Household wiring types refer to the specific combinations of conductor insulation, outer jacketing, and physical shielding used to route electrical power safely through residential structures based on environmental exposure and physical protection needs. When you walk into an electrical supply house, you aren't just buying 'wire'; you are buying an engineered system with strict limits on temperature, moisture tolerance, and physical abuse. Choosing the wrong type doesn't just mean a failed inspection—it means risking insulation breakdown, voltage drop, or a concealed fire.

The Core Household Wiring Types Defined

To make sound decisions on the jobsite or at the workbench, you need to know the four primary cable and wire assemblies used in residential construction:

  • NM-B (Non-Metallic Sheathed Cable): Commonly known by the brand name Romex. It consists of two or more insulated conductors (usually THHN/THWN-2), a bare copper ground, and a paper filler, all wrapped in a moisture-resistant PVC outer jacket. It is strictly rated for dry, indoor locations.
  • THHN/THWN-2 (Individual Conductors): Single wires with a tough nylon outer skin over PVC insulation. They cannot be run bare through studs; they must be pulled through a raceway like EMT conduit or PVC pipe.
  • UF-B (Underground Feeder): Looks like NM-B but features a solid, gray PVC jacket that encapsulates the wires completely with no paper filler. It is rated for damp locations and direct burial underground.
  • MC (Metal-Clad Cable): Individual THHN conductors wrapped in an interlocking aluminum armor jacket. It provides the physical protection of metal conduit with the pull-through ease of a flexible cable.

What the Jacket and Insulation Actually Change

The wiring type you select dictates three critical variables in your installation: the fastening method, the allowable environment, and the ampacity column you must use from NEC Table 310.16.

The 60°C Rule for NM-B: Even though the individual wires inside a standard 12/2 NM-B cable are rated for 90°C, the National Electrical Code (NEC) requires you to size the overcurrent protection based on the 60°C ampacity column for cables sized 14, 12, and 10 AWG. This is because the standard residential breakers and receptacles they terminate on are typically only rated for 60°C or 75°C.

If you run individual THHN wires in conduit, you gain access to the 75°C or 90°C columns for derating purposes, though your final termination ampacity is still capped by the device rating. Furthermore, the jacket type dictates environmental limits. Running NM-B through a wet concrete slab or burying it in a trench will cause the paper filler to wick moisture, destroying the dielectric integrity of the cable. For a deep dive into code-mandated installation methods, refer to the NFPA National Electrical Code guidelines.

Where You Meet This in Practice

You will encounter specific wiring types based on the physical boundaries of the building:

  • Interior finished walls and ceilings: NM-B is the undisputed king here. It is fast to staple to studs and easy to strip.
  • Unfinished basements and garages: Where cables are run below 8 feet on walls and subject to physical damage, NM-B is prohibited. You must step up to MC cable or pull THHN through EMT (Electrical Metallic Tubing) conduit.
  • Trenching to a detached structure: UF-B is used for direct burial (minimum 24 inches deep for residential branch circuits), while THWN-2 in Schedule 40 or 80 PVC conduit is used when you want the option to pull new wires later without digging up the yard.
  • Masonry and concrete: You can never bury NM-B or UF-B directly in concrete. You must use THHN/THWN-2 pulled through rigid or PVC conduit.

Worked Numeric Example: 30A Shed Feeder (UF-B vs. THWN-2)

Let's look at how wiring type changes your material pick when sizing a 50-foot, 240V feeder to a detached shed subpanel on a 30A breaker. We need to pull two hots and a ground (equipment grounding conductor sized per NEC 250.122).

Option A: 10 AWG UF-B Direct Burial
Because UF-B is limited to the 60°C column, 10 AWG copper is rated for exactly 30A.
Voltage Drop Check: Using the standard formula $VD = \frac{2 \times K \times I \times L}{CM}$ (where K=12.9 for copper, I=24A at 80% load, L=50ft, CM=10380 for 10 AWG):
$VD = \frac{2 \times 12.9 \times 24 \times 50}{10380} = 2.98V$.
At 240V, this is a 1.24% drop, which is excellent. Result: 10/2 UF-B is legally and electrically sufficient.

Option B: 10 AWG THWN-2 in 1-inch PVC Conduit
THWN-2 in conduit allows us to use the 75°C column for the wire's inherent ampacity (35A for 10 AWG), though the 30A breaker limits the circuit to 30A anyway. However, if we decide to upgrade to a 40A breaker later, the THWN-2 in conduit can handle it (provided we upsize to 8 AWG for the 40A termination limit), whereas the UF-B would require a complete trenching redo. Furthermore, pulling three individual 10 AWG THWN-2 wires through conduit is often cheaper per foot than buying heavy UF-B cable, and it provides a dedicated ground wire if you choose to pull a 4-wire setup (two hots, neutral, ground) for a 120/240V subpanel.

For exact voltage drop calculations on your specific runs, the Southwire Voltage Drop Calculator is an industry-standard bench tool.

Decision Tree: Which Cable to Buy for Your Project

Use this matrix to terminate your decision process quickly. Do not overthink the environment; match the cable to the physical space.

Installation Environment Physical Protection Needed? Moisture Exposure? Concrete Pick (AWG dependent on load)
Interior drywall, standard studs No No (Dry) NM-B (e.g., 12/2 or 14/2)
Unfinished garage/basement walls Yes (Subject to damage) No (Dry) MC Cable or THHN in EMT
Trench to shed (Direct Burial) No (Buried >24') Yes (Wet) UF-B
Trench to shed (In Conduit) Yes (PVC raceway) Yes (Wet) THWN-2 individual wires
Embedded in concrete slab Yes (Rigid/PVC raceway) Yes (Wet) THWN-2 individual wires
The Default Baseline: If you are wiring standard 15A or 20A interior branch circuits for receptacles and lighting in a dry, finished home, stop evaluating and buy 250-foot coils of 12/2 NM-B with a ground. Using 12 AWG instead of 14 AWG for 15A circuits costs roughly $15 more per 250ft coil but eliminates voltage drop concerns on long runs and allows for future breaker upgrades to 20A without rewiring.

Common Confusions and Mistakes to Avoid

Confusing the assembly with the conductor: The most frequent mistake DIYers make is assuming that because the wires inside NM-B are stamped 'THHN', they can strip the yellow jacket off and use the internal wires in an outdoor conduit. You cannot. Once you remove the NM-B jacket, the individual wires lack the UV and moisture ratings required for wet locations, and the paper filler ruins the pull. Always buy dedicated THWN-2 for conduit.

The 'Conduit is Always Better' Fallacy: Some builders assume pulling THHN in conduit is universally superior to NM-B. While conduit offers physical protection and future expandability, it introduces ampacity derating. If you pull more than three current-carrying conductors through a single conduit (e.g., sharing a pipe for two separate 20A circuits), NEC Table 310.15(C)(1) requires you to derate the ampacity to 80%. Four 12 AWG THHN wires in one pipe derate to 20A at the 90°C column, which perfectly matches a 20A breaker, but if you add a fifth circuit, you must upsize to 10 AWG. NM-B cables stapled to studs do not suffer this bundling derating unless you pass multiple cables through a single bored hole in a fire-blocked stud.

Using NM-B in wet conduit: Never run NM-B through a conduit that exits the building. Even if the conduit is sealed, condensation will form inside the pipe outdoors, wicking into the NM-B paper filler and causing a ground fault or short circuit. Transition to THWN-2 in a junction box before the conduit exits the structure.

For further reading on how local inspectors interpret these wiring methods, the EC&M National Electrical Code resource center provides excellent field-level breakdowns of inspector hot-buttons.