Most wiring in a modern American home comes down to three products: non-metallic (NM) cable for dry indoor runs, underground feeder (UF) cable for wet and buried locations, and individual THHN/THWN conductors pulled through conduit where durability matters. Each is defined by the National Electrical Code (NEC) for specific environments, and using the wrong one in the wrong place is not a minor mistake — NM cable is explicitly prohibited in wet or buried locations. Here is how the three differ, and how to match them to a project.

Understanding Common Residential Electrical Wires

Non-metallic cable — sold commercially as Romex, though Romex is a registered trademark of Southwire Corporation — is the workhorse of North American residential wiring. According to Dongguan Greater Wire & Cable, an electrical cable manufacturer, NM-B cable carries two to four THHN-insulated copper conductors inside a standard PVC jacket, usually with paper or plastic fillers between them. That jacket is thinner than the one on UF-B cable and has no special moisture-proof treatment, which makes the cable lighter and easier to handle but limits where it can be installed.

The NEC treats that limitation as a hard rule. Under Article 334, as summarized by Greater Wire, NM-B wire is limited to dry locations and is prohibited in wet or damp locations, buried applications, commercial garages, special occupancies such as movie theaters, and environments above 80°C. It must also be installed where it will not be subject to physical damage. Where it is permitted — chiefly wood-frame residential construction — its lower cost relative to conduit-and-wire systems drives what US Electrical Authority describes as nearly universal adoption.

Underground feeder cable looks similar but is built for the opposite job. The NEC's Article 340, again per Greater Wire's summary, allows UF-B in various indoor and outdoor environments: it can be directly buried, subject to minimum burial depth requirements, and is suitable for damp, wet, and dry locations. Its current-carrying capacity must be corrected in accordance with NEC 310.15(B)(3)(a). Practically, that makes UF the natural choice for outdoor lighting or running power to a shed, where the cable will be exposed to moisture or soil.

THHN and THHN/THWN conductors are a different category altogether: individual insulated wires rather than a finished cable. The letters describe the wire's coating properties and temperature rating. These conductors often live inside a cable assembly such as NM-B, but they are also pulled individually through conduit, which makes them suitable for exposed or outdoor areas where the conduit, not a plastic jacket, provides the protection.

Wire gauge matters as much as cable type. Conductors are sized by the American Wire Gauge (AWG) standard, and a conductor that is too thin for its load can overheat — which is why the NEC ties ampacity to conductor size and installation conditions, requiring derating corrections such as those in 310.15(B)(3)(a) when multiple current-carrying conductors share a raceway or cable. Heavier household loads demand heavier wire, and the gauge printed on a cable's jacket should always be matched to the circuit it will serve.

Comparative Guide to Residential Wiring Types

The table below contrasts the three wire types across the selection criteria that actually determine suitability: construction, code-approved location, and typical application. Read it as a decision framework — first identify where the wire will run (indoors and dry, outdoors or underground, or in conduit), then confirm the construction matches that environment.

Common residential wire types compared by construction, approved location, and application
Wire Type Construction / Sheathing Approved Location / Use Common Applications
Non-Metallic (NM) Cable Standard PVC jacket, thinner than UF-B, without special moisture-proof treatment; contains 2–4 THHN-insulated copper conductors with paper or plastic fillers. Dry, indoor locations only (NEC Article 334). Prohibited in wet or damp locations, buried, commercial garages, special occupancies such as movie theaters, or above 80°C; must be protected from physical damage. The most common home wiring; branch circuits in wood-frame residential construction where permitted by code.
Underground Feeder (UF) Cable Similar to NM but built with a heavier, moisture-resistant jacket for underground and wet service. Various indoor and outdoor environments (NEC Article 340): may be directly buried subject to minimum burial depth; suitable for damp, wet, and dry locations; ampacity corrected per 310.15(B)(3)(a). Outdoor lighting and supplying power to a shed or other outbuilding exposed to moisture or soil.
THHN/THWN Wire Individual insulated conductors, often copper; the letters denote coating properties and temperature rating. Installed in conduit; suitable for exposed or outdoor areas where durability is crucial. Pulled through conduit for protected runs; also found as the conductors inside NM-B cable assemblies.

The comparison also clarifies a common question: whether UF-B can substitute for NM-B. Because UF-B is permitted in damp, wet, and dry locations alike, it can serve where NM-B is allowed — but the reverse is false, since NM-B is barred from every wet or buried environment. The trade-off is practical rather than legal: UF-B's heavier jacket makes it less flexible and typically more costly for routine indoor branch-circuit work.

Codes, Certification, and Local Rules

The NEC sets the baseline, but it is not the last word. Greater Wire notes that many local jurisdictions add requirements on top of the code: some areas prohibit NM-B in residential use entirely, seismic regions may require additional fixing measures, and some cities impose special rules on outdoor wiring. Before any actual construction, the latest local electrical regulations must be consulted — and this article is informational only, not a substitute for a licensed electrician or local code approval.

Third-party certification is the other half of compliance. Recognized listing marks, such as UL Listing, indicate that a wire or cable product has been evaluated against a safety standard, and the NEC itself is built around listed products used within their marked ratings. A cable's jacket and conductor markings — type, gauge, temperature rating, and listing mark — are what an inspector reads, and they should be what a buyer reads too.

Ensuring Safety and Compliance in Your Wiring Project

Always select the correct wire type and gauge for your specific application and location, ensuring it meets local electrical codes and bears recognized safety certifications. The scope here is deliberately narrow: these are the most common wire types in modern U.S. residential construction, and specialized wiring, commercial or industrial applications, and low-voltage systems for security or data fall outside it. No step-by-step installation guidance is offered, because the decisions that precede installation — type, gauge, location, and certification — are the ones that determine whether a project is safe.

The checkable indicators are concrete: look for a UL Listing or other recognized safety certification mark on the wire's packaging or printed along the cable itself, and verify that the chosen wire type and installation method comply with local code before work begins. If either check fails — no listing mark, or a local prohibition on the planned cable — stop and consult a licensed electrician rather than proceeding.

Sources