Understanding Electrical Wire Types: NM-B, THHN, and UF-B Explained

Electrical wire types are categorized by their conductor material, insulation chemistry, and outer jacketing, which together dictate their maximum safe current capacity (ampacity) and approved installation environments. Choosing the correct type for your project is not just about physical fit; it fundamentally changes the thermal limits of your circuit, dictates your voltage drop calculations, and determines whether your installation will pass a municipal inspection or fail due to code violations. For instance, NM-B cable is strictly limited to the 60°C ampacity column in the NEC, while individual THHN conductors allow you to utilize the 90°C column for derating calculations.

The Core Differences: Insulation, Jacketing, and Construction

When you walk down the electrical aisle, you are primarily looking at three distinct categories of wire and cable used in residential and light commercial work. The distinction between them comes down to how the copper is protected from heat, moisture, and physical damage.

NM-B (often referred to by the brand name Romex) is the standard for interior wiring. It is easy to strip and staple to studs. THHN (Thermoplastic High Heat-resistant Nylon-coated) is a single-conductor wire designed to be pulled through conduit. The nylon coating makes it slick, reducing friction during pulls. UF-B looks like NM-B but features a solid, moisture-resistant plastic jacket that encases the wires completely, allowing it to be buried directly in the dirt without a conduit sweep.

Worked Example: How Wire Type Dictates Ampacity and Derating

To understand why wire type matters beyond just 'indoor vs. outdoor', let us look at a real-world ampacity derating scenario. Suppose you are running a 240V circuit to a garage subpanel and need to pull six current-carrying conductors (CCCs) through a single 1-inch PVC conduit. You need a final safe ampacity of at least 30A.

Scenario A: Using 10 AWG NM-B
If you somehow tried to use 10 AWG NM-B (which you should not pull through conduit, but let us look at the math), the NEC restricts NM-B ampacity to the 60°C column regardless of its 90°C insulation rating. Looking at NEC Table 310.16, 10 AWG in the 60°C column is rated for exactly 30A. However, because you have six CCCs in a single raceway, NEC Table 310.15(C)(1) requires an 80% derating factor. 30A × 0.80 = 24A. Your wire is now undersized for a 30A breaker.

Scenario B: Using 10 AWG THHN
When using individual 10 AWG THHN conductors in conduit, you are allowed to use the 90°C column for your starting derating calculations. In the 90°C column, 10 AWG is rated for 40A. Applying the same 80% derating factor for six CCCs: 40A × 0.80 = 32A. Because 32A is greater than your 30A requirement, 10 AWG THHN passes the calculation and is perfectly safe to use on a 30A breaker.

This mathematical reality is why electricians use THHN in conduit for complex pulls and subpanel feeders, reserving NM-B strictly for simple, single-circuit runs inside walls where conduit fill and heat buildup are not factors.

Where You Meet These Electrical Wire Types in Practice

Knowing the theory is useful, but recognizing where each type belongs on the jobsite prevents costly rework and safety hazards.

• NM-B: You will use this for 90% of interior residential rough-ins. It runs from the breaker panel through bored holes in wooden studs to feed 15A lighting circuits (14/2 NM-B) and 20A receptacle circuits (12/2 NM-B). It must remain protected inside walls, ceilings, or floors.
• THHN/THWN-2: You meet this when surface-mounting EMT conduit in a basement, running a feeder to a detached garage, or wiring a commercial drop-ceiling. It is always sold on spools as individual conductors, meaning you must buy separate spools for black (hot), white (neutral), and green (ground).
• UF-B: You will use this when trenching a line to a backyard shed, wiring an outdoor post light, or connecting a well pump. It must be buried at the proper depth (typically 24 inches for direct burial residential branch circuits, per OSHA and NEC guidelines) and protected by conduit where it emerges above ground.

Common Confusions: Wire vs. Cable and Solid vs. Stranded

When discussing electrical wire types, people frequently confuse the physical construction of the copper with the insulation type, or they misuse the terms 'wire' and 'cable'.

Wire vs. Cable: In electrical terminology, a wire is a single conductor (like a spool of bare copper or a single THHN strand). A cable is a group of two or more wires bundled together inside an outer sheath or jacket (like NM-B or UF-B). When you buy '12/2 Romex', you are buying a cable containing two 12 AWG insulated wires and one bare ground wire.

Solid vs. Stranded: This refers to the copper core, not the insulation type. Solid wire is a single, stiff piece of copper, preferred for pushing through holes in studs and terminating on standard residential receptacle screws. Stranded wire is made of many thin copper threads, making it highly flexible. You will almost always find stranded copper in THHN pulled through conduit with tight bends, or in automotive and low-voltage DC applications. Both can carry the same current if the total cross-sectional AWG area is identical, but stranded wire requires careful termination (often with crimped ferrules or lugs) to avoid stray strands causing short circuits.