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Residential wiring uses NM-B for dry indoor circuits, UF-B for buried runs, MC for tough spots, and THHN/THWN in conduit.
You can’t choose house wire by gauge alone — the correct choice among the types of wire for house wiring depends on where it runs, how much physical protection it needs, and what the circuit feeds. Four cable families handle nearly every residential job: NM-B, UF-B, MC, and THHN/THWN conductors. This article covers what each one is for, which size matches which circuit, and the code rules that decide what you can install.
The Four Main Cable Types For House Wiring
Nearly every residential branch circuit uses one of four cable types: NM-B, UF-B, MC, or THHN/THWN conductors. Each one exists for a different set of conditions, and choosing the wrong one is the most common avoidable mistake in house wiring.
Type NM-B, or nonmetallic-sheathed cable, is the standard for interior dry locations. It bundles two or more insulated conductors plus a bare equipment grounding conductor under a nonmetallic sheath, is rated 600 volts, and is permitted in exposed or concealed dry locations. It also appears in the air voids of masonry block walls when those spaces are not damp. It feeds outlets, switches, and lighting circuits in walls and attics that stay dry. Romex is a brand, not a cable type; the correct generic term for this cable is NM-B.
UF-B cable is the underground version. It is built for direct burial, outdoor runs, and other damp or wet locations where NM-B is not allowed. MC cable (metal-clad) wraps the conductors in armor and is used where wiring needs more mechanical protection than NM-B offers, such as exposed runs in a garage or basement. THHN and THWN are individual insulated conductors pulled through conduit when a protected raceway is required; they are not a cable assembly on their own.
| Cable Type | Best Use | Watch Out For |
|---|---|---|
| NM-B | Dry indoor branch circuits: outlets, switches, lighting | Not rated for wet, outdoor, or underground use |
| UF-B | Direct burial and damp outdoor locations | Costlier and harder to bend than NM-B |
| MC | Areas needing extra physical protection | Heavier and pricier than NM-B |
| THHN/THWN | Individual wires inside conduit | Needs a raceway; not a stand-alone cable |
Lighting fixtures have one extra NM-B rule. NEMA’s guidance on NM-B and lighting fixtures notes that when existing dwelling wiring is 60°C-rated, a junction box may be required before transitioning to Type NM-B near the fixture.
Copper is the standard conductor for residential branch circuits. Aluminum shows up on larger service and feeder conductors, but 12- and 14-gauge residential circuits are almost always copper.
If you are ready to buy, compare the best electrical wire for house wiring in our product roundup, which breaks down the top NM-B, UF-B, and MC options.
What Size Wire Goes With Each Circuit?
Residential branch circuits follow a simple pattern: 14 AWG copper feeds 15-amp lighting circuits, 12 AWG copper feeds 20-amp receptacle circuits, and 10 AWG copper handles 30-amp loads.
In U.S. practice, 12-gauge wire is required for 20-amp breakers or fuses, including at least two kitchen countertop circuits. The breaker size has to match the wire: a 15-amp breaker protects 14 AWG, and a 20-amp breaker protects 12 AWG. Upsizing the breaker to get rid of nuisance trips is a fire hazard and a code violation. For large appliances, 10 AWG is common on 30-amp dedicated circuits.
Code Rules And Common Mistakes
Residential wiring must follow the National Electrical Code, and local code amendments can make the rules stricter. Type NM-B is governed by NEC Article 334 and is limited to normally dry locations.
Use NM-B for outlets, switches, and lighting only in dry indoor spaces. For underground or wet locations, use UF-B. Run THHN/THWN inside conduit when a raceway is required, and choose MC cable where physical protection matters. Don’t confuse cable assemblies with individual conductors: NM-B, UF-B, and MC are complete cables, while THHN/THWN are single wires that need a conduit. Local inspectors can add their own requirements, so the jurisdiction’s amendments always win.
The 90°C marking on NM-B conductors is a common source of confusion. The conductors may be rated 90°C, but under NEC practice the ampacity of a typical residential NM-B installation is capped by the 60°C column. Older Type NM cable made before the 1984 code revision was rated 60°C; Type NM-B is the 90°C version. That higher rating does not let you install a larger breaker.
FAQs
Can I use NM-B cable outside?
No. Type NM-B is rated for dry locations only. For direct burial or damp outdoor runs, use UF-B cable; for exposed areas that need mechanical protection, MC cable is the usual choice. Local code and the authority having jurisdiction determine what is acceptable on a specific job.
Should I use 12-gauge or 14-gauge wire for outlets?
14 AWG copper is rated for 15-amp lighting circuits, while 12 AWG copper is required for 20-amp receptacle circuits. The gauge must match the breaker rating. Never install a 20-amp breaker on 14-gauge wire, and never use 14-gauge wire on a circuit that needs 12.
What does the 90°C rating on NM-B mean?
The individual conductors inside Type NM-B are rated 90°C, but the allowable ampacity for typical residential installation is limited to the 60°C column under NEC practice. In plain terms, the 90°C number does not allow a larger breaker than the 60°C rating permits.