Electrical Wire Types Guide: Choosing the Right Wiring Method

Electrical wire is selected by more than gauge. The insulation system, cable construction, conductor material, environment, wiring method, and equipment terminations determine where a product can be used. A 12 AWG conductor pulled through conduit and a 12 AWG conductor inside NM-B cable share a gauge, but they are not interchangeable wiring methods.

Lumera Electric’s electrical wire and cable collection includes building wire and cable for residential, commercial, and industrial work. The guide below explains the families you are most likely to encounter and the decision behind each one.

Wire vs. Cable: Start With the Terminology

A wire is generally a single conductor, bare or insulated. A cable is an assembly of two or more conductors or insulated components under a common jacket or armor. In everyday jobsite language, the terms overlap, but the distinction matters because the product listing and installation rules apply to the complete wiring method.

THHN and THWN-2 Building Wire

THHN/THWN-2 is one of the most common individual-conductor choices for conduit. Southwire’s current THHN/THWN-2 copper product is rated 600 V and lists THHN for dry locations up to 90°C and THWN-2 for wet or dry locations up to 90°C, subject to the conditions of use and termination limitations.

Because the conductor is installed as part of a raceway system, the conduit type, fill, bending, pulling conditions, conductor count, ambient temperature, and terminal ratings all matter. THHN/THWN-2 is not a substitute for a cable assembly where the wiring method requires one.

NM-B Cable

NM-B is the familiar nonmetallic-sheathed cable used heavily in residential interior wiring. It packages insulated current-carrying conductors and an equipment grounding conductor under one outer jacket, which makes routing through wood framing fast and practical.

NM-B is a dry-location wiring method. Southwire specifies its NM-B cable for exposed or concealed work in dry locations and limits ampacity to the 60°C conductor column even though the individual insulated conductors inside are rated 90°C. For a deeper explanation, see Lumera’s What Is Romex (NM-B) Cable?.

UF-B Cable

UF-B is designed for environments that NM-B cannot handle. Its construction is moisture resistant, and listed UF-B products may be used underground, including direct burial, when installed within their listing and the adopted code. It is commonly used for outdoor feeders, post lights, pumps, and other exterior loads.

UF-B is tougher to strip and work with than NM-B because the conductors are embedded in the jacket, but that construction is part of what gives the cable its wet-location capability. Do not assume that any gray cable is automatically direct-burial rated; read the printed designation.

MC Cable

Metal-clad cable contains insulated conductors inside interlocked metal armor. Current Southwire MC products are listed for branch, feeder, and service power distribution across commercial, industrial, institutional, and multi-residential buildings, with specific product constructions determining whether the cable is suitable for dry, wet, hazardous, or other specialized locations.

MC cable can reduce raceway installation time while still providing mechanical protection, but it requires listed fittings and correct grounding/bonding provisions. The armor construction and grounding method are product-specific, so installers should follow the cable listing instead of assuming all armored cable behaves the same way.

XHHW-2 Conductors

XHHW-2 is a thermoset-insulated conductor commonly used for feeders and services in conduit or other recognized raceways. Copper and aluminum versions are available. Its insulation is well suited to wet and dry locations when the product is listed for that use, and larger feeder sizes often make aluminum XHHW-2 attractive where weight and material cost matter.

Copper vs. Aluminum Conductors

Copper and aluminum are both legitimate conductor materials when the system is designed for them, but they do not have identical ampacity at the same AWG size. Aluminum conductors also require terminals and connectors identified for aluminum or dual-rated use. Never substitute metal type without rechecking conductor size, terminal rating, equipment listing, and installation requirements.

Solid vs. Stranded Wire

Solid conductors hold their shape and are common in smaller residential branch-circuit cable. Stranded conductors are more flexible and easier to pull through conduit, especially as conductor size increases or the raceway has several bends. Large building-wire sizes are commonly stranded because a solid conductor would be difficult to handle.

How to Read Wire and Cable Markings

The printing on the insulation or jacket is more reliable than appearance. Look for the conductor size, copper or aluminum designation, insulation or cable type, voltage rating, wet/dry or sunlight-resistant markings, listing marks, and the number of conductors in a cable assembly.

• AWG size tells you conductor size, not allowable current by itself.
• THHN, THWN-2, XHHW-2, NM-B, UF-B, and MC identify different insulation or wiring-method families.
• A “W” or wet-location listing matters anywhere the raceway or cable can be exposed to moisture.
• Jacket color can be a useful manufacturer convention, but the printed legend is the controlling identification.

How to Choose the Right Electrical Wire Type

1. Identify the environment: dry, damp, wet, underground, outdoors, exposed, or protected inside a building.
2. Identify the wiring method: individual conductors in raceway, nonmetallic cable, armored cable, service cable, or another listed system.
3. Calculate the load and choose conductor ampacity under the adopted electrical code.
4. Check the equipment terminal temperature rating and conductor material rating.
5. Apply correction or adjustment factors for ambient temperature and current-carrying conductor count where required.
6. Check voltage drop on long runs and upsize when design performance requires it.
7. Use connectors, fittings, and boxes listed for the wiring method and conductor material.

If the primary question is gauge rather than wiring method, Lumera’s Wire Gauge and Ampacity Guide covers conductor sizing, while 12 vs 14 Gauge Wire focuses on the two most common residential branch-circuit sizes.

Why Wire Type Matters Beyond Ampacity

NFPA fire data cited in Lumera’s wire-gauge guide estimated an average of 32,620 U.S. home fires per year involving electrical distribution and lighting equipment during 2015-2019, with wire or cable insulation identified as the first item ignited in 32% of those fires. That statistic covers many failure modes, not just wrong wire selection, but it reinforces why conductor type, insulation condition, termination quality, and overcurrent protection all matter together.

Common Wire Selection Mistakes

• Using NM-B in a wet or direct-burial application.
• Treating THHN as a complete wiring method without the required raceway.
• Choosing gauge from a breaker number without checking cable type and termination temperature.
• Assuming every armored cable is suitable for the same locations.
• Mixing copper and aluminum conductors with terminals not listed for both metals.
• Relying on jacket color instead of reading the printed product marking.
• Ignoring voltage drop on long runs.