Introduction to cable insulation designations

Cable insulation is often known to nuclear electrical engineers by an industry shorthand. The different shortcodes describe different materials and properties. Here you can learn where to find the decoding cipher . . . .

Before sizing a cable, its insulation type must be selected to ensure it will be compatible with its environment. It is a failed design that installs cable intended for dry conditions into a wet environment. Its insulation type will also govern its ampacity.

Class 1E cable is qualified for use in the harshest, wettest environments, so it is compatible with most environments (though it may be overkill for your needs).
You can refer to Table 310.104(A) of the National Electric Code (2014) for a summary of the various types of cable insulation and their applications. Two common types are THHN and THHW. THHN cables are rated at 90ºC for use in dry or damp locations. THHW is rated at 90ºC in a dry location but 75ºC when used in a wet location.

This is critical: notice the distinction between damp and wet. A damp location is one which is usually dry and protected from weather but can routinely experience wetness, such as under a roofed-in porch or in damp basements. Wet locations are those that are routinely saturated with or submersed in water, such as underground duct banks, direct-burial in the earth, or at the carwash. Installing THHN cables inside of an underground duct bank is bad design because THHN cables are not meant to operate at all when submerged. Duct banks flood frequently.

Class 1E-rated cables generally have something like XHHW-2 insulation. XHHW-2 is rated at 90ºC for use in dry or wet locations. Class 1E-rated cables are expected to be used during and following design basis accidents that turn normally-dry areas into flooded ones.