Wire Insulation and Cable Coding


The wire manufacturers’wire markings give:

  • Wire gauge
  • Maximum voltage rating
  • Insulation code (R for thermoset, T for thermoplastic, X for cross-linked polyethylene thermoset)
  • Maximum temperature code (H for 75°C, HH for 90°C, nothing for 60°C)
  • Abbreviation for sheath if present (e.g., N for nylon)

The identification codes for wires with thermoplastic insulation approved for use in conduit are given in Figure (a) for THHN and in (b) for THWN.


Cables are identified by their wire gauge and the number of insulated wires they contain. In addition, all cables have a grounding wire. The codes printed on the jackets of the major classes of cable for lighting and power applications, service entrance (SE), nonmetallic (NM or Romex), underground feeder (UF), flexible armored (BX or AC), and metal-clad (MC) are given in below Figure.

The code for SE cable shown in below Figure. (a) provides the following information:

  • Cable type (e.g., service entrance)
  • Insulation code
  • Maximum voltage rating
  • Number and gauge of “hot” conductors
  • Number and gauge of ground conductors
  • UL listing
  • Year of manufacture

The codes on nonmetallic (NM) and underground feeder (UF) cable, as shown in in below Figure. (b,c), provide the following information:

  • Wire gauge
  • Number of conductors
  • Presence of ground wire (W/G)
  • Function (e.g., nonmetallic sheath or underground feeder)
  • Maximum operating temperature (H for 75°C, HH for 90°C, none for 60°C)
  • Identification of sheath, if present (e.g., N for nylon)

Service Entrance Cables

Service entrance cable, type SE, , conducts power into the building from the splice with the utility’s service cables. Standard SE cable has two

insulated conductors and a bare ground wire. It is approved for powering 240-V AC heat pumps and electric furnaces, but it cannot be used where an insulated neutral is required. Two of the insulated cables are for the 240-V AC load and a return neutral is for the 120-V AC load. SE cable is available in three different styles, U, R, and USE, all rated for 600 V AC. The letter X on the insulation indicates that it is crosslinked polyethylene

NEC 2002, Article 338, “Service-Entrance Cable: Types SE and USE,” covers the use, installation, and construction specifications for SE and USE cable.

Service entrance cable, type SE, style U,is a flat cable approved only for above-ground use. It has two insulated conductors. Both are black, but one might have a red stripe for phase identification. The neutral is multistranded braid enclosing the two insulated conductors.
Conductors and braid are wrapped in glass-reinforced tape and jacketed in polyvinyl chloride (PVC) pipe. The jacket is flame-retardant, moisture-resistant, and has ultraviolet-resistant properties. It is available in AWG 6 and larger. U-style SE cable cannot be used to power 120/240-V AC appliances such as electric dryers and stoves, because these appliances require three insulated conductors and ground.

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