According to the National Electric Code (NEC), Article 110.3(7), wires are classified by type, size, voltage, current capacity and specific use. Article 200 goes on to describe the identification on green grounding wire, gray-colored wire, or wires with three continuous white stripes. Wires are rated according to their temperature as well as size. Each wire must be labeled, identified and tested for different environmental conditions and then approved for specific uses. Wire usages, ratings, sizes, as well as special usages must be labeled every 18 inches on the outer jacket of the wire. Article 310.13(A) of the NEC shows all types of insulation ratings and special usages based on the minimum safety requirements for electrical insulation.
FEP or FEPB Outer Jackets
Fluorinated ethylene propylene, or FEP, conductor insulations are rated for use under 90 degrees Celsius, or 194 degree Fahrenheit. Applications and usages for FEP can be in both dry and damp locations. The insulation of FEP must be at least 0.51 millimeter (mm). FEPB is rated for higher temperatures and can be used up to 392 degree Fahrenheit. However, it is rated for dry locations only and can be used for special applications. The "B" in FEPB stands for the "braid material" that is integrated with the chemical used to manufacture the outer jacket, which makes it more durable and more heat-resistant.
TBS Outer Jackets
The thermoplastic and fibrous outer braid jackets are used for wires that can only be used in switchboard wiring. The actual insulation is made out of thermoplastic and can operate at temperatures up to 194 degrees Fahrenheit. The insulation is about 0.76 mm. The outer jacket for TBS is also rated as flame-retardant and nonmetallic.
THHN Outer Jackets
Heat-resistant thermoplastic is rated to operate under a maximum temperature of 194 degrees Fahrenheit and can be used in both dry and damp locations. This is perhaps the most commonly used outer jacket and can be recognized easily because of its various usages. The outer insulation is made of flame-retardant, heat-resistant thermoplastic with a minimum of 0.38 mm for No.14 to No.12 wires. The outer cover also has to be covered in nylon jacket or equivalent.
THHW Outer Jackets
This is the second most commonly used wire insulation. The moisture and heat-resistant thermoplastic is rated for wet locations and is rated for up to 167 degree Fahrenheit. This type of wire can also be used for dry locations. However, in this case the temperature ratings will be up to 194 degree Fahrenheit. The THHW insulation is flame-retardant, moisture and heat-resistant thermoplastic.
Each wire will have markings for everything that the wire is rated for. Some markings to consider are: "O" for oil-resistant, "S" for standard services, "SEOW" for oil-resistant and weather-resistant, "SJ" for service junior, "SPT-1 and SPT-2" for parallel-jacketed, two-conductor, light-duty cord; both wires are rated for 300 volts and "W' for being weather-resistant.
- "NEC Code"; NFPA; 2011
- "Insulating Materials: Principles, Materials, Applications"; Margit Pfundstein; 2008
- "Electrical Wiring, Residential"; Ray C. Mullin; 2001
- Photo Credit Jupiterimages/Photos.com/Getty Images
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