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Decoder System Design

"T" Splicing Decoder Wire

  • It is permissible to T-splice decoder wire paths
  • All T-splices must be made in valve boxes with DBRY-6 or equal connectors
  • T-splices consist of making a three way connection in the red wire, and a three way connection in the blue wire
  • It is especially important to allow adequate slack in a three-way splice. Each splice should be able to be withdrawn from the valve box for above-ground inspection and service.

Where possible, it is desirable to run the two-wire paths in the same trenches as irrigation pipe. The pipe will provide some protection to the wire, and logically leads to the valves where decoders will be positioned (see illustration).

The maximum run of a given wire type is valid from the controller to the end of each arm of the T.

If the total distance from the controller to the end of each arm of the T is under 10,000 ft / 3 km (using ID1 wires as an example), the system meets specification, even if the total amount of wire exceeds 10,000 ft / 3 km.

For example (using the more common 14 AWG/2.5 mm2 ID1 wires), if a T is placed 5,000 ft/1.5 km from the controller, and 2 arms in different directions each run an additional 5,000 ft / 1.5 km, the wire is within specifications. It is only 10,000 ft / 3 km to the end of each arm of the T from the controller, even though there is 15,000 ft / 4.5 km total wire connected to the output.

It is possible to have more than one splice in a wire run, provided all the above conditions are met. With DUAL systems, the distances differ but the same principles apply.

In very large systems, the length of the wire run and the number of intervening devices (other decoders) may affect the ability to run simultaneous stations near the end of the wire run. This will not damage the equipment but may require adjusting station timing to prevent underpowering the solenoid outputs. The calculations near the end of this document help determine if there is adequate power for any given wiring scenario.

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