Understanding 2-Wire Decoder Systems

By Igin Staff

Imagine fifty people who have to get to the same place, each driving their own car. This scene represents the expense of the wire and labor for a conventional installation, with a separate wire running to each valve.

Now, imagine if they could all take a bus. The bus makes a stop near each one’s home for pick up, but takes them all to the same destination. The 2-Wire path is the bus, and each bus stop is a decoder. Everyone has a short walk to the bus stop, but the overall cost to the destination is much lower.

Chances are, you’ve already installed or inherited a 2-Wire decoder irrigation system, or at least heard about them. This game-changing valve control technology is here to stay, and for good reason. Here’s a little of the what, why, and how, that makes 2-Wire decoders a powerful option.

Decoders are electronic devices that are connected to a wire path from an irrigation controller. They operate irrigation valve solenoids by receiving both signal and power from the same wire path. This allows the installation of many irrigation stations (or valves) down the same wire run, rather than requiring a separate power wire for each solenoid.

Decoder systems differ from “conventional” wiring because they allow control of many valves over very long distances, with a corresponding savings in wire and labor. However, the installer must invest in the decoder electronics themselves which will be offset by some of the wire savings.

Most systems have a “break even” point at which the savings in copper wire offsets the cost of the decoders. The break-even point depends on the controller selected (there’s a large range of decoder controllers available, suited to variously-sized projects). Most contractors find this point to be anywhere between 18 and 30 valves; after that, they’re putting money back into the company’s pocket.

Wire savings are only the beginning of the decoder story, however.

Experienced decoder installers point to other real-world benefits of decoder wiring.

Flexibility: Because decoders can be positioned anywhere along the 2- Wire path(s), and their station numbers can be assigned by the installer, it is easy to accommodate changes to the original landscape plan. Decoder stations generally do not need to be assigned in physical order—station 3 can be located between station 1 and station 2, for example.

Expandability: Irrigation systems can be easily expanded or subdivided after the original installation without running spare wires, or retrofitting new wiring back to the controller. The installer simply finds the closest point in an existing 2- Wire path and splices in a new decoder station whenever additional valve control is desired.

Ease of troubleshooting: Many irrigation systems experience problems over time due to degradation or faults in the field wiring. The industry has evolved, and there are new techniques and devices to simplify fault-finding. In decoder wiring, there are fewer wires to troubleshoot. Some brands offer handheld devices to aid programming and diagnostics of decoders, which can save many man-hours. A forty-station cut wire path would require several hours to repair, while a 2-Wire path would require one splice and waterproof wire connection.

Reliability: Less wire means fewer problems. Exposure to shovels, rodents, and even lightning is minimized with 2-Wire paths.

While there are now several brands of decoders available to the designer or installer, most share some common characteristics:

•The decoders themselves are waterproof, being filled with an epoxy that insulates the electronics from the elements. They are designed to survive in a valve box, along with the valves they control.

•They are low-voltage, meaning they are safe, and do not require conduit for wiring in direct burial installation.

•They are uniquely addressed.

Any station on the path can be turned on individually, without affecting the other decoders and stations on the same wire path.

•They receive on and off commands from a programmable irrigation controller. Most decoders do not store irrigation schedules (that’s the controller’s job). A decoder simply listens to the wires, and waits for its name to be called to start a station.

Many decoder systems are designed to operate the standard 24V AC solenoids commonly found in landscaping installations. Some others are designed to operate with DC latching solenoids. It is important to know which type of solenoid you’ll be using when choosing a decoder technology, and vice versa.

Generally, AC and DC solenoids may not be mixed within a controller installation, and a decoder system of a given type is either AC, or DC latching.

Planning: Each manufacturer has rules and specifications for decoder layout, but one principle is generally true: your decoder wire is going to be installed along with your pipes. The idea is for the wire to go wherever you’ll need valves, so the pipes and the wire can share the same trenches.

Most modern decoder controllers permit more than one wire path, to further increase their flexibility. Many of them also permit T- splices in the wire paths, to facilitate following pipe runs on a project.

The best designs make the most efficient use of copper wire within a specific design. The pipe layout and overall irrigation plan will determine how many decoders you need on a single path (within the specifications of the control system). It is generally not necessary to ‘loop’ the wire path back to the controller from the end, but individual manufacturers provide specific guidance on their products. The idea, after all, is to save wire.

Generally, the heavier the wire gauge, the further you can run a 2-Wire path.

While specifications vary, path runs of up to 15,000 feet (4500 m) are possible with 12 AWG (3.3 mm2) wiring in large control systems. Smaller systems may use 14 AWG (2 mm2), or even 16 AWG, for distances that would require much heavier wire in conventional solenoid wiring.

