Repairing Corroded Field Wire Under Concrete: Splice and Waterproof

Your 2-zone system has one dead zone. The solenoid tests good, but you've found a broken field wire leading under a concrete patio. The exposed wires are corroded.

Cause of Corrosion

Nick or cut in the wire insulation allowed moisture intrusion, causing copper corrosion that increased resistance until the solenoid couldn't pull in. This is the #1 cause of intermittent zone failures.

Repair Procedure

1. Locate the damaged section. Dig back from the valve until you find clean, uncorroded wire on both sides of the break.
2. Cut out 1-2 feet of the corroded section.
3. Splice in new wire of the same gauge (typically 18 AWG for residential). Use waterproof gel-filled wire nuts (3M DBR/Y or King DryConn) on both splices.
4. If under concrete: Consider directional boring a new wire run, or abandon the old wire and run new conduit under/around the slab.

Pro Tips

• Use irrigation-grade wire with polyethylene jacket (not THHN) for direct burial.
• Label both ends of the splice with zone number and date.
• Apply silicone sealant over wire nuts for extra protection in wet locations.
• Test solenoid resistance after repair—should read 20-60 ohms.

When to Run New Wire

If corrosion is widespread, the wire run is over 20 years old, or you have multiple nicks, pulling a new multi-conductor cable is more reliable than multiple splices.