I have always been fascinated by cables and conductors—sometimes referred to as wires in the National Electrical Code (NEC) and other standards—and the insulation for these cables and conductors, as well as their multiple uses. The uses and applications of these cables and conductors permit us to install conductors in all different locations, such as in extreme temperatures, wet and dry areas, and other similar harsh conditions.
The key behind all of these installations—in normal as well as extreme conditions—is the ability of the insulation to withstand the various conditions the cables and conductors will be subjected to while still providing adequate and functional operation. Let’s look at the various types of insulation that are commonly used in the electrical industry with a focus on the pros and cons of these insulations.
Two of the most common insulations used on conductors covered by the NEC are thermoplastic and thermoset materials. Of course, there are many more types of insulation, and becoming familiar with the different types will help the user provide the correct conductor and insulation for their particular condition and usage. The conductor insulation construction applications and specifications are covered in 310.104. Tables 310.104(A) through 310.104(E) provide specifics about each different type of insulation and other pertinent information about the conductors and insulation.
There is an informational note at the beginning of 310.104 that states “thermoplastic insulation may stiffen at temperatures lower than –10°C (+14°F). Thermoplastic insulation may also be deformed at normal temperatures where subject to pressure, such as at points of support.”
Table 310.104(A) is by far the longest table in the five table series and applies to conductor applications and insulations rated at 600 volts (V). Table 310.104(B) covers the thickness of insulation for nonshielded Types RHH and RHW solid dielectric insulated conductors rated 2,000V. Table 310.104(C) covers the conductor application and insulation rated 2,001V and higher. Table 310.104(D) covers the thickness of insulation and jacket for nonshielded solid dielectric insulated conductors rated at 2,001V to 5,000V. The last table in 310.104 is Table 310.104(E) covering the thickness of insulation for shielded solid dielectric insulated conductors rated 2,001V to 35,000V.
Thermoplastic insulation is defined as a classification for a solid insulation material that can be softened and made to flow by heating, extruded onto the wire, and quenched in cool water to make solid again. It can be readily softened and resoftened by repeated heating but remains in a safely usable solid form when operated within its rated temperature limits.
This material commonly labeled as THHN/THWN is flame-retardant and heat-resistant, and it can be installed in dry and damp locations with a maximum operating temperature of 90°C (194°F) and in wet locations with a maximum operating temperature of 75°C (167°F). The 90°C (194°F) conductor amperage rating can be used for adjustment factors where using more than three current-carrying conductors based on Table 310.15(B)(16) and ambient temperature correction factors, but due to termination at the circuit breakers and the load termination as provided in 110.14(C), the ampacity used for the conductor cannot exceed the 75°C (167°F) ampacity column in Table 310.15(B)(16). Heat adversely affects thermoplastic insulation more than thermoset insulation as shown in the 310.15(B)(3)(c) exception, covering raceways and cables exposed to sunlight on rooftops. The exception states that Type XHHW-2 insulated conductors shall not be subject to the temperature adjustment adder of 33°C (60°F) when these conductors are installed in a raceway and located closer than 7/8-inch from the roof.
Thermoset insulation consists of polymer structures that are cured or vulcanized to become natural or synthetic rubber materials. Irradiation, heat or chemical reactions can be used to cure the material. During the curing process, polymer chains are cross-linked with other molecules and are often called cross-linked polyethylene (XLPE). Once cured, thermoset materials are irreversibly molded.
Thermoset materials have a very high melting point with the materials degrading and decomposing before reaching their melting point. Thermoset insulation is much thicker than thermoplastic insulation and may be more difficult to pull since the thermoset conductors have a greater square inch area raceway fill than THHN. XHHW insulation is resistant to chemicals, ozone, abrasions and has more mechanical strength but is not as flexible as THHN/THWN. Since XHHW has thicker insulation than THHN/THWN and has the additional polymer cross linking, there is less dielectric leakage in the XHHW conductor insulation. Both XHHW and THHN/THWN are adversely affected by wet locations and water, so both insulation types must be rated at 75°C (167°F), unless using XHHW-2 or THWN-2, which are then both rated at 90°C (194°F) for wet and dry locations.