Building wire is the electrical wiring installed inside walls, ceilings, floors, and conduits to carry power throughout a residential or commercial structure. It connects your electrical panel to every outlet, light switch, and appliance in the building, forming the backbone of the entire electrical system. Most building wire is rated for 600 volts or less and comes in several varieties designed for different environments, from dry interior walls to underground burial.
How Building Wire Works
At its simplest, building wire consists of a metal conductor surrounded by insulation. The conductor carries electrical current from your service panel to wherever it’s needed. The insulation prevents that current from escaping into surrounding materials, which would create a fire or shock hazard. In most residential and light commercial settings, building wire operates at 120, 208, or 240 volts.
There’s an important distinction between “wire” and “cable” that often gets blurred. A wire is a single conductor. A cable is a group of conductors bundled together inside a protective sheath. When you see a thick white or yellow cord running through exposed framing in a house under construction, that’s a cable containing multiple wires inside it. When an electrician pulls individual colored wires through a metal or plastic tube (conduit), those are single conductors. Both fall under the umbrella of “building wire.”
Copper vs. Aluminum Conductors
The two metals used in building wire are copper and aluminum, and each has clear trade-offs. Copper is the benchmark for conductivity. Aluminum conducts only about 61% as well as copper, meaning you need a thicker aluminum wire to carry the same amount of current. For a 60-amp circuit, for example, you’d use a size 6 aluminum conductor instead of a size 8 copper one.
Aluminum’s advantage is weight and cost. It weighs only 30% as much as copper, so a bare aluminum wire with the same electrical capacity as a copper wire weighs roughly half as much. It’s also cheaper. That weight and cost savings matter most in large commercial buildings and long feeder runs, where aluminum is common. In most residential branch circuits (the wiring behind your walls going to outlets and lights), copper remains the standard choice.
Common Types of Building Wire
NM-B Cable (Romex)
This is the wire most people picture when they think of household wiring. NM-B stands for non-metallic sheathed cable, and Romex is the most widely known brand name. Inside its PVC outer jacket, you’ll find two or more insulated copper conductors plus a bare copper ground wire. It’s cost-effective, easy to install, and the go-to choice for interior walls, ceilings, and floors in residential construction.
The key limitation: NM-B cable is designed strictly for indoor, dry locations. Its plastic jacket isn’t built to handle moisture, direct sunlight, or physical abuse from outdoor exposure. You can’t bury it in the ground or run it through damp environments.
THHN/THWN-2 Wire
THHN wire is a single conductor with thermoplastic insulation, high heat resistance, and a nylon outer jacket. The full designation, THWN-2, adds water resistance and a 90°C temperature rating in both wet and dry conditions. This is the wire electricians pull through conduit in commercial buildings, industrial spaces, and any application where individual conductors need to be routed through protective tubing. It handles up to 600 volts and holds up well in dry, damp, and wet locations depending on the specific rating.
UF-B Cable (Underground Feeder)
UF-B cable is purpose-built for outdoor and underground use. Its conductors are encased in a tough, moisture-resistant plastic jacket designed to withstand sunlight, soil contact, and weather without deteriorating. Electricians can bury it directly in the ground without running it through conduit, which simplifies installation for landscape lighting, outdoor outlets, and irrigation systems. It typically comes in larger sizes like 6/3 and 8/3 to handle the demands of outdoor circuits.
Solid Wire vs. Stranded Wire
Building wire conductors come in two physical forms. Solid wire is a single, rigid piece of metal. Stranded wire is made of many thin filaments twisted together. The choice between them depends on the size of the wire and how it needs to be routed.
For smaller gauge wires (size 10 and below), electricians typically use solid conductors. Solid wire is cheaper, more durable for permanent installations, and easier to terminate at outlets and switches. For larger gauges (size 8 and up), stranded wire becomes the practical choice because it’s far more flexible. Pulling a thick, rigid solid conductor through conduit with multiple bends would be extremely difficult. Stranded wire bends and routes through tight spaces much more easily.
What the Letters on Wire Mean
The alphabet soup printed on building wire isn’t random. Each letter tells you something about the insulation’s properties:
- T: Thermoplastic insulation
- H: Heat resistant (a single H means 75°C; double H means 90°C)
- W: Suitable for wet locations
- N: Nylon outer jacket for extra durability
- X: Cross-linked polyethylene insulation (a tougher alternative to thermoplastic)
So THHN means thermoplastic, high heat resistant (90°C), with a nylon jacket. XHHW-2 means cross-linked polyethylene, high heat resistant, water resistant, rated for 90°C in both wet and dry conditions. Once you know the code, you can read any wire label and understand what environment it’s designed for.
Wire Color Coding
In North American electrical systems, the color of a wire’s insulation tells you its function in the circuit:
- Black: Hot (carries current, Phase 1)
- Red: Hot (secondary hot, Phase 2)
- Blue: Hot (Phase 3, used in commercial three-phase systems)
- White or gray: Neutral (return path for current)
- Green, green with a yellow stripe, or bare copper: Ground (safety path to earth)
Yellow insulation also indicates a hot wire in certain applications. A white wire wrapped with black or red tape has been re-identified as a hot conductor, which is a common practice when wiring switches. These color conventions aren’t just helpful labels. They’re safety standards that prevent dangerous wiring mistakes and allow any electrician to quickly understand a circuit they didn’t install.
Where Each Type Gets Used
The environment dictates which building wire is appropriate. Interior residential walls, where conditions are dry and the cable is protected by framing, call for NM-B cable. Commercial buildings, which often require conduit systems for fire protection and future maintenance access, use individual THHN/THWN-2 conductors pulled through metal or PVC conduit. Outdoor circuits and direct-burial applications use UF-B cable.
Other specialized categories fill specific niches. Armored cable (often called MC cable) wraps conductors in a flexible metal sheath for extra physical protection in commercial and industrial settings. Service entrance cable connects your utility’s meter to your main electrical panel. Each type is defined and regulated by the National Electrical Code, which local jurisdictions adopt and enforce through building permits and inspections.
Temperature ratings also matter for wire selection. Standard insulation types handle 60°C (140°F), while high-heat varieties like THHN are rated for 90°C (194°F). Specialty wires used near industrial heat sources can handle 150°C to 200°C. Using wire with an insufficient temperature rating in a hot environment degrades the insulation over time, creating a serious fire risk.