Wires are single solid or stranded conductors that carry electrical current, while cables are two or more insulated wires bundled together inside a single protective outer jacket. Choosing the correct type dictates your ampacity limits, voltage drop over distance, and whether the run requires conduit or can be stapled directly to framing. Despite this clear distinction, DIYers routinely confuse a single "wire" (like a spool of THHN) with a "cable" (like NM-B), leading to dangerous mistakes like pulling three single THHN wires through a stud hole without conduit, which directly violates NEC Article 300 protection requirements.
The Core Differences: Single Conductors vs. Bundled Cables
At the workbench, the distinction comes down to insulation versus jacketing. A wire (such as THHN or XHHW) consists of a single copper or aluminum conductor wrapped in a thin layer of dielectric insulation. It is designed to be pulled through a raceway, conduit, or cable tray where the conduit itself provides the physical protection. A cable (such as NM-B, UF-B, or MC) takes multiple insulated wires, adds a bare equipment grounding conductor, and wraps the entire assembly in a thick outer jacket or armor. This outer layer allows the cable to be run through open framing, buried underground, or exposed to physical environments without needing a separate conduit.
The most critical trap for beginners involves temperature ratings. You will often hear that THHN wire is rated for 90°C. However, NEC Section 110.14(C) dictates that your ampacity is almost always limited by the terminations (the breakers and receptacles), which are typically rated for 60°C or 75°C. Therefore, even if you use 90°C THHN wire inside a cable assembly like NM-B, you must size your breaker using the 60°C column of NEC Table 310.16. Ignoring this rule is a primary cause of melted breaker lugs and residential fires.
Spec Sheet: Common Kinds of Wires and Cables
Use this reference table to select the right material for your rough-in. Ampacities listed assume copper conductors in a standard 30°C ambient environment.
| Type | Construction | Max Temp Rating (Ampacity Column) | Typical Use Case | Conduit Required? |
|---|---|---|---|---|
| NM-B (Romex) | 2+ insulated THHN wires, bare ground, PVC jacket | 60°C (for ampacity sizing) | Indoor dry branch circuits (15A/20A outlets, lighting) | No (stapled to framing) |
| THHN / THWN-2 | Single solid/stranded conductor, nylon/PVC insulation | 90°C (wire), 60/75°C (terminations) | Panel wiring, conduit pulls, 240V heavy appliances | Yes (must be in raceway) |
| UF-B | 2+ insulated wires, bare ground, solid gray PVC jacket | 60°C (for ampacity sizing) | Outdoor lighting, direct burial to detached sheds | No (direct burial rated) |
| MC (Metal Clad) | 2+ insulated THHN wires, green ground, interlocking metal armor | 90°C (wire), 75°C (terminations) | Commercial, exposed garage walls, hospitals | No (armor acts as raceway) |
Where You Meet This in Practice
On a standard residential jobsite, you will encounter these materials in distinct phases of the build. During the initial rough-in of interior walls, NM-B cable is king. It is fast to pull, easy to strip with a standard cable ripper, and requires no conduit bending. You will use 14/2 NM-B for 15A lighting circuits and 12/2 NM-B for 20A receptacle circuits.
When you move to the electrical panel or run a dedicated line for a 240V electric dryer or range, you transition to single THHN/THWN-2 wires. These are pulled through EMT (metal) or PVC conduit. The conduit provides physical protection and a pathway to pull the wires after the drywall is up. For a 50A range circuit, you will pull three 6 AWG THHN wires (two hots, one neutral) and a 10 AWG ground through a 1-inch conduit.
For exterior runs, such as wiring a landscape transformer or a detached garage subpanel, you meet UF-B cable. Unlike NM-B, the gray PVC jacket on UF-B is tightly extruded around the individual wires with no paper filler, making it impervious to moisture and rated for direct burial at a depth of 24 inches (or 12 inches if protected by GFCI and limited to 120V/20A, per NEC Table 300.5).
Worked Numeric Example: Sizing a Long-Run Workshop Circuit
Theory is useful, but voltage drop is where wire sizing gets real. Let us calculate the voltage drop for a 120V, 20A dedicated circuit running from your main panel to a workshop receptacle 150 feet away. You plan to run a continuous 16A load (like a dust collector or space heater) for more than three hours.
The Formula:
Voltage Drop (VD) = (2 × K × I × D) / CM
- K = 12.9 (resistivity constant for copper)
