DC power were installed. The DC current accelerated the failure through electrolysis, which is often seen in the posts of car batteries. In those days, the problem was solved by developing a cable with PVC insulated wires covered by a PE jacket. The PE jacket was found to have better insulating properties and to give excellent protection from shovel abuse and rock penetration. PE is also used today as a jacketing material for Rainbird MAXI communication cable. Because of its much higher resistance, PE wire insulation will be less likely to be damaged by lightning and its associated high voltage.

CHEMICAL RESISTANCE
UL tests show that when subjected to chemicals normally found in the landscape environment such as fertilizers, herbicides, and fungicides, PE wire has superior resistance, which means that the deterioration and aging process is much slower.

ABRASION RESISTANCE
This property measures how well the insulation resists surface wear, as in situations when wire is "pulled" or "laid" during installation.

CUT-THROUGH RESISTANCE
This is a measure of how well the insulation resists crushing abuse from heavy equipment, rocks, shovels, trench back-filling, and trench settling.

ELONGATION
In installations where wire is pulled or laid into the ground, longitudinal forces are exerted on the wire that could stretch it to the breaking point. This also happens when the spools are placed on a vehicle for unwinding the wire into an open trench and the wire gets tangled and stretched. Sometimes the copper conductor breaks inside the insulation and it is not apparent by visual inspection. PE wire is a bit more forgiving in these situations.

RODENT RESISTANCE
The teeth of a gopher grow about 17 inches per year and the chewing of hard objects is essential to keep the teeth trimmed and at a comfortable length. Unfortunately, neither PVC nor PE insulated conductors offer much resistance to the sharp teeth of rodents. Some believe that rodents prefer one to the other because of taste, but tests have shown that this is not true. These tests were performed by Dr. Cadjillo of Bell Laboratories in the late sixties and early seventies. The results of the tests showed that gophers will chew on any kind of insulation with a diameter of up to 1¼ inches, which appears to be the limitation posed by the size of their mouth. It was also learned that in addition to wearing down their teeth, the gophers actually use the wire insulation grinds as a bedding material for their underground nests. Several experiments were also conducted to find recommendations for solving this nagging problem. All of the recommendations require the protection of the wire with either a 5 mil stainless steel tape, an 8 mil bronze tape , fine braided wire, or concrete. The braided wire was found to be particularly annoying to the little creatures because the fine wires got caught between their teeth. Cal-Trans, the organization that is responsible for the irrigation systems along the California highways has been successfully using a steel tape armored cable, developed by Paige Electric, in those areas that are infested with gophers. The armored cable could be either a PE or PVC insulated copper conductor with a steel covering and a PVC outer jacket. It is possible to enclose more than one insulated conductor within the armor and either PVC or PE insulation will do the job. The product with PE insulation offers the above mentioned benefits at a slightly lower cost.

Upon reviewing this information it would seem that the whole industry ought to be using PE insulated wire. So why isn't everybody using this product? The possible answers are many. This would include the many specifications that were written long ago and haven't been updated. Some say that it would require stocking two products at higher inventory costs. Others would rather not fix it because "it ain't broke." Here are some additional facts about PE wire:

  • It was listed by Underwriters Laboratories (UL) in 1986.
  • Over one billion feet of Paige Electric PE wire have been installed since 1986.
  • Utility companies bury only PE and XLPE insulated and jacketed wire.

In conclusion, when it comes to 14, 12, and 10 gauge irrigation valve wire, there is no reason for not completely switching to PE insulated products. The superior mechanical strength and insulation properties combined with excellent chemical resistance to landscape fertilizers, fungicides and herbicides, and slightly lower cost makes for an easy decision in favor of PE insulated wire.

Rain Bird, Maxi and Maxicom are trademarks of Rain Bird Corporation.