Thermostat Wire and Thermostats Wiring For HVAC
Thermostat wiring colors help designate which wire goes where on a thermostat
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For many, wiring a thermostat can be a very big hurdle especially if they do not understand basic HVAC electrical control circuits, thermostat wire color designations, or thermostat terminal designations. No matter what type of thermostat you have, Honeywell, White Rogers, Robertshaw, Maple Chase, or whatever brand you have, High Performance HVAC Thermostat Wiring page will help you better understand this often confusing subject of thermostat wiring.
Thermostat is typically 18 gauge solid wire and is usually designated by different colors of many wires wrapped in an outer insulating jacket which hold the multi-color wires together. Reading the label on different wire reels one will notice 18-2, 18-3, 18-5, or even 18-10. The first numbers represents the gauge of the wire inside the outer jacket. The second number represents the number of wires or conductors inside the outer jacket. A reel labeled 18-2 will have two wires or conductors which are 18 gauge wire. A reel labeled 18-10 will have 10 wires or conductors which are 18 gauge.
Depending on the type of equipment you have will determine the number of wires needed to run from the thermostat to the equipment. The HVAC Technician who installed the equipment made the decision on the number of conductors needed for the installation of the equipment. It is common for the installer to pull more conductors than necessary therefore there may be some unused wires or conductors which can be used as spares in case the conductors which are in service are damaged in some way. In some cases, if you look behind the thermostat you will find these spare wires wrapped around the outer jacket or stuffed back into the hole or chase where the thermostat wire runs through the wall.
Thermostat Terminal Designations
It is important to note that before working on any type of wiring that the power for all the circuits be disabled. Typically, the air handler contains the control voltage for the thermostat (but not always). It is advisable that all HVAC equipment be turned off from the main power disconnect or breaker panel before proceeding with any wiring. Additionally, read the site disclaimer page for more information.
R - The R terminal on many thermostats typically is the 24 volt control power which feeds power to the other terminals (except the common terminal) when the thermostat is manually or automatically switched to an on position.
RC - The RC terminal is not on all thermostats. It is usually on thermostats which allow for separate equipment control transformers where the two different control circuits need to be isolated. The RC terminal is usually fed from the cooling or air conditioning equipment and is used to energize all the components necessary for cooling or air conditioning.
RH - Just as the RC terminal, the RH terminal is not on all thermostats. This will usually accompany the RC terminal where the thermostat will isolated two different control transformers. The RH terminal will feed all the heating components necessary to run the heat according to thermostat manual and automatic settings.
Typically, the 24 volts control power is fed directly from a control transformer located in the HVAC equipment. For best results with modern state of the art solid-state controls and circuitry always use National Electrical Manufacturers Association (NEMA) rated transformers.
The commonly used wire colors for the R, RC, and RH terminal is red. It is possible, when the circuits are isolated, to have more than one red wire in the thermostat. It is also possible that only one control transformer is being used to control the equipment and the RC and RH terminals have a jumper wire running between so that they may share the control power. If you are unsure about this always consult an HVAC professional as miswiring can damage components and cost more to repair than the cost of paying an HVAC professional technician to install a thermostat correctly.
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Thermostats 
