Behind every outlet and switch in your home runs a specific type of electrical cable chosen for that exact job, and using the wrong one is a genuine fire hazard. Understanding the common types of wiring, from the NM cable in your walls to the THHN in conduit and the UF cable buried in your yard, helps you talk intelligently with an electrician and recognize when something has been done wrong. This guide is informational; any actual electrical work should follow local code and, for anything beyond simple swaps, be handled by a licensed professional.
NM Cable (Romex): The Household Standard
Non-metallic sheathed cable, universally known by the brand name Romex, is the wiring inside the walls of nearly every modern home. It bundles two or three insulated conductors plus a bare ground wire inside a flat plastic jacket, making it easy to run through wall studs and ceiling joists.
NM cable is rated for dry, indoor, protected locations only, so it cannot be exposed, buried, or run outdoors. Manufacturers color-code the jacket by gauge: white is 14-gauge for 15-amp circuits, yellow is 12-gauge for 20-amp circuits, and orange is 10-gauge for 30-amp circuits. That color system makes it easy to verify the right cable is feeding the right breaker.
THHN and THWN Wire
THHN (thermoplastic high heat-resistant nylon-coated) is single-conductor wire pulled through metal or PVC conduit rather than run as a bundled cable. You will find it in unfinished basements, garages, commercial buildings, and anywhere wires need the physical protection of conduit.
The closely related THWN version carries a “W” rating for wet locations, allowing use in damp environments and underground conduit. Because each conductor is separate, electricians can pull exactly the colors and gauges a circuit needs, which is why conduit-and-THHN wiring offers flexibility that bundled cable cannot.
UF Cable for Underground and Outdoor Use
Underground feeder cable, or UF, looks similar to Romex but is built for the outdoors. Its conductors are molded into a solid, moisture-resistant plastic that lets it be direct-buried in the ground without conduit, making it the standard for running power to detached garages, sheds, landscape lighting, and outdoor outlets.
UF cable must still be buried at code-required depths, generally 18 to 24 inches depending on protection, and connected through proper weatherproof fittings. Never substitute indoor NM cable outdoors; its jacket degrades quickly when exposed to moisture and sunlight.
Armored and Metal-Clad Cable
For situations needing more physical protection than a plastic jacket provides, armored cable steps in. AC (BX) and MC (metal-clad) cable wrap the conductors in a flexible spiral metal sheath that resists crushing and rodent damage.
MC cable, which includes a dedicated ground wire, is common in commercial work and in exposed residential runs like basements and utility areas. The metal armor itself serves as additional mechanical protection, so the cable can be run exposed where bare NM would be prohibited.
Low-Voltage and Specialty Cables
Not all home wiring carries 120 volts. A whole category of low-voltage cable handles communications and controls, and these run separately from power wiring.
- Coaxial cable: Carries cable TV and internet signals; the familiar round black cable with a single center pin.
- Cat5e and Cat6 (Ethernet): Twisted-pair data cable for wired networking.
- Speaker wire: Simple two-conductor wire for audio.
- Thermostat wire: Multi-conductor low-voltage cable for HVAC controls.
These carry little to no shock hazard but should still be routed cleanly and kept separated from line-voltage wiring to avoid interference.
Understanding Wire Gauge
Gauge, measured on the American Wire Gauge (AWG) scale, is the most safety-critical spec in any cable. Counterintuitively, a smaller number means a thicker wire that carries more current.
- 14-gauge: Rated for 15-amp circuits, lighting and standard outlets.
- 12-gauge: Rated for 20-amp circuits, kitchen and bathroom outlets.
- 10-gauge: Rated for 30-amp circuits, water heaters and dryers.
- 8- and 6-gauge: Heavy 40- to 60-amp loads like ranges and EV chargers.
Matching gauge to breaker is non-negotiable. Putting 14-gauge wire on a 20-amp breaker lets the wire overheat before the breaker trips, a classic cause of electrical fires.
Wire Color Codes
The insulation color on individual conductors tells you each wire’s job, and these conventions are consistent across US residential wiring. Black and red wires are hot (live) conductors, white is the neutral that returns current, and green or bare copper is the safety ground.
In a typical circuit, black connects to the brass terminal, white to the silver terminal, and ground to the green screw. Never assume a wire is dead based on color alone; always test with a voltage tester before touching any conductor, because older or amateur wiring may not follow the code.
Safety and When to Call a Pro
Knowing these cable types is useful, but electricity is unforgiving of mistakes. Simple tasks like replacing a light fixture or an outlet, with the breaker off and verified dead, are within reach for a careful homeowner. Anything involving new circuits, panel work, aluminum wiring, or anything you are unsure about belongs to a licensed electrician.
Permits and inspections exist for good reason; they catch the undersized wire, missing ground, or overloaded circuit that could otherwise smolder inside a wall for years. If you ever find old knob-and-tube or aluminum branch wiring, have a professional evaluate it rather than tackling it yourself.
The Bottom Line
The common types of wiring, NM cable indoors, THHN in conduit, UF underground, armored MC for protection, and low-voltage cable for data, each exist because they are engineered for a specific environment and load. Match the cable, gauge, and color code to the job, respect the safety rules, and lean on a licensed electrician for anything beyond basic swaps. Doing so keeps your home both code-compliant and safe for the long haul.
