Understanding Electrical Wire Labeling

What do the Letters on Electrical Wire Mean?

Electrical wire labels
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Wiring sold for electrical projects often carries labeling to help you choose the right product for your needs. Letters, numbers, and wording on wiring labels tell you important information, such as the wire material, the size of the wire, and the type of insulation used on insulated wires. Labels are found on individual insulated wires and on insulated cable. Cables carry labels indicating the cable type or construction as well as the number of wires inside the cable.

Labels on Non-Metallic Cable (Romex)

The most common type of wiring used in homes is non-metallic (NM) cable, commonly called "Romex," after the popular brand name. New NM cable contains two or more insulated wires and usually a bare ground wire. The wires may be wrapped in paper, and all of the wires are encased in a flexible plastic jacket, or sheathing. 

Labels on NM cable indicate the size (gauge) of the individual wires, the wire material, the number of wires contained inside the cable, the maximum voltage rating, and whether there is a ground wire present. The wire size and number of wires are indicated with numbers. A ground wire is indicated by "G," "w/G," or "with Ground." The wire material is indicated by "CU" for copper and "AL" for aluminum.

Here are some examples of labels on common cable types used in home wiring:

  • 14-2G: Cable contains two insulated wires plus a ground wire; the wires are 14-gauge.
  • 14-3G: Cable contains three insulated wires plus a ground wire; the wires are 14-gauge.
  • 12-2 w/G: Cable contains two insulated wires plus a ground wire; the wires are 12-gauge.
  • 12-3 w/G: Cable contains three insulated wires plus a ground wire; the wires are 12-gauge.
  • 600 V: Cable is rated for a maximum of 600 volts; this is standard for residential NM cable.
  • TYPE NM-B: Non-metallic type-B cable; this is the current standard for residential installations. "NM-B" cable is more heat-resistant than older "NM" cable.

Underground Feeder Cable

Most NM cable is used in "dry," or interior, locations, where the cable is protected inside wall, ceiling, and floor cavities. Underground feeder (UF) cable is a special type of non-metallic cable that is suitable for "wet" locations and for direct burial in the ground. UF cable is usually gray (not white, yellow, orange, or black—like standard NM cable) and is labeled "UF-B" and may include "Sunlight Resistant" or similar wording. UF cable uses the same symbols as standard NM cable to indicate the number and gauge of wires.

Labels on Individual Wire

Individual insulated wires are used in home wiring when an installation calls for conduit, a rigid or flexible protective pipe or tubing through which the wires are run. Electricians buy the individual wires by the spool so they can pull different wires from different spools as needed.

The important labeling on individual wires relates to the wire insulation, the plastic coating that covers the metal part of the wire. The most common types of wire used in home wiring include:

  • THHN
  • THWN
  • THW
  • XHHN

Here's what the letters on the labels mean:

  • T: Thermoplastic insulation, a fire-resistant material
  • H: Heat-resistant; able to withstand temperatures up to 167 F
  • HH: Highly heat-resistant; able to withstand temperatures up to 194 F
  • W: "Wet," or approved for damp and wet locations; this wire is also suitable for dry locations
  • X: Insulation made of a synthetic polymer that is flame-retardant
  • N: Nylon-coated for resistance to oil and gasoline  

Labels on Low-Voltage and Thermostat Wires

Low-voltage wiring used around the home includes small non-metallic cable used for thermostats and other control devices and paired insulated wire used for landscape lighting systems. Wire for landscape lights usually is black and has labeling stamped into the wire insulation. Labels typically include:

  • Wire size: Indicated by a number (such as 12, for 12-gauge) or a number followed by "AWG," for American Wire Gauge.
  • Number of wires: Usually indicated by the number 2; landscape wiring typically has two insulated wires stuck together (similar to a lamp cord) and contains no ground wire.
  • Properties: Wording indicating sunlight-resistance or suitability for underground installation.

Thermostat cable is similar to NM cable but contains four or more small insulated wires and no ground wire. The cable may or may not be labeled. Each wire has its own color to help you connect to the appropriate terminal at the thermostat and the equipment it controls. While color coding is not universal, the lettering on the thermostat terminals is relatively standard:

  • C: Common wire; allows for continuous power flow from the R wire; not all thermostats use this terminal
  • R: 24-volt power supply from the furnace transformer
  • Rc: Calls for heat or cooling; there may be more than one Rc terminal
  • G: Fan
  • W: Heat
  • Y: Air conditioner