Understanding How Electrical Wiring Is Labeled

An image of electrical wire lettering.
Electrical Wire Lettering. Timothy Thiele

Insulation on electrical wires is stamped with various codes and numbering that indicates the wire type and performance characteristics of the insulation. Similarly, nonmetallic (NM) cable, which contains multiple wires, has markings on the outer sheathing of the cable. Understanding the basic markings helps you choose the right type of wire or cable for your project.

Labels On Individual Wires

Individual insulated wires typically are used for installation inside of conduit or flexible metal cable.

The coding system relates to the performance characteristics of the wire insulation. The most common types of wire used in residential construction are THHN and THWN. Here's what the letter mean:

  • T: Thermoplastic, a heat-resistant plastic used on many types of wire
  • H: Heat-resistant up to 167 degrees F
  • HH: Heat-resistant up to 194 degrees F
  • W: Moisture-resistant; suitable for damp and wet environments
  • N: Nylon-coated to resist damage from oil and gasoline 

Labels On Nonmetallic Cable

NM cable (including Romex and other types) is labeled on the outside of its plastic outer jacket, or sheathing. Cables may carry a variety of numerical codes and lettering, and this varies by manufacturer and cable type. But the most important labels indicate the number and size of wires inside the cable as well as the appropriate use of the cable. The most common types of cable used in homes include:

  • NM-B: Standard NM cable suitable for indoor use in dry locations; older versions of NM cable labeled "NM" (without the "B") have a slightly lower temperature rating than today's NM-B cable
  • UF: Underground Feeder cable suitable for outdoor exposure and direct burial in the ground
  • SE: Service Entrance cable; outdoor-rated cable for above-ground applications; commonly used to bring power from the utility's transformer to the customer's house
  • USE: Underground Service Entrance cable; similar to SE cable but rated for direct burial

    The numbering on NM cable indicates the wiring size and the number of wires inside the cable. The first number is the wire size, or gauge; the second number is the number of insulated wires. For example, "14/2" cable contains two 14-gauge insulated wires. Cable labeled 12/3 contains three 12-gauge insulated wires.

    In addition to the insulated wires, most NM cable also includes a bare copper ground wire. The ground wire is not included in the labeled number but is usually indicated as "G," "w/G," or simply "with Ground." For example, "12-2 WITH GROUND" means the cable contains two 12-gauge insulated wires and a bare copper ground wire.

    Finally, cables usually include the name of the manufacturer and a maximum voltage rating, which is typically 600 volts—well above the 240 volts that is standard for home electrical service.

    The Importance of Wire Size 

    Wire size relates to the diameter of the metal conductor of the wire, excluding any insulation. It is important because the size (along with the wire material and a few other factors) determines how much electrical current the wire can safely carry. Wire size is measured by the American Wire Gauge (AWG) system. The smaller the AWG number the larger the wire and, generally speaking, the more current the wire can carry without overheating.

    Current-carrying capacity is rated in amperes, or amps. The wiring in any circuit must have the proper amp rating for the circuit devices and the circuit breaker protecting the circuit. For example, 14 AWG wire is rated for 15 amps and should used on standard 15-amp circuits. Other common wire sizes and their amperage ratings include:

    • 12 AWG—20 amps
    • 10 AWG—30 amps
    • 8 AWG—40 amps
    • 6 AWG—55 amps