House Electrical Wiring Types Explained: NM-B, THHN, and UF-B Selection Guide

House electrical wiring types refer to the specific insulation classifications and physical constructions of conductors—such as NM-B, THHN, and UF-B—that the National Electrical Code (NEC) mandates for safely carrying current across distinct residential environments. Selecting the correct type dictates your allowable ampacity, whether the run requires physical conduit protection, and if the installation will legally pass inspection in damp versus dry locations. Default Pick: 12 AWG NM-B for standard 20A indoor branch circuits.

The Core House Electrical Wiring Types (and What They Actually Do)

When you walk into an electrical supply house, the spools and coils are segregated by their NEC article classifications. Understanding these three primary categories is the baseline for any residential wiring project.

NM-B (Non-Metallic Sheathed Cable)

Commonly referred to by the brand name Romex, NM-B is a flat cable containing two or more insulated conductors and a bare copper ground, all wrapped in a flexible PVC jacket. It is rated for 90°C, but its ampacity is strictly limited to the 60°C column of the NEC ampacity tables (per NFPA NEC Article 334). It is the undisputed standard for interior, dry-location branch circuits.

THHN / THWN-2 (Thermoplastic High Heat-resistant Nylon-coated)

These are individual, single-conductor wires with a tough nylon outer skin. THHN stands for Thermoplastic High Heat-resistant Nylon-coated, while THWN-2 adds water resistance. Because they are installed inside raceways (conduit), they are evaluated using the 75°C or 90°C columns of NEC Table 310.16, making them highly efficient for packing multiple circuits into a single pipe.

UF-B (Underground Feeder)

UF-B looks like NM-B but features a solid, moisture-resistant PVC jacket that encapsulates the individual conductors entirely. Governed by NEC Article 339, it is designed for direct burial and damp locations without the need for conduit, though it still requires physical protection (like PVC) where it transitions above ground.

Where You Meet These Wiring Types in Practice

Choosing the right wire is entirely dependent on the physical environment of the run. Here is where each type earns its keep on the jobsite:

• NM-B: Fished through interior drywall, stapled to wooden studs in basements, and run through ceiling joists for receptacles, switches, and hardwired appliances.
• THHN: Pulled through EMT (metal) or ENT (smurf tube) conduit for exposed basement ceilings, surface-mounted garage runs, and panel-to-panel pigtails.
• UF-B: Trenched directly into the soil to feed detached sheds, outdoor post lights, and well pumps.

Worked Example: Ampacity Derating with THHN in Conduit

The most common mistake DIYers make with THHN is assuming they can use the 90°C ampacity column for their breaker sizing. The 90°C column is strictly used for derating when you have multiple current-carrying conductors in a single raceway. Let's run the numbers for a real-world scenario.

The Scenario: You are running three separate 20A circuits to a detached workshop through a single piece of 1-inch PVC conduit underground. This means you have 6 current-carrying conductors (3 hots, 3 neutrals) plus a ground wire in the pipe. You plan to use 12 AWG THHN.

1. Base Ampacity: According to NEC Table 310.16, 12 AWG THHN in the 90°C column is rated for 30A.
2. Derating Factor: NEC Table 310.15(C)(1) states that for 6 current-carrying conductors, you must apply an 80% adjustment factor.
3. Derated Ampacity: 30A × 0.80 = 24A.
4. Termination Limit Check: NEC 110.14(C) dictates that your final allowable ampacity cannot exceed the temperature rating of the terminations (usually 60°C for residential breakers and receptacles). In the 60°C column, 12 AWG is capped at 20A.

The Result: You must use the lower of the two values (24A derated vs. 20A termination limit). Therefore, your 12 AWG THHN is legally capped at 20A. It works perfectly for your 20A breakers.

What if you added a 4th circuit? That brings you to 8 current-carrying conductors, triggering a 70% derating factor. 30A × 0.70 = 21A. You are still safe. But if you pushed to 10 conductors (50% factor), 30A × 0.50 = 15A. Since 15A is lower than the 20A termination limit, your 12 AWG wire is now only good for a 15A breaker. To maintain a 20A circuit with 10 conductors, you would be forced to upsize to 10 AWG THHN (90°C base of 40A × 0.50 = 20A).

Decision Tree: Picking the Exact Wire for Your Next Run

Stop guessing at the hardware store. Use this decision matrix to select the exact wire type and part number for your specific installation environment.

Common Confusions and Mistakes to Avoid

Even experienced hobbyists trip over the nuances of residential wire types. Here is what people commonly confuse, and how to avoid the resulting failed inspections.

Confusion 1: Solid vs. Stranded Conductors

NM-B and UF-B use solid copper conductors, which bite cleanly into the screw terminals of standard residential receptacles and switches. THHN is available in both solid and stranded. Stranded THHN is much easier to pull through long conduit runs with bends, but it tends to fray and slip out of standard residential side-wire screw terminals. If you use stranded THHN for receptacle drops, either crimp a ferrule onto the end or use the back-wire push-in holes (if rated for stranded) to prevent loose connections and arcing.

Confusion 2: The 'Wet Location' Trap

Many DIYers assume that if NM-B is inside a sealed PVC conduit outdoors, it is protected from moisture. This is false. Temperature swings cause condensation inside sealed conduit. NM-B contains a paper separator that wicks moisture, leading to insulation breakdown and nuisance GFCI trips. Once a wire leaves the interior envelope and enters a wet location, it must be THWN-2 or UF-B.

Always verify your local amendments to the National Electrical Code before pulling a permit, as some municipalities have specific restrictions on wire types in certain fire-rated assemblies.