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:
|Temperature Range (°C)
||-55 to +600
||-10 to +60
|Weight (pounds per 2500 feet of 14AWG)
|Insulation Thickness (mils)
|Insulation Resistance (Meg-Ohms/1000')
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.
PE wire is approximately 28% lighter than PVC wire. This allows for easier
handling and reduces the risk of injury.
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.
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.
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.
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.
This property measures how well the insulation resists surface wear, as
in situations when wire is "pulled" or "laid" during installation.
This is a measure of how well the insulation resists crushing abuse from
heavy equipment, rocks, shovels, trench back-filling, and trench settling.
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.
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
- 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.