Installation: The difference between happy decoder systems and sad ones usually has more to do with the installation than with the design. Installing 2-Wire is not the same as installing traditional 24V solenoid wiring. It’s best to approach the manufacturer’s specifications with an open mind, no matter how many standard 24V systems you have done in the past.

Here are a few installation tactics that apply in general to all decoder systems.

Box it: All decoders go in valve boxes. They will almost always go in the boxes with the valves they control, so this adds little or no extra cost. Don’t bury decoders in the dirt unless mine-sweeping is your hobby.

The same rules apply for T-splices in the 2-Wire path. Even a small, round box will save you hours of hunting later on. And if you’re adding an unplanned splice, why not wire in a short three-foot piece of extra decoder wire, to give it the extra slack we will mention below?

Waterproof it: This is the Achilles heel of almost all decoder systems. The smallest short circuits from the wire path to earth will cause decoder systems to malfunction (even though they weren’t that big of a deal in conventional solenoid wires).

Most manufacturers call out specific waterproof connections, or even include them with the decoders. A plain wire nut is a formula for disaster, and the cheapest grease-filled connectors often used at solenoid connections are not much better over the long haul.

Read and believe manufacturer connection specs.

Slack at the splice: Leave enough slack at each splice in the wire, including decoder splices, to withstand years of ground settling, expansive soils, and frost. Pull up a three-foot slack loop at each decoder location and at each T-splice, and loop the extra around the inside of the box for expansion and future service. Tight connections will pull themselves apart over time.

There are some pretty slick third-party splice connections that allow you to isolate each arm of a decoder T (without undoing the waterproof connections) for diagnostics. Some manufacturers offer one that is compatible with virtually all decoder systems.

Ground it: Virtually all decoder systems require some form of in-line earth grounding for lightning surge protection, in addition to grounding the controllers. Techniques and devices vary, but proper grounding is vital to the longevity and reliability of decoder systems.

Generally, grounding is placed along the wire run at specified intervals, and at the end of each 2-Wire path. Ground lead wires are connected to earth hardware (rod or plate) which is located off the 2-Wire path. Do not place earth ground hardware in the same box as a decoder; it should be well away (at least eight feet from the 2-Wire path itself, to prevent reintroduction of the surge onto the wire. In sandy soils, use soil amendments to improve earth contact at the grounding hardware.

Map it: Best practice is to provide the customer (and yourself) with a rough map of the finished system, including station number locations, and the general run of the 2-Wire paths. If you want to modify, service, or expand a system in a few weeks, months, or years, the details that seemed so obvious today will be quickly forgotten.

Mapping does not require satellite imagery or fancy graphics. A Sharpie pen and a cardboard lid have been known to work well. Ideally, create a simple map in your design process, and then update it as modifications occur in the build process. Some systems use serial numbers for decoder stations, and this is a good place to note them, too. Ditto for earth ground locations.

Save it somewhere you can find it later. This will pay you back when you revisit the project for expansion. Expandability is a key feature of decoder systems, so make it easy down the road, with an informal “as-built.”

Undocumented secrets: When you get right into the nitty-gritty, the little things are what save or cost you extra time.

Connect short sections of decoders, and test them as you build. It is much easier to find a wiring fault over a short distance, in a smaller set of decoder stations, than after an entire project or long path is installed. Do a few, test a few, and continue.

Test first, then waterproof. The best connectors have high quality connections for the splice, which are then inserted in waterproof gel-filled devices. Make your connections, keep or prop them up out of the dirt, and test from the controller before adding the waterproof components. You will find them much easier to test and resolve. Just remember to add the waterproofing after they check out.

Strip back enough jacket. If using sleeved or jacketed wire, strip enough of the outer jacket back from the inner conductors to work within the box. Also, remember you might want to get a clamp-type amp meter around those wires someday. Strip the outer jacket back at least 12 inches from the splice point.

Use the right tool for stripping decoder wire, and do not nick or cut the inner conductors when removing the outer jacket or sleeve. With some decoder wire, you can just score the outside of the outer jacket without cutting through anything, then just bend, snap, and slide the jacket off the conductors. Foam-filled wires often require special tools, so make sure to use one, if required. The knife is not always your friend.

One splice, one connection. Do not attempt to cram more than one splice into a single waterproof connector. It causes poor connections and bad waterproofing.

Search up some decoder installation videos (there are several to choose from) before your first project. The saying, “There’s never enough time to do it right, but there’s always enough time to do it twice,” applies very well to decoder installations.

EDITOR’S NOTE: Dave Shoup is product manager of controllers for Hunter Industries.