Cable wires are assemblies of two or more insulated conductors bound together within a common protective jacket or armor, designed to route electrical power from a source to a load. Choosing the correct cable type fundamentally changes your installation method, environmental resistance, and physical protection level on a jobsite. Beginners most commonly confuse a single "wire" (like a loose THHN conductor) with a "cable" (like NM-B), or mistakenly assume all jacketed cables are rated for outdoor and direct-burial use.
The Core Types of Residential Cable Wires
Before pulling any wire, you need to match the cable assembly to the environment. The National Electrical Code (NEC) categorizes these by their jacket material, temperature rating, and physical construction. Below is the reference table for the most common residential cable types you will encounter in 2026.
| Cable Type | Jacket / Armor Material | Max Temp Rating (Conductor) | Primary Use Case | NEC Article |
|---|---|---|---|---|
| NM-B (Nonmetallic) | PVC (Polyvinyl Chloride) | 90°C (but ampacity limited to 60°C) | Indoor, dry-location branch circuits (walls, ceilings) | Article 334 |
| UF-B (Underground Feeder) | Moisture-resistant PVC (solid matrix) | 90°C (but ampacity limited to 60°C) | Direct burial, outdoor wet locations, outbuildings | Article 339 |
| MC (Metal Clad) | Interlocking aluminum or steel armor | 90°C (ampacity based on 75°C terminations) | Commercial, exposed residential, physical damage zones | Article 330 |
| SE (Service Entrance) | Reinforced PVC or metal braided sheath | 90°C (75°C for ampacity) | Feeder runs from meter to main panel, or subpanels | Article 338 |
Row-by-Row Notes for the Jobsite
NM-B: Often generically called "Romex" (a trademark of Southwire), NM-B is the default for interior walls. The "B" denotes the 90°C conductor insulation, but per NEC 334.80, you must strictly use the 60°C column for ampacity calculations.
UF-B: Unlike NM-B, where the conductors float loosely inside a hollow jacket, UF-B conductors are embedded in a solid extrusion of plastic. This prevents moisture wicking but makes stripping the jacket significantly harder. Use a dedicated UF cable stripper, not standard wire strippers.
MC: With copper prices fluctuating through the mid-2020s, MC cable has become highly competitive for exposed basement and garage runs because it eliminates the need for EMT conduit while offering superior crush resistance.
Where You Meet This in Practice: Installation Rules
Theoretical ratings only matter if you follow the physical installation rules. Here is how these cable types behave when you are actually framing walls or trenching yards.
Securing and Supporting (Stapling)
For NM-B and UF-B, the NEC mandates strict support intervals. You must secure the cable within 12 inches of every outlet box, junction box, or cabinet. After that, you must staple it at intervals not exceeding 4.5 feet (NEC 334.30). Use insulated cable staples sized exactly for your gauge (e.g., 1/2-inch staples for 14/2 or 12/2) to avoid crushing the dielectric insulation.
Physical Damage Zones
NM-B is strictly forbidden in areas subject to physical damage. If you are wiring a garage, an unfinished basement, or a crawlspace where the cables run below 8 feet on the face of a stud, NM-B is a code violation. In these zones, you must either run individual THHN wires inside EMT conduit or switch to MC (Metal Clad) cable. The metal armor of MC cable acts as an integral ground path (if listed as such) and protects the conductors from stray nails, impact, and rodents.
Wet vs. Damp Locations
A common failure point for DIYers is running NM-B to an exterior receptacle. NM-B is rated only for dry locations. If a cable is exposed to weather, condensation inside a conduit, or direct soil contact, it must be UF-B or individual wet-rated THWN-2 conductors in conduit. Even running NM-B inside a conduit that exits a building is a violation, because condensation will accumulate inside the pipe and wick into the paper wrapper of the NM-B, causing eventual short circuits.
Worked Example: Ampacity Derating in Multi-Wire Runs
To understand what cable choice changes in a real circuit, let us look at ampacity derating when bundling multiple circuits together. This is where confusing cable types leads to failed inspections or fire hazards.
The Scenario: You need to run two separate 20A, 120V circuits through a single 24-inch sleeve of conduit to protect them through a masonry wall. This creates a total of four current-carrying conductors (two black hots, two white neutrals) in a confined space.
Attempt 1: Using 12 AWG NM-B (Sleeved)
By NEC 334.80, NM-B ampacity is locked to the 60°C column. Looking at NEC Table 310.16, 12 AWG at 60°C is rated for 20A.
Because you have four current-carrying conductors bundled together, heat dissipation drops. NEC Table 310.15(C)(1) requires an 80% derating factor for 4-6 conductors.
Math: 20A × 0.80 = 16A.
Result: 16A is less than your 20A breaker. This installation fails. The breaker will not trip before the wire overheats.
Attempt 2: Using 12 AWG THHN/THWN-2 (Individual Wires in Conduit)
THHN insulation is rated for 90°C. At 90°C, 12 AWG is rated for 30A.
Apply the same 80% derating factor for bundling.
Math: 30A × 0.80 = 24A.
Result: 24A is greater than the 20A breaker. This installation passes. (Note: The final termination at the breaker and receptacle is still limited to 75°C or 60°C, but the derating calculation is permitted to start from the 90°C column for THHN).
Frequently Asked Questions
What is the exact difference between a "wire" and a "cable"?
In electrical terminology, a wire is a single conductor (insulated or bare), such as a spool of 12 AWG THHN or a bare copper grounding wire. A cable is an assembly of two or more wires bound together inside a single outer jacket or armor, such as 12/2 NM-B (which contains a black hot, white neutral, and bare ground). Calling NM-B a "wire" is colloquial but technically incorrect.
Can I use indoor NM-B if I wrap it in electrical tape for outdoor use?
No. Electrical tape does not provide a watertight seal, nor does it stop UV degradation or moisture wicking. The paper wrapping inside NM-B acts like a sponge if water breaches the tape. You must use UF-B for direct burial or THWN-2 inside liquid-tight or PVC conduit for outdoor wet locations.
Does the color of the NM-B jacket mean anything?
Yes, manufacturers use jacket colors as a quick visual identifier for the wire gauge inside, though the printed text on the jacket is the legal identifier. Standard industry practice (pioneered by Southwire and adopted by others) uses:
• White: 14 AWG (15A circuits)
• Yellow: 12 AWG (20A circuits)
• Orange: 10 AWG (30A circuits)
• Black: 8 AWG or 6 AWG (40A-60A circuits)
Why is my 12/2 NM-B limited to 20A when the copper can handle more?
The copper itself could handle more heat, but the PVC jacket and the insulation materials begin to degrade and off-gas at sustained high temperatures. Furthermore, standard residential breakers and receptacles are typically rated for 60°C or 75°C terminations. The NEC limits the entire assembly to the weakest link in the thermal chain, which is why 12 AWG NM-B is hard-capped at 20A.
