The wires are typically arranged as follows: red for 24-volt hot, white for heat, yellow for cooling, green for the fan, and blue for common (although the common wire may be a different color).
Red: The red wire in your thermostat is for power. It will provide a 24-hour volt of power and will originate from the system's transformer. Every air conditioning system will have a red power wire. In some systems with a dual transformer, there will be two: Rc wire and Rh wire.
Power (R-Wire)
This wire is Red in color and connects to the R terminal on the thermostat, which then connects to the transformer of your HVAC system. There are typically various versions of the Red/Power wire depending on the specific function of the wire.
RH: This terminal is the 24-volt heating power supply. Y/O: This terminal controls the cooling condenser by sending signals to power up the condenser and the blower fan. W/B: This terminal controls the heat relay or valve by sending signals to power up the furnace and the blower fan or the boiler.
Common wiring practice is black for hot and white for common/neutral. In wiring jargon there are 2 power buses. 120 volt AC black is hot and white common, 0n 240 volt single phase red is other phase. For 24 volts thermostats use black for R and RT RC and white for C common.
FOUR WIRES
The standard configuration consists of 24-volt hot (red), 24-volt common (blue), heat (white), and a fan (green).
article 504 of NEC, light blue is intrinsically safe, other than that your 24 VAC can be any color except green. Don't use white or grey either unless it's grounded, and make sure the wire is the type allowed for your application.
Typically, the "hot" wires, or the wires that provide power to the unit, are the heating and cooling wires, labeled Rc and Rh respectively. They provide the source of 24V power that comes into the main HVAC control board itself.
In most systems, the Y or Y1 wire controls your cooling system. If you have a heat pump, your Y or Y1 wire controls your compressor. Your compressor is responsible for heating and cooling your home. The G or G1 wire controls your fan.
Line (high) voltage operates on 120 volts and can be found in our home outlets, while a low voltage system typically operates on 12 or 24 volts and it requires a small transformer to lower the 120 volts too low voltage.
If you wire a thermostat wrong, you may damage the thermostat, the electrical system, or the HVAC unit itself, which makes the entire installation process more frustrating. You may find yourself testing thermostat wires for hours, trying to get them right.
US AC power circuit wiring color codes
The protective ground is green or green with yellow stripe. The neutral is white, the hot (live or active) single phase wires are black , and red in the case of a second active. Three-phase lines are red, black, and blue.
24 Volt Systems - Distance in Ft
The minimum wire gauge recommended is 14 gauge for compatibility with the terminals inside the power distribution boxes.
Yellow is the most common color for cooling wires, but Blue is also used in some systems. The cooling wire connects the thermostat to a compressor. Black (Or Blue) Wire is the so-called C wire or common wire. It is a 24V common wire that is linked to the transformer.
The FreezeAlarm acts as a switch between your main thermostat and a brand new second thermostat. When you call up the FreezeAlarm, you can tell it to supply the low voltage power to either thermostat #1 (T1) or to thermostat #2 (T2). Whichever ther- mostat is getting power is the one controlling the furnace.
In conventional cooling systems, Y/Y1 controls the first stage of cooling and Y2 controls the second stage, which helps cool down the home faster. In heat pump systems, Y1 controls your compressor, which heats and cools your home.
R (red) is the 24V “hot” wire and C (black) is the common. G (green) is the fan relay. W (white) is the call for heat relay. Y (yellow) is the call for cooling relay.
Perhaps you've heard of the C Wire (aka the common wire), a power source frequently found in thermostat installations. This wire provides a continuous power flow of 24 V to the thermostat.
Red (R, RC, or RH): The red wire supplies 24V AC power to the thermostat. A wire labeled RH carries power to the heating system, while an RC wire provides a power connection between the thermostat and your cooling unit.
The neutral is bound to ground only at the 24V-supply's negative output terminal, so current coming from a sensor going to neutral always has the path with least resistance back to the supply through the neutral wire and then the supply's negative terminal, going through ground only makes sense if there's a disconnect ...
Per NFPA79 and UL508A, the suggested (but not required) color code for 24VDC grounded conductors is white with a blue stripe.