Electrical and Control Systems: Controllers, Valves, Wiring, Sensors
Control circuits, valves, and controllers
Irrigation control is low voltage: a 24 VAC transformer in the controller powers solenoids through a common wire plus one station wire per valve. The controller switches the common side, so a single loose common conductor can knock out the entire field system.
Size wire for voltage drop, not just current. Long runs on 18 AWG cause solenoids to hum or fail to open. Check transformer VA against the load: a typical solenoid draws about 0.2–0.35 A holding (roughly 5–8 VA) with several times that in inrush, so two valves opening together can exceed a small transformer even though they run one at a time.
Know the accessories: rain sensors interrupt the common or signal the controller, flow sensors detect a burst main and shut down, master valves and pump-start relays isolate the system, and decoders replace multiwire runs on two-wire systems. A sensor wired to the wrong terminal or left disabled in programming is simply ignored by the controller.
Worked example: a 24 VAC, 40 VA transformer running one 0.3 A solenoid uses ~7.2 VA, leaving plenty of margin. Add a pump-start relay (10–15 VA) and a second simultaneous zone, and the total crowds the rating - verify before adding stations.
Traps: no wire nuts buried in soil, no 120 V and 24 V conductors sharing a box, and GFCI protection on all outdoor circuits.