Types of Wire for House Wiring | Four Types You Should Know

When you’re wiring a house, the type of wire you choose matters as much as the size. Use the wrong type in the wrong location—like running standard NM cable outdoors—and you’re looking at failed inspections at best and fire hazards at worst. The National Electrical Code (NEC) governs which wire goes where, and the distinctions come down to insulation, jacket materials, and where the cable or conductor can safely run. Here is what the four common residential types are, how they differ, and which size pairs with which breaker.

The Four Common Wire Types For House Wiring

NM-B (Romex) is the standard indoor residential cable. It bundles two or more insulated conductors with a bare grounding wire inside a PVC jacket. The NEC covers it under Article 334, and it is approved only for dry indoor locations—walls, joist bays, attics, and crawlspaces that stay dry. Never run it outdoors, underground, or in wet areas.

UF-B looks similar to NM-B but uses a solid, moisture-resistant jacket that stands up to soil and water. Governed by NEC Article 339, it is rated for wet locations and direct burial underground, making it the choice for outdoor feeds, landscape lighting, and damp environments.

THHN/THWN-2 are individual insulated conductors designed to be pulled through conduit, not pre-assembled into a jacket. THWN-2 carries a wet-location rating, and both types fall under NEC Article 310. Use them in conduit runs in basements, garages, panel wiring, and other protected raceways.

MC Cable wraps the conductors in a flexible metal armor for extra mechanical protection. Covered by NEC Article 330, it is common for exposed runs in garages, basements, and commercial spaces where the wiring needs physical defense against impact or puncture.

Wire Sizes and Breaker Pairings

The conductor gauge determines how much current the wire can safely carry. The table below shows the standard copper-wire pairings for residential circuits. Always match the wire size to the breaker rating—oversizing the breaker for the wire gauge creates an unsafe condition that code will not allow.

Larger appliances like electric ranges and air conditioners may require 6 AWG or heavier. Wire designation numbers like 14/2 mean two insulated 14-gauge conductors plus a ground—the gauge is what governs ampacity, while the count tells you how many hot/neutral conductors are in the jacket.

Choosing The Right Wire For The Location

The golden rule: match the wire’s insulation and jacket rating to the environment where it will be installed. NM-B stays inside dry walls. UF-B goes outdoors or underground. THHN/THWN-2 runs through conduit. MC cable protects exposed runs. And low-voltage wiring (under 50 volts) covers doorbells, thermostats, and landscape lighting with its own set of rules.

If you’re ready to buy, our recommended electrical wire picks compare the top options for each location and circuit type, so you can grab the right cable on the first try.

Keep these rules in mind to avoid the most common mistakes:

• Color code: black and red conductors are hot, white and gray are neutral, green and bare are ground.
• Never substitute NM-B for UF-B in wet or outdoor locations.
• Conduit wiring uses individual conductors like THHN/THWN-2, not jacketed cable.
• Aluminum wire requires different sizing than copper—consult an electrician if your home has aluminum branch circuits.

Common homeowner errors include confusing “wire” (the conductor material) with “cable” (the assembled bundle), running Romex outdoors, and assuming any 12 AWG wire works for any 20-amp application without checking the full circuit rating and the device’s load.

FAQs

Can I use Romex outdoors?

No. NM-B (Romex) is rated for dry indoor locations only. For outdoor runs, wet areas, or direct burial, use UF-B cable, which has a moisture-resistant jacket and meets NEC Article 339 requirements.

What does 12/2 mean on a wire label?

It means the cable contains two insulated 12-gauge conductors (typically one black hot and one white neutral) plus a bare copper ground wire. The gauge number determines how much current the wire can safely carry.

Do I need conduit for home wiring?

It depends on the wire type. NM-B and UF-B are jacketed cables that do not require conduit inside finished walls. THHN and THWN-2 are individual conductors and must be pulled through conduit for physical protection and code compliance.