Electricity wires are insulated metal conductors designed to carry electrical current from a power source to a load, categorized primarily by their insulation material, jacket type, and installation environment. Choosing the correct type changes your allowable ampacity, dictates whether the run can be buried, pulled through conduit, or stapled to framing, and ultimately decides if your installation passes inspection or becomes a fire hazard. Beginners most commonly confuse the outer cable jacket (like the yellow sheath on Romex) with the actual wire insulation (THHN) hidden inside it. For 95% of standard indoor residential branch circuits, your default pick is NM-B (Non-Metallic Sheathed Cable)—specifically 14/2 for 15A lighting and 12/2 for 20A receptacles.
The Core Distinction: NM-B Cable vs. Individual Conductors
When discussing the types of electricity wires, the most critical fork in the road is whether you are buying a pre-assembled cable or individual conductors.
NM-B (Non-Metallic Sheathed Cable), widely known by the trademarked name Romex, is a factory-assembled bundle. It contains two or more insulated THHN/THWN-2 conductors and a bare copper ground, all wrapped in a moisture-resistant PVC jacket. It is designed to be stapled directly to wooden studs and concealed behind drywall.
THHN/THWN-2 (Thermoplastic High Heat-resistant Nylon-coated / Thermoplastic Heat and Water-resistant Nylon-coated) refers to the individual wires themselves. These cannot be run bare inside a wall cavity; they must be pulled through a continuous raceway, such as EMT (metal conduit) or PVC pipe, which provides the physical protection that the NM-B jacket otherwise supplies.
Worked Numeric Example: Sizing a 20A Kitchen Appliance Circuit
Let's look at how wire type interacts with the National Electrical Code (NEC) ampacity tables in a real scenario. We are running a new 20A, 120V small appliance branch circuit in a kitchen to handle a 16A continuous load (like a high-end toaster oven running alongside a coffee maker).
16A × 1.25 = 20A minimum circuit ampacity.
We need a wire that can safely carry 20A. We will use 12 AWG copper. Here is how the wire type changes the math:
- Scenario A: Using 12/2 NM-B Cable. Even though the individual conductors inside the yellow jacket have 90°C insulation, NEC Article 334.80 strictly mandates that NM-B ampacity must be determined using the 60°C column of NEC Table 310.16. Looking at the 60°C column for 12 AWG copper, the ampacity is exactly 20A. This perfectly matches our 20A breaker.
- Scenario B: Using 12 AWG THHN in EMT Conduit. THHN is rated for 90°C. In the 90°C column, 12 AWG is rated for 30A. However, standard residential breakers and receptacles are only rated for 75°C (or 60°C for devices under 100A). You can use the 90°C column for derating (e.g., if you have four current-carrying conductors in the pipe), but the final overcurrent protection cannot exceed the termination rating. In the 75°C column, 12 AWG is rated for 25A. A 20A breaker is well within this limit.
Assumptions: Copper conductors, 30°C ambient temperature, no more than 3 current-carrying conductors in the conduit, and standard residential 75°C rated terminations.
Where You Meet These Wire Types in Practice
Understanding the theory is useless if you don't know where the code allows you to physically install them. Here is where you meet this in practice on the jobsite:
- NM-B (Romex): Dry, indoor locations only. Concealed behind drywall, stapled to wooden framing, or fished through finished walls. It is strictly forbidden in wet locations, embedded in concrete, or used as aerial cable on messenger wires.
- THHN/THWN-2: Inside EMT, PVC, or flexible metal conduit. You will also see this as individual pigtails inside your main service panel, or running through exposed conduit in unfinished basements and garages.
- UF-B (Underground Feeder): Direct burial applications. It looks like NM-B but has a solid gray PVC jacket that completely encapsulates the wires, making it resistant to moisture and soil chemicals. Used for feeding outdoor post lights or detached sheds (when buried at the NEC-mandated depth, typically 24 inches for direct burial without a GFCI).
- MC (Metal-Clad) Cable: Often required in commercial construction or multi-family residential (Type V construction) where fire ratings prohibit exposed NM-B. It features an aluminum or steel interlocking armor jacket.
Decision Tree: Which Wire Type to Buy for Your Project
Stop guessing at the hardware store. Use this decision-tree-table to terminate your choice in a concrete pick based on your specific installation environment.
| Installation Environment | Required Wire/Cable Type | Concrete Default Pick (Copper) |
|---|---|---|
| Indoor, concealed behind drywall on wood studs | NM-B (Non-Metallic Sheathed) | Southwire 12/2 NM-B with Ground (Yellow) |
| Inside EMT or PVC conduit (exposed or underground) | THHN/THWN-2 Individual Conductors | 12 AWG Solid THHN (Buy Black, White, Green) |
| Underground direct burial (no conduit) | UF-B (Underground Feeder) | 12/2 UF-B (Gray Jacket) |
| Exposed indoor surface in a commercial garage/shop | MC (Metal Clad) or Conduit + THHN | 12/2 MC Cable with Aluminum Armor |
| Heavy feeder to a subpanel (e.g., 60A) | THHN in conduit or SER Cable | 6 AWG THHN (for conduit) or 6-6-6-6 SER (indoor) |
Common Confusions and Code Traps
When selecting the types of electricity wires, DIYers frequently fall into three specific traps that result in failed inspections or unsafe conditions.
1. The '90°C Ampacity' Myth
Every spool of THHN and every roll of NM-B is stamped with '90°C'. Beginners look at NEC Table 310.16, see that 14 AWG at 90°C is rated for 25A, and assume they can put it on a 25A breaker. This is a massive code violation. Because standard residential devices (receptacles, switches, breakers under 100A) are only tested and rated for 60°C or 75°C terminations, you must size your overcurrent protection based on the lowest temperature rating in the circuit. For NM-B, that is strictly the 60°C column. 14 AWG is always limited to 15A in residential branch circuits.
2. Stranded vs. Solid Core
NM-B cable uses solid copper conductors. Solid wire is stiff, holds its shape when pushed into a receptacle's push-in terminals, and is easy to strip with standard gauge-specific wire strippers. THHN pulled through conduit is often purchased in stranded form because it is vastly easier to pull around 90-degree conduit sweeps. However, stranded wire can fray when pushed into standard back-stab receptacle terminals; you must use the screw terminals or apply a ferrule.
3. Using Indoor Cable Outdoors
NM-B jacket is not UV resistant and degrades rapidly when exposed to sunlight or constant moisture. Running standard yellow Romex out a hole in your siding to power a patio outlet is a guaranteed way to crack the insulation and create a ground fault. You must transition to UF-B or THWN-2 in conduit before the wire exits the building envelope.
Frequently Asked Questions
Can I run THHN wire through a wall cavity without conduit?
No. The NEC requires individual conductors to be installed within a recognized raceway (conduit, tubing, or cable tray). Running bare THHN through a stud bay leaves the thin nylon coating vulnerable to puncture from drywall screws and physical abrasion, creating an immediate arc-fault hazard.
Why is NM-B cable sometimes banned in my city?
While the National Electrical Code allows NM-B in most residential construction, local Amendments (AHJ) can override it. Cities like Chicago and New York have historically banned NM-B in favor of EMT conduit or MC cable to provide superior fire containment and physical protection in dense, multi-story construction. Always check with your local building department.
What does the 'B' in NM-B stand for?
The 'B' designates that the cable jacket is rated for 90°C (194°F). Older NM and NMC cables were rated for only 60°C or 75°C. You will almost never find anything other than NM-B in modern hardware stores, as it has been the manufacturing standard since the 1980s. For deeper installation specs, refer to the Southwire installation guidelines.
Can I mix solid and stranded wire on the same circuit?
Yes, but only inside a junction box using proper connectors. You cannot splice stranded and solid wire together with a standard wire nut without ensuring the stranded wire is wrapped tightly around the solid wire before twisting the nut. A better practice is to use WAGO 221 lever nuts, which clamp both solid and stranded conductors independently and securely.
When in doubt, default to 12/2 NM-B for indoor concealed runs and 12 AWG THWN-2 in PVC conduit for outdoor runs. This baseline covers nearly all residential upgrades while keeping you safely within the 60°C and 75°C ampacity limits mandated by the NEC.
