Carrier RTU OPEN 11-808-427-01 - Manual (Page 38)

Installation
32 RTU Open
To wire an enthalpy switch (differential) to the controller
Wiring diagram for optional enthalpy sensor mounted in the return air for differential enthalpy:
Wiring a status switch
Filter - Part #CRSTATUS005A00 or field-supplied
Fan status - Part #CRSTATUS005A00 or field-supplied
Filter and/or fan status switches may be installed to provide a Dirty Filter indication or Fan Running status.
Wiring specifications
Cable from sensor to controller: If <100 ft (30.5 meters) 22 AWG, unshielded
If >100 ft (30.5 meters) 22 AWG, shielded
Maximum length: 500 feet (152 meters)