Understanding Electrical Wire Labeling

What do the Letters on Electrical Wire Mean?

Electrical wire labels
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The wires sold for electrical wiring carry several different labels to help you choose the right product for your needs. For example, whether you are buying individual conductors or sheathed cable, the wire gauge labeling will help you buy wires that are sized correctly for the electrical load they will carry. The labeling on the wire also tells what the wire is made of--either aluminum or copper.

But wires also carry another form of labeling that serves to identify the nature of the plastic or vinyl insulating sheath that covers the individual conductors. There are many different types of wire insulation used for residential use. Some of the most commonly used are THHN, THWN, THW and XHHN.

So exactly what do the letters on the wire mean? Here's a breakdown to help make some sense of the lettering.

  •  A "T" stands for thermoplastic insulated cable, a fire-resistant material. 
  • A single "H" means the wire is heat resistant, able to withstand heat up to 167°F. 
  • "HH" means that the wire is heat resistant and can withstand a higher temperature. This wire can withstand heat up to 194° F.
  • A "W" means that the wire is approved for damp and wet locations. Of course, this wire is also suitable for dry locations.
  • The "X" means the cable is made of a synthetic polymer that is flame-retardant.
  • The "N" is for the nylon coating that covers the wire insulation. Nylon coating gives the wire oil- and gasoline resistance.  

    The labeling on a wire's insulating jacket tells the story of the wire's ruggedness. You'll likely see labels like THHN or THWN written on the wire.  For example, THHN wire stands for thermoplastic high heat-resistant nylon coated wire. THWN stands for thermoplastic heat- and moisture-resistant nylon coated wire.

    As you can see, these wires are built to take on many different conditions. And with a little prior knowledge, you can read the labeling at a glance and understand the conditions appropriate for any wire. Some examples:  

    • THW: With these letters, you know the wire is flame-retardant (the T indicates a flame-resistant thermo-plastic), heat-resistant (indicated by the W) and moisture-resistant (indicated by the W). 
    • THHN wire is fire retardant (T), extra heat resistant (HH), and has a nylon coating (N) that makes it resistant to oil and gasoline. 
    • THWN is flame-retardant (T) heat-resistant (H) moisture-resistant (W)  and gasoline-resistant, and oil-resistant (N).  

    Labels on Low-Voltage and Thermostat Wires:

    Like high-voltage wiring, low-voltage wiring has its own color coding and terminal lettering. Heating and cooling technicians know these well and that makes their job easier. Connecting a set of thermostat wires is fairly easy if you know what the terminal letters stand for and what they control.

    • G: This terminal controls the fan relay and is responsible for turning the blower fan on and off automatically or manually via the thermostat.
    • RC: This terminal is the 24-volt cooling power supply.
    • RH: This terminal is the 24-volt heating power supply.  Note: The RC and RH terminals are jumpered together in a four-wire heat/cool system and a  single-stage heat pump system, but not in a five-wire heat/cool system.
    • Y/O: This terminal is used to control the cooling contractor. When the thermostat calls for cooling, power is fed to pull in the cooling contractor and the fan relay. This powers up the condenser and the blower fan, cooling your home.