What exactly is a two-wire system, or as it is often referred to, a decoder system?

Think of the last time you were caught up in peak traffic with the hundreds of other people all driving individual cars, mostly alone in their vehicles, and all trying to get to the same destination at the same time. This is akin to the expense, extra time and sometimes frustration of running hundreds of wires.

Now, imagine if we could all take the train. The train makes a stop near everyone’s home for pick up, but takes them all to the same destination in a calm orderly fashion. The two-wire path is the train, and each train stop is a decoder. Everyone has a short walk to the stop, but the overall cost to the destination is much lower now as well as any future costs. Troubleshooting time which we will explore further.

A two-wire decoder allows control over conventional 24 VAC solenoid valves, often in larger numbers, over relatively long distances. By inserting waterproof encapsulated decoder modules prior to each solenoid valve as needed in a low voltage, direct burial two-wire path, it streamlines everything. A wire is “cut” wherever a station control is needed, and the decoder wires are spliced into the path. The decoders are then connected to the off-the-shelf local standard 24 VAC solenoids for individual operation of valves.

Each decoder is uniquely addressed for both the signal for their address and the power required for solenoid operation, and each uses the same single pair of wires. Systems can cater from 24 through to hundreds of decoders all of which can be individually operated. This allows the installation of many irrigation stations (or valves) down the same wire run rather than requiring separate power wire for each solenoid. Decoder systems differ from “conventional” wiring because they allow control of many valves over very long distances, with corresponding savings in wire and labour. However, the installer must invest in the decoder electronics themselves which will be offset by some of the wire savings.

Like all new things, it takes a while for landscape contractors to trust new types of irrigation technology, so traditional ones will always have their place. However, over the last few years in particular, two-wire systems are becoming more and more popular in the irrigation industry with costs being reduced as more systems and more volume are introduced.

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 various sizes of 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 or theirs.


Traditional systems have been proven but with two-wire realistically being around for over 20 years these are now seen as reliable and easy to use with people seeing the benefits. Let’ s start to look at those advantages, as the more you use the decoders, the more other real-world benefits of decoder wiring become apparent.


Because decoders can be positioned anywhere along the two-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 three can be located between station 12 and station 16, for example. This also allows for expanding irrigation systems easily by subdividing after the initial installation without running spare wires, or retrofitting new wiring back to the controller. The installer simply finds the closest point in an existing two-ire 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 two-wire path would require one splice and waterproof wire connection in minutes. This increases reliability due to less wire meaning fewer problems. Exposure to shovels, rodents, and even lightning is minimized with two-wire paths.


Each manufacturer has rules and specifications for decoder layouts, but one principle is generally true in having your decoder wire installed along with your pipes. The idea is for the wire to go wherever you’ll need valves so that 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.

While specifications vary, path runs of up to 4,500 metres are possible with 3.3 square millimetre wiring in large control systems. Smaller systems may use 2.1 square millimetres for distances that would normally require much heavier wire in conventional solenoid wiring.


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, usually to 10 ohms or less.

Generally, grounding is placed along the wire run at specified intervals, and at the end of each two-wire path. Ground lead wires are connected to earth hardware (rod or plate) which usually located away from the two-wire path. In sandy soils, the use of soil amendments to improve earth contact at the grounding hardware is sometime required. The grounding could be considered a disadvantage as they require more hardware than traditional systems but some extra work here protects the system for years to come.

Two-wire systems have long been attractive because they require less copper wire than traditional systems. You’re probably going to save 60 to 80 per cent on wire with a two-wire system, compared to a traditional system.

Two-wire systems can be used on a variety of sites, but there are certain types they are specifically designed to manage. Typically, two-wire systems make economic sense for projects that are constructed in phases over any given amount of time. Large estate residential, light commercial and commercial projects are well suited for two-wire systems. This type of system has recently found a new home in vertical wall gardens in order to keep conduit size to a minimum in reduced area cavities.

Installers can cap the wire at the end of phase one and add to it in future phases.

With a traditional system, you have to plan everything out and wire it up from the beginning,  Installers also have to be more careful with the wire compared to traditional systems, because little cuts or nicks can cause communication problems with the system and even complete failure.

There is now something else to consider when preparing quotes and desired outcomes, and it takes time for more education for contractors and more importantly to customers. A lot of people don’t install two-wire systems because they don’t understand the technology, but the installation portion is the simplest part. The repair and maintenance is what sometimes gives them trouble, but with more awareness and reducing the myths the repair and diagnose becomes straight forward.

A contractor who takes this time may prove to have a tremendous competitive advantage over their competitors in overall price.