Changing over from a Hunter XCore to Rachio 3 with outdoor enclosure. The power supply cord on the Hunter is a 16 AWG cord. Is this fine to wire into the Rachio 3 outdoor box to then plug into the GFCI receptacle for power? I thought I read somewhere it needed to be 18 AWG but wasn’t sure if it made much difference. 
I believe you should be fine. The lower the ga., the thicker the wire. The concern would be going to smaller wire or possibly where the wire connects in. Wait, are you talking about the 120VAC? If so, either sounds small to me. If I remember correctly, the sprinkler wire itself is typically18ga.
@Harley27 - I would post pictures to make sure that everyone is on the same page. Since you’re using the Rachio outdoor enclosure - that needs 120 volt AC to it’s wiring block to plug the Rachio transformer into. I believe the National Electrical Code NEC) requires a permanent/hard wired connection here and not an extension cord or temporary corded plug. That said, it should work. Will the enclosure be exposed to the weather or is it in a garage?
It’s not an extension cord. It is an outdoor power supply cord that can be hard wired into the outdoor enclosure and then plugged in. This is the exact directions from Rachio. My power supply cord from my Hunter was 16 AWG though and Rachio linked a 18 AWG on their support page. So I was inquiring if that made much difference. 16AWG is a heavier duty cord so I don’t see why it would.
The set up will a 16 gauge supply cord wired to the Rachio outdoor enclosure. Plugged into a GFCI receptacle that’s wired into the house with 14gauge.
I cannot speak for your exact wiring and would suggest an electrician. I believe I found the link that talks about the wiring guage. How do I install a Rachio Controller outdoors? Weatherproof Outdoor Enclosure which has a link to an 18 AWG pigtail for the enclosure. Larger wire should be fine as mentioned, but would suggest the electrician to be safe and knows the local code.
I think this is the part that you are reading, and this is in reference to the low voltage valve and sensor wiring, NOT the power wiring. Whatever was used to hardwire your Hunter XCore should be fine if it worked, but like @DLane said, electrical code would require something like this to pass code. I’d be curious to see a picture of what type of “cord” you have.
- Mount the controller
Pull valve and sensor wires through the center opening on the controller’s base. Mount the controller to the enclosure using the included bolts
- Mounting holes: top center + two bottom (do not use the center middle)
- DO NOT install the cover on the base of the controller after it’s mounted in the enclosure.
Note
The Rachio 3 Smart Sprinkler Controller has streamlined hardware for quick and easy installation. Our new easy-press connectors accommodate 14 & 18 gauge wire, making Rachio 3 installation a breeze compared to other controllers on the market. Most installations can be completed in 30 minutes or less, with no special tools or expertise required! The Rachio app provides guided installation every step of the way. Steps are easy – replace your old controller, plug in existing wires, and complete setup within the free Rachio app.