The "type of wire" in residential electrical refers to the specific combination of conductor material, insulation rating, and outer jacketing designed for a particular installation environment, which dictates how much current it can safely carry and where it can be legally routed. Choosing the correct type of wire changes the physical routing method (cable staples vs. conduit pulls), the maximum allowable ampacity under the National Electrical Code (NFPA 70), and the overall material cost of your project. Most DIYers commonly confuse wire type (the insulation and jacket, like NM-B or THHN) with wire gauge (the physical thickness of the copper, like 14 AWG or 12 AWG), but both must be selected correctly to prevent a fire.
The Core Difference: Cable Assemblies vs. Individual Conductors
When you walk into a hardware store, the two most common wire types you will encounter for branch circuits are NM-B and THHN. They serve fundamentally different purposes based on their physical construction.
NM-B (Non-Metallic Sheathed Cable), often referred to by the brand name Romex, is a pre-assembled cable. It contains two or more insulated conductors and a bare copper ground wire, all wrapped in a paper binder and encased in a flexible PVC outer jacket. According to Southwire Technical Resources, the individual conductors inside NM-B are rated for 90°C, but NEC Article 334 strictly limits the allowable ampacity of the entire cable assembly to the 60°C column of the ampacity tables. This means a 12 AWG NM-B cable is capped at 20 amps, regardless of the 90°C insulation on the inner wires.
THHN (Thermoplastic High Heat-resistant Nylon-coated) is an individual conductor. It features a single copper wire insulated with PVC and coated in a tough, slick nylon outer skin. THHN is rated for 90°C in dry locations and is almost always pulled inside a raceway, such as metal EMT or PVC conduit. Because it dissipates heat differently and is protected by the conduit, you can utilize its higher temperature rating for derating calculations, though your final overcurrent protection is still limited by the termination ratings of your breakers and receptacles (usually 60°C or 75°C).
Where You Meet Wire Types in Practice
You cannot simply swap NM-B and THHN based on preference; the installation environment dictates the legal and safe choice.
- Dry, Finished Interior Spaces: Inside walls, floors, and ceilings covered by drywall or plaster, NM-B is the standard. It is fast to install, requires no conduit, and is stapled directly to the face of wooden studs.
- Exposed or Wet Locations: In unfinished basements, garages, or exterior walls where the wire might be subject to physical damage, NM-B is prohibited if run below 8 feet or exposed on the surface of walls. Here, you must use THHN (or THWN-2) pulled inside rigid metal, EMT, or PVC conduit.
- Underground Burial: Standard NM-B cannot be buried. For direct burial, you must use UF-B (Underground Feeder) cable, or pull THWN-2 rated conductors inside a continuous run of schedule 80 PVC conduit.
Worked Example: Ampacity Derating in a Hot Attic
To understand why wire type matters beyond just physical protection, let us look at a real-world numeric example involving ambient temperature derating. This is a scenario where choosing the wrong type of wire leads to a dangerous, code-violating installation.
Option A: 12/2 NM-B Cable
Because NM-B ampacity is locked to the 60°C column, its base ampacity is 20A. According to NEC Table 310.15(B)(1), at an ambient temperature of 120°F (49°C), we must apply a correction factor of 0.58 to the 60°C column.
Calculation: 20A × 0.58 = 11.6A.
Result: The 12/2 NM-B cable can now only safely carry 11.6 amps. If you put this on a 20A breaker, the wire will overheat and melt before the breaker ever trips. You would be forced to upsize to 8 AWG NM-B to safely carry 20A in this heat.
Option B: 12 AWG THHN in Conduit
THHN allows us to use the 90°C column for derating purposes. The base ampacity of 12 AWG at 90°C is 30A. The correction factor for 120°F in the 90°C column is 0.87.
Calculation: 30A × 0.87 = 26.1A.
Result: The derated ampacity is 26.1A. Because this is higher than our 20A breaker, the 12 AWG THHN wire is perfectly safe and code-compliant in this hot attic.
This example highlights a massive practical difference: THHN's higher thermal rating provides a crucial buffer for temperature derating that NM-B simply does not have.
Decision Tree: Picking the Exact Type of Wire
Use this decision path to select the exact wire type for your specific project. Follow the "If" condition to find your mandatory concrete pick.
| Project Scenario (If...) | Routing Environment | Concrete Pick (Then buy...) |
|---|---|---|
| Wiring standard interior bedroom/living room outlets | Inside finished drywall, wooden studs | 12/2 NM-B (Yellow jacket) with bare ground |
| Wiring a 240V electric dryer or range | Inside finished drywall, wooden studs | 10/3 or 8/3 NM-B (Orange or Black jacket) |
| Running exposed circuits in a garage or shop | Surface-mounted EMT or PVC conduit | 12 AWG THWN-2 (Individual black, white, green wires) |
| Feeding a detached shed or outdoor post | Underground PVC conduit | 10 AWG or 8 AWG THWN-2 (Individual conductors) |
| Burying power to a landscape lighting transformer | Direct burial in trench (no conduit) | 12/2 or 10/2 UF-B (Gray jacket, solid PVC) |
Common Confusions and the Default Recommendation
Before you head to the electrical supply house, clear up these two common points of confusion that trip up even experienced hobbyists:
- THHN vs. THWN-2: You will rarely find wire labeled solely as "THHN" anymore. Almost all modern building wire is dual-rated and stamped as THHN/THWN-2. The "W" stands for Water-resistant. If your wire is dual-rated, it is legal to pull inside wet locations (like outdoor conduit) and underground PVC. If it only says THHN, it is strictly for dry, indoor conduit.
- Solid vs. Stranded: NM-B almost always uses solid copper conductors, which are stiff but terminate easily under standard receptacle screw terminals. THHN is available in both solid and stranded. Stranded THHN is vastly easier to pull through long conduit runs with multiple bends, but the frayed ends can be tricky to terminate on standard residential devices without using ferrules or pigtailing to a solid wire.
For more visual guidance on identifying cable jackets and stripping techniques, The Family Handyman offers excellent photographic breakdowns of residential wire types and their specific applications.
