Use no. 18 AWG color-coded, insulated (35_C minimum) wire to make the low-voltage connections between the thermostat, the unit, and the outdoor equipment.

A standard category cable is 24 AWG, while a thermostat wire is 18 AWG. Using a lighter wire in a thermostat may result in a voltage drop that is too significant for effective use. This is not an issue for a very short cable run, but it is a problem for a standard or long cable run.

Evaporator coils are found inside the indoor air handler and contain refrigerant that plays a role in extracting heat from the air in your home. The fan coils are in the condenser and help the unit disperse the heat energy gathered inside into the outdoors.

Carrier systems made in the last 20 years or so encode the week and year of manufacture within the first 4 digits. The first 2 are the week of the year (1-52) and the second are the year. For example 0199 would mean the system was made the first week of January in 1999.

The most common thermostat cabling is 18/5 (18 gauge, 5 conductor), because 5 conductors typically offer enough functions for a modern thermostat/low voltage device.

You can use pretty much any wire you like for a thermostat as long as it is thick enough. 20 gauge will work okay for short runs put is prone to breaking. 18 gauge is more popular. You can go thicker it is just more expensive and more difficult to handle.

Remove the front door and look inside the unit. You should see the model number printed on rating-plate or decal.