While many homeowners may only examine their house’s interior or exterior electrical cables and wires once in a blue moon, it’s still useful to know what they’re looking at. One of the easiest ways to familiarize yourself with your home’s wiring system is by understanding cable labels. Below, we’ll explain what some of the most common markings on electrical wires mean.
Common Labels on Home Cables
In residential locations, the most common type of cable used is called non-metallic (NM) cable, or “Romex.” NM cables are two or more insulated conducting wires, and typically a bare ground wire encased within a jacket—a plastic sleeve that envelops the wires. On that cable are either laser wire or hot stamped markings that relate the wire size, number of wires in the cable, whether it’s a ground wire, and the wire material—“CU” for copper and “AL” for aluminum.
For example, a homeowner may see a cable marked “12-2G CU” on the insulation; this means the wire is 12 gauge in size, has two insulated wires, a ground wire, and is copper material. Cables and wires are also labeled with the voltage, like 600V or 1000V.
Pro Tip: If the cable is marked “NM-B,” that means it’s a non-metallic cable that’s also heat-resistant.
Underground Feeder Cables
While the typical NM cables are usually reserved for interior and protected locations in residential environments—within walls, ceilings, or floors—there are also what’s known as underground feeder (UF) cables. A UF cable is a special kind of NM cable that is most common for outdoor or wet locations; if you need to bury a cable, it’ll likely be a UF.
While a standard home NM cable we mentioned earlier is usually a bright color like yellow, white, or orange, a UF is almost always gray, so it’s instantly recognizable. UF cables feature the same markings as residential NM cables but may also have additional labels like “Sunlight-Resistant” to show that they’re capable of functioning in direct sunlight.
Individual Wire Markings
Finally, the individual electrical wires within a cable also have their own markings; if you need to install a conduit, for instance, it’s wise to know what they mean. These individual wires’ vital labeling is their insulation—the brightly-colored coating around the conductor. These labels will tell homeowners whether the insulation is heat-resistant, how heat-resistant it is, and if it’s approved for damp locations.
- H: Heat-resistant (Up to 167 degrees Fahrenheit)
- HH: Highly heat-resistant (Up to 194 degrees Fahrenheit)
- T: Thermoplastic (Fire-resistant)
- W: Wet label, can be used in damp locations
- X: Synthetic polymer insulation (Fire-retardant)
- N: Nylon-coated insulation (Resistant to oil and gasoline)
On a typical wire, you may find a label like “THWN;” this means the wire has thermoplastic insulation, is heat-resistant (up to 167 degrees Fahrenheit), can be used in damp locations, and is nylon-coated for oil resistance.
We hope our guide has been enlightening! Knowing about these labels is important, but if you’re doing electrical work for your home, it’s always wise to consult a professional electrician first.
