Using the correct type of electrical wire is essential for safety, performance, and code compliance. Each wire type is designed for specific environments—indoor, outdoor, underground, or exposed runs—and comes with insulation, protection, and current ratings suited for its role. Choosing the wrong wire can lead to overheating, electrical failures, or hazards. Understanding the differences gives you confidence to select and install wiring that suits your project.
NM / Nonmetallic (Romex) Cable
One of the most common wire types in residential wiring is NM (nonmetallic) cable, often called Romex. It consists of multiple insulated conductors plus a ground wire, all enclosed in a plastic sheath. NM cable is suitable for dry, interior locations such as walls, ceilings, and floors. Because it’s flexible and relatively easy to work with, it’s popular for most indoor branch circuits.
UF / Underground Feeder Cable
UF cable is similar in structure to NM but built stronger for wet or outdoor environments. Its insulation is solid around each conductor (rather than just a separate sheath), making it suitable for direct burial or use in damp or exposed areas. UF is often used for outdoor lighting, garden wiring, or power to outbuildings. The added insulation gives it greater protection against moisture.
THHN / THWN Single Conductors
THHN and THWN are insulation codes used for individual conductors, not cables. These wires are typically pulled through metal or plastic conduits rather than run inside sheathed cables. THHN stands for thermoplastic high heat-resistant nylon-coated, and THWN stands for thermoplastic heat- and water-resistant for wet locations. They allow more flexibility in routing wires in exposed or unfinished spaces like basements, garages, or through conduit systems.
Armored Cable (AC or BX) & Metal-Clad (MC) Cable
Armored cable, sometimes called BX or AC cable, wraps insulated conductors in a metallic sheath for added physical protection. It’s useful in exposed or more rugged installations where extra protection against damage is beneficial. Metal-clad (MC) cable is similar but offers additional flexibility and shielding, combining insulation and mechanical protection in one assembly. These types of wire are common in commercial or industrial settings and places where wiring could be exposed to harm.
Low-Voltage Wires, Data, and Specialty Conductors
Beyond high-voltage building wiring, many types of wire carry low-voltage signals. These include speaker wire, thermostat wire, doorbell wiring, and landscape lighting cables. Data and communication wiring like Cat5, Cat6, or coaxial cables carry signals rather than power and follow specific performance requirements. Specialty wires—like ribbon cables, tinsel wire, or litz wire—are used in electronics, flexible connections, or high-frequency applications. Each is designed for its intended use and environment.
Solid vs Stranded Wire
Another important distinction is whether the conductor is solid (a single metal core) or stranded (many small wires twisted together). Solid wire is better for fixed, rigid runs as it’s stable and simple. Stranded wire offers flexibility and resists fatigue—ideal for movable parts, fixture leads, or areas where vibration or bending occurs. Often, wire types like THHN, speaker wire, and low-voltage conductors come in stranded form.
Choosing the Right Wire Type
When selecting wire, consider where it will be used (indoor, underground, exposed, in conduit), the current load (gauge size), insulation type (resistance to heat, moisture), and applicable codes. Always ensure the wire’s rating (voltage and temperature) meets or exceeds what your installation requires. Using the correct type ensures safety, durability, and performance over time.
Conclusion
There are many types of electrical wire—NM (Romex), UF, THHN/THWN, armored/metal-clad, low-voltage, and specialty wiring—each tailored for particular applications. Solid and stranded constructions offer choices in stability versus flexibility. By matching the wire type to the environment and intended use, you ensure a safer, more reliable electrical system. Knowing wire types well lets you plan and execute wiring projects properly.
FAQs
Can I use indoor NM cable outdoors?
No. NM cable is designed for dry, protected indoor environments. Use UF or conduit with THHN/THWN for outdoor or wet applications.
Is armored cable always better than NM?
Not always. Armored cable offers extra mechanical protection in exposed locations, but it’s more expensive and harder to work with. NM is fine when wiring is protected.
Does wire gauge matter with different wire types?
Yes. All wire types come in different gauges (thickness). Choose a gauge that handles the current load and matches the circuit’s breaker rating.
Can I mix solid and stranded wire in one circuit?
It’s possible, but junctions must be properly made using connectors rated for both types. Mixing types incorrectly can cause poor connections or overheating.
How do I know if wire is rated for wet locations?
Look for markings like “W” (for wet) or “THWN” on insulation. If a wire lacks wet-location rating, it should not be used in damp or exposed areas.
