Electrical wires and cables types refer to the specific classifications of individual conductors and multi-conductor assemblies, defined by their insulation material, jacket rating, and intended installation environment. While a "wire" is technically a single conductor and a "cable" is two or more conductors bundled under a single outer jacket, the National Electrical Code (NEC) treats them as distinct entities with entirely different ampacity tables and routing rules. Choosing the wrong type doesn't just change the physical flexibility of your run; it fundamentally changes the thermal limits of the circuit, the allowable conduit fill, and whether an inspector will legally sign off on your installation.
The Core Definition: Wire vs. Cable and Insulation Limits
The most common mistake DIYers and junior apprentices make is confusing the conductor material (copper vs. aluminum) with the cable type (NM-B vs. THHN). The metal inside dictates the base resistance, but the insulation type dictates the maximum operating temperature before the jacket melts, off-gasses, or causes a short circuit.
What this changes in a real installation is your allowable ampacity. A 10 AWG copper wire has the exact same physical thickness whether it is bare, wrapped in THHN plastic, or buried inside a thick NM-B sheath. However, the NEC assigns different current-carrying capacities to that same 10 AWG wire based on how well its specific insulation dissipates heat. NEC Article 310 governs general conductors like THHN, while NEC Article 334 strictly governs Nonmetallic-Sheathed Cable (NM-B).
What people commonly confuse: Many builders see that THHN insulation is rated for 90°C and assume they can use the 90°C ampacity column for their entire circuit. In reality, the terminations (breakers, lugs, and receptacles) are almost universally rated for 75°C or 60°C. You must use the lowest temperature rating in the entire circuit chain to determine your final wire size.
The Big Three Residential Electrical Wires and Cables Types
If you are wiring a home, shop, or outbuilding in 2026, 95% of your material list will consist of three specific types. Here is how they stack up regarding construction, temperature limits, and current market pricing.
| Type | Construction | Temp Rating (Insulation) | Max Ampacity (12 AWG Cu) | Best Use Case | Approx Cost (100ft) |
|---|---|---|---|---|---|
| NM-B (Romex) | 2+ insulated wires, bare ground, PVC jacket | 90°C (but limited to 60°C ampacity) | 20A (60°C column limit) | Interior dry-wall framing, outlets, lighting | $42 - $48 |
| THHN / THWN-2 | Single conductor, nylon outer skin | 90°C dry / 75°C wet | 25A (75°C col) / 20A (60°C col) | Conduit runs, subpanel feeders, commercial | $16 - $22 |
| UF-B | 2+ insulated wires, bare ground, solid gray PVC | 60°C (strictly limited) | 20A (60°C column limit) | Direct burial, outdoor lighting, sheds | $60 - $75 |
Note: Pricing reflects early 2026 copper and PVC market averages for Southwire and Cerro brands. Always check local big-box or electrical supply house rates.
Where You Meet This in Practice: Sizing and Derating
Let's look at a worked numeric example that trips up even experienced hobbyists when pulling wire through conduit. Suppose you are wiring a 20A receptacle circuit in a garage and pulling individual wires through 1/2-inch EMT conduit.
- The Base Wire: You choose 12 AWG THHN. Looking at NEC Table 310.16, the 90°C column lists 12 AWG copper at 30A.
- The Termination Limit: Your standard 20A breaker and duplex receptacles are rated for a maximum of 60°C (per NEC 240.4(D) for small conductors). Therefore, your baseline usable ampacity is capped at 20A (the 60°C column value for 12 AWG).
- The Derating Factor: You decide to pull six current-carrying conductors (two 120V circuits and two 240V circuits) through the same conduit. NEC Chapter 9, Table 310.15(C)(1) requires an 80% derating factor for 4-6 conductors.
- The Math: Derating is applied to the 90°C column, not the termination-limited column. So, 30A (from 90°C) × 0.80 = 24A.
- The Verdict: Since 24A is greater than your required 20A load, the 12 AWG THHN run is perfectly legal and safe, even though the final termination is limited to 20A.
If you had attempted this same pull using 12 AWG NM-B cable, it would be illegal. NM-B cannot be pulled inside conduit as a sleeved cable assembly for long distances due to heat dissipation issues, and you cannot strip the outer jacket to use the individual THHN wires inside it, as the internal wires lack the nylon outer skin required for conduit pulling.
Real-World Scenario Walkthrough: The Melted Subpanel Lug
To understand why electrical wires and cables types matter beyond simple code compliance, let's walk through a real-world failure scenario involving a detached garage subpanel.
The Setup: A homeowner decides to install a 48A continuous-load Level 2 EV charger in their detached garage. They need to feed a 60A subpanel. To save money and avoid digging a trench for conduit, they bore through the framing and run 6 AWG NM-B cable from the main house panel to the garage subpanel, terminating at 75°C rated lugs on both ends.
The Numbers: The homeowner looks at the 75°C column in the NEC ampacity table and sees that 6 AWG copper is rated for 65A. They install a 60A breaker, assuming they have a 5A safety buffer. However, NEC 334.80 explicitly states that the ampacity of NM-B cable shall not exceed that for 60°C rated conductors. In the 60°C column, 6 AWG copper is only rated for 55A. Furthermore, because the EV charger is a continuous load (running for 3+ hours), NEC 210.20(A) requires the circuit to be sized at 125% of the load: 48A × 1.25 = 60A minimum required ampacity.
The Outcome: During the first long charging session, the 6 AWG NM-B cable is forced to carry 48A continuously. The wire operates above its 55A (60°C column) safety limit. The thick PVC jacket of the NM-B traps the heat. After three hours, the insulation softens, and the heat travels down the copper conductor into the subpanel lug.
What Went Wrong: The homeowner confused the 75°C termination rating of the lug with the strict 60°C ampacity limitation of the NM-B cable type. The lug overheated, melting the NM-B jacket at the termination point and creating a severe fire hazard. The Fix: They should have used 4 AWG NM-B (rated 70A in the 60°C column) or, much better, pulled three strands of 6 AWG THHN inside a PVC conduit, which legally allows the use of the 75°C column (65A) and provides superior heat dissipation.
Safety Caveat: Always de-energize the main breaker, lock out the panel, and verify zero voltage with a tested multimeter before inspecting or replacing melted lugs. If your main service panel shows signs of thermal damage, defer immediately to a licensed electrician; do not attempt to re-terminate main bus lugs yourself.
FAQ: Common Selection and Routing Questions
Can I strip the gray jacket off UF-B cable and use the inner wires indoors like NM-B?
No. While the inner conductors of UF-B look similar to THHN, NEC Article 339 does not permit stripping UF-B to use as individual conductors. Furthermore, UF-B lacks the nylon outer skin that protects THHN from abrasion when pulled through conduit or stapled to studs. Use it only as a complete, intact cable assembly.
Why is my indoor wire labeled THHN/THWN-2? What does the "W" mean?
The "W" stands for water-resistant. Modern manufacturing processes allow the same PVC/nylon insulation to meet both the 90°C dry rating (THHN) and the 75°C wet rating (THWN-2). This dual-rating is standard in 2026 and means you can safely use that single wire type in both dry interior conduit and wet exterior conduit without changing spools.
Does the bare ground wire count toward conduit fill and derating?
For physical conduit fill (Chapter 9, Table 1), yes—the ground wire takes up physical space and must be counted. However, for ampacity derating (Table 310.15(C)(1)), the equipment grounding conductor does not" count as a current-carrying conductor, because under normal operating conditions, it carries zero current. Only the hot and neutral (on 120V circuits) or hots (on 240V circuits) count for derating.
Is aluminum wire still used in residential wiring?
Yes, but almost exclusively for large feeders (like 2/0 or 4/0 for 200A service entrances) where copper pricing becomes prohibitive. For branch circuits (15A to 60A), copper remains the undisputed standard due to its superior termination reliability and resistance to thermal creep at standard breaker lugs.
