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    Tractor Supply Company

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    How to Energize an Electric Fence

    Authored by Tractor Supply Company

    If you've experienced the frustration of chasing down a runaway horse or cow, you know that good fencing is essential to keeping livestock, whether you have one or 100.

    Electric fences are good for containing livestock and are excellent for keeping predators away.

    Because an electric fence works partly as a psychological barrier, rather than solely as a strong physical barrier, it can be made of lighter materials than a non-electrified fence. This makes electric fences less expensive and easier to install and maintain. Moreover, livestock rarely push or rub up against an electric fence trying to scratch an itch. This makes electric fences last longer.

    Use this guide to decide if an electric fence is the right choice for your place.

    Fence energizers
    To electrify the fence, you will need an electric fence energizer, which is also referred to as a charger, fencer, or shock box. An energizer produces pulses of electricity on a fence wire so that animals touching the fence receive an intense shock.

    Because the pulse duration is so short — it lasts only microseconds — the shock is safe for humans and animals of all sizes. However, animals inside and outside the enclosure quickly learn to fear the shock and stay away from the fence.

    Energizers produce electrical pulses on the fence about once every second. In between, there is a one-second period with no electrical energy on the fence. This is called the off time or recovery time. Off time allows the animal to recover from the shock and get away from the fence.

    Fence energizers are classified as either low-impedance or high-impedance. Here's the difference:

    • Low-impedance — Low-impedance fence energizers can increase their energy output as the fence load increases. They have a large reserve of current and are able to charge very long fences or fences that are overgrown by green growth. As weeds touch the fence and draw voltage and amperage to earth, a low­ impedance fence will begin to produce higher amounts of energy. These higher energy levels will overcome the voltage and current loss caused by the weeds, as well as maintain energy levels on the fence for controlling animals.
    • High-impedance (also known as solid state) — High-impedance, or solid state, fence energizers do not have the capability of increasing their output energy as fence load increases the way low-impedance fences do. High-impedance fences cannot overcome high levels of fence load; they "short out" when too many weeds touch the fence.

    Choosing the Right Energizer
    You should purchase the largest, most powerful energizer you can afford. Otherwise, as you expand, introduce hard-to-control animals, or face weedy fence conditions, you may find yourself with a fence that's not doing the job.

    Ask yourself these questions when choosing a fence energizer:

    • Do you have access to a 110-volt outlet? If so, use a 110-volt operated energizer. If not, use a battery- or solar-powered unit.
    • What kind of fence wire are you using? High-tensile fences require a low-impedance energizer. Poly wire, tape, and rope also require a low-impedance energizer. As a rule of thumb, the larger the diameter of the wire, the easier electricity will flow over and through it.
    • What kind of animals are you keeping in or out? We recommend a high-impedance energizer only if you have animals that are easier to control, such as shorthaired livestock, horses, pigs, small animals, and pets. Long-haired animals, sheep, goats, bulls, deer, and predators, such as coyotes and wolves, require intense shocks, like those delivered by a low-impedance energizer, to truly fear the fence.
    • What type of weed growth is expected along the fence? Weeds zap the energy from the fence. If weeds are allowed to grow up and touch the fence, you need a low-impedance fence energizer. Some high-impedance energizers can also power through moderate weed growth. With no weed growth, either high- or low­ impedance energizers can easily energize conventional steel and aluminum fences.
    • How long is the fence? Long fences require more powerful energizers to energize them adequately. This is why the mileage ratings are so helpful. Mileage ratings are the maximum amount of fence recommended and are based on the total length of all electrified wires on the fence. For example, a 1-mile long fence with four strands of electrified wire is a 4-mile fence, not a 1-mile fence.
    • Is the fence new or old? Rusty fence wire impedes the flow of electricity. New, shiny wire is easier to energize.
    • How many splices do you have on your fence? Tests have shown that every hand splice eventually will loosen with age, temperature, humidity, and other factors. Every splice can cause the fence to lose 300 to 500 volts once it loosens. Use a low-impedance energizer if the fence has lots of hand-tied splices.
    • Is the fence built in a dry, rocky, or frozen area? These conditions do not allow electricity to travel through the earth efficiently. More than three ground rods may be required or you may need to build the fence with a hot/ground return system. Without ground rods, there is no current and consequently no shock, which defeats the purpose of an electric fence.

    Frequently Asked Questions

    What is fence load?
    Fence load is anything that reduces the amount of electricity on the fence wire. Weeds growing up and touching the fence are the most common fence load.

    How are fence energizers rated?
    Low-impedance energizers have both a mileage rating and a joule rating. Mileage is the total length of all electrified wires on the fence. A joule is a unit of electric energy; one joule is equal to one watt of power for one second of time. The higher the joule rating of the low-impedance energizer: 

    • The more energy it can supply to the fence
    • The more intense the shock
    • The more fence load it can overcome

    The higher the mileage rating, the greater the capability to power longer runs of fence wire. High-impedance energizers are rated with mileage ratings only.

    Where should I mount my energizer?
    If possible, mount the energizer in the middle of the run of fence, so that the energy goes in both directions, effectively cutting the distance it must travel in half.

    Tools Checklist

    • Fence energizer
    • Galvanized steel or aluminum electric fence wire
    • Studded "T" posts
    • 48" steel electric fence post
    • Step-in poly fence post
    • CCA pressure-treated wood posts
    • Deluxe post driver
    • Insulators
    • Ton Come Along
    • Fence pliers
    • Post hole digger, gas powered or manual
    • Gloves
    • Quikrete
    • Ground rods
    • Crimping sleeves
    • Lightning protection device