What are the two standard gauges of thermostat wire?

The two standard gauges of thermostat wire typically used in HVAC systems are 18 and 16 AWG (American Wire Gauge). These gauges are sufficiently thick to handle the low-voltage requirements of thermostat controls while also offering the necessary durability to withstand various environmental conditions inside and outside of walls.

Using 18 and 16 AWG wire ensures minimal voltage drop over the length of the wires, which is crucial for the effective operation of thermostats. A lower gauge (meaning a thicker wire) can be particularly beneficial when the wire runs over longer distances, as it helps ensure that the electrical signals from the thermostat to the HVAC equipment remain strong and reliable.

The other options involve gauges that are either too fine for typical thermostat applications or do not conform to the common practices in the HVAC industry for thermostat wiring. It is important to select the correct gauge to ensure proper system operation and safety.