Types of wires and cables refer to the specific combinations of conductor material, insulation type, and jacket configuration used to safely route electrical current from a source to a load. In one sentence, it is the physical medium that dictates how much current can flow, where it can be routed, and how it survives environmental stress. What this changes in a real installation is your allowable ampacity, voltage drop over distance, and whether the run passes inspection. People most commonly confuse 'wire' (a single conductor) with 'cable' (multiple conductors bound in a jacket), and conflate physical build (solid vs. stranded) with insulation type (THHN vs. NM-B).
Think of the conductor as a water pipe, the insulation as the pipe wall, and the outer jacket as a protective trench. Getting the combination wrong doesn't just trip breakers; it causes insulation meltdown, voltage drop that destroys appliance motors, and failed inspections. Here is exactly how to decode the options and pick the right one for your workbench or jobsite.
The Anatomy of Home Wiring: Insulation and Jacketing
Before you buy, you need to understand the four dominant cable types used in residential and light commercial work. The critical differentiator isn't just the copper gauge; it is the thermal rating of the insulation and the physical protection of the jacket.
- Non-Metallic Sheathed (NM-B / Romex): The standard for indoor, dry-location branch circuits. It contains bare copper conductors wrapped in PVC insulation and a paper filler, all bound in a PVC jacket. Thermal limit: 90°C conductor rating, but ampacity is strictly limited to the 60°C column by NEC 334.80.
- Thermoplastic High Heat-resistant Nylon-coated (THHN/THWN-2): Single-conductor wire used inside conduit. The nylon coating makes it slick for pulling and highly resistant to oil and moisture. Thermal limit: 90°C dry / 75°C wet. Ampacity is typically calculated using the 75°C column due to terminal temperature limits.
- Underground Feeder (UF-B): Designed for direct burial. The conductors are embedded directly in a solid gray PVC jacket with no paper filler. It is UV and moisture resistant but stiff to work with.
- Metal-Clad (MC): Conductors wrapped in a continuous corrugated aluminum or steel armor. Used where physical damage is a risk or in commercial exposed runs. Requires special anti-short bushings and connectors.
Worked Numeric Example: Sizing a 48A EV Charger Circuit
Let's look at a real-world scenario that trips up many DIYers: wiring a Level 2 EV charger. The charger draws a continuous 48A load. According to NEC Article 210.20, continuous loads require the branch circuit to be rated at 125% of the load.
The Requirement: We need wire with an allowable ampacity of at least 60A.
Here is where understanding the types of wires and cables saves you from a failed inspection or a melted terminal. We have two routing options for a 50-foot run from the panel to the garage:
Option A: NM-B (Romex) Cable
Even though the individual wires inside NM-B are rated for 90°C, NEC 334.80 mandates that we must use the 60°C column of Table 310.16 for ampacity. Looking at the 60°C column for copper: 6 AWG is only rated for 55A (too small). We must step up to 4 AWG NM-B, which is rated for 70A. This cable is thick, incredibly stiff, and costs roughly $2.80 per foot.
Option B: THHN in PVC Conduit
Because THHN terminations at the breaker and EV disconnect are typically rated for 75°C, we can use the 75°C column. In the 75°C column, 6 AWG THHN is rated for 65A. This meets our 60A requirement. The 6 AWG THHN wire costs about $0.90 per foot, plus $1.20 per foot for 3/4-inch Schedule 40 PVC conduit.
The Verdict: Option B (THHN in conduit) is cheaper, significantly easier to pull through the garage walls, and leaves room for future heat derating. If you chose 6 AWG NM-B because 'it's 6 gauge,' the inspector will fail you for being 5 amps short on the 60°C column.
Where You Meet This In Practice
You will encounter these specific cable types in distinct zones of a building. Matching the cable to the physical environment is just as important as matching the gauge to the breaker.
- Interior Walls and Ceilings (Dry): NM-B is the undisputed king here. It is fast to staple, easy to strip, and cheap. You will use 14/2 for 15A lighting circuits and 12/2 for 20A receptacle circuits.
- Panel Feeders and Subpanels: You will almost exclusively use individual THHN/THWN-2 wires pulled through large PVC or EMT conduit, or large-gauge SER (Service Entrance) cable for short, exposed runs.
- Outdoor and Underground Runs: UF-B is used for direct burial to detached sheds or landscape lighting transformers. It must be buried at least 24 inches deep (or 12 inches if protected by a 2-inch concrete pad) per NEC 300.5.
- Exposed Basement/Workshop Walls: Where NM-B is subject to physical damage (like a workshop wall where you might bang a workbench), code requires MC cable or running the wires inside EMT conduit.
Decision Tree: Picking the Right Cable for Your Run
Stop guessing at the hardware store. Use this decision path to terminate your choice in a concrete pick.
| Installation Environment | Circuit Type / Load | Required Cable Type | Concrete Default Pick |
|---|---|---|---|
| Indoor, dry, concealed behind drywall | Standard 15A/20A branch (lights, plugs) | NM-B (Romex) | Southwire 12/2 NM-B (20A) or 14/2 NM-B (15A) |
| Indoor, exposed to physical damage (workshop) | 20A receptacle circuit | MC Cable or THHN in EMT | Southwire 12/2 MC (Metal Clad) |
| Long runs, high ampacity (EV charger, subpanel) | 40A to 100A continuous | THHN/THWN-2 in PVC Conduit | Cerrowire THHN-2 (Size calculated via 75°C col) |
| Direct burial, no conduit | Outdoor lighting, detached shed feeder | UF-B | Cerrowire 12/2 or 10/2 UF-B (buried 24" deep) |
| Wet locations (inside conduit outdoors) | Outdoor receptacle, pool equipment | THWN-2 (must have the '-2' suffix) | Cerrowire 10 AWG THWN-2 (dual-rated) |
Critical Code Caveats and Derating
When selecting your cable, you must account for NEC adjustment factors. If you run more than three current-carrying conductors in a single conduit, or if your ambient temperature exceeds 86°F (30°C), you must derate the ampacity of THHN wires.
For example, if you pull four 12 AWG THHN wires through a conduit in an attic that reaches 110°F, you apply a 90°C ambient temperature correction factor of 0.87, and an adjustment factor of 0.80 for 4-6 conductors. 30A (base 90°C ampacity) × 0.87 × 0.80 = 20.8A. While this still barely covers a 20A breaker, it shows how quickly thermal environments eat into your safety margin. NM-B, however, cannot use the 90°C column for derating corrections; you must start from the 60°C baseline, making it a poor choice for hot attic runs with multiple cables bundled together.
Always verify local amendments. While the National Electrical Code (NFPA 70) provides the baseline, your local Authority Having Jurisdiction (AHJ) may ban NM-B entirely in certain municipalities (like Chicago, which mandates EMT conduit for almost all branch circuits).
FAQ: Wire and Cable Selection
Can I use THHN wire without conduit inside a wall?
No. Single-conductor THHN is not rated for physical protection on its own. Running it loose inside a wall cavity violates NEC 300.4 and leaves the thin nylon insulation vulnerable to drywall screws and rodent damage. It must be inside a raceway (conduit) or part of a manufactured cable assembly.
Why is the ground wire in NM-B bare, but THHN requires a green or yellow/green ground?
In NM-B, the bare copper serves as the equipment grounding conductor (EGC) because it is permanently protected by the outer PVC jacket. When pulling individual THHN wires in conduit, the code requires the EGC to be identifiable, meaning it must be green, green with a yellow stripe, or bare (though bare is rarely used in conduit as it can snag and is hard to identify). Refer to the Cerrowire ampacity and identification charts for exact color coding standards.
Is it okay to mix solid and stranded wire on the same terminal?
Avoid it if possible. If you must pigtail a stranded THHN wire to a solid NM-B wire under a single wire nut, use a high-quality winged connector (like an Ideal Wire-Nut) and ensure the stranded wire is wrapped slightly tighter and sits flush. Never put both solid and stranded under a single standard receptacle screw terminal; the solid wire will crush the stranded wire, leading to a loose connection and a potential arc fault.
Final Recommendation: If you are wiring a standard indoor 120V/240V branch circuit in a dry, concealed residential wall, your default pick is 12/2 NM-B for 20A circuits and 14/2 NM-B for 15A circuits. Do not overcomplicate it with THHN and conduit unless your run exceeds 40 amps, requires physical armor, or must survive a wet environment.
