THHN and THWN identify common types of electrical wires. These are wires with thermoplastic insulation (T), heat resistance (H), high heat resistance (H), appropriate for wet conditions (W), and not damaged by gas or oil (N). Non-metallic cable, NM, is the most common electrical wiring used inside homes.
THHN and THWN wires are marked CU, indicating that they are copper, as opposed to aluminum. Copper wires are insulated because electricity runs on the outside of the wire. Numerical ratings indicate the maximum voltage these wires can carry, and gauge indicates the thickness of the wire. The smaller the gauge number, the larger the wire. THHN and THWN wires are also color-coded to identify the wire's properties, such as hot, neutral or ground wires. These types of wires are usually encased in conduit and used outdoors or in unfinished interior spaces.
NM cable has varying numbers of wires, indicated by gauge, inside a flexible non-metal sheath. An 8-gauge cable contains eight wires. It is often labeled NM-B indicating that it is resistant to temperatures up to 194 degrees Fahrenheit without overheating or melting the insulation, which can cause fires. NM-B wiring is used inside walls and other covered areas. Other types of electrical wiring cable are used underground or for nonresidential indoor wiring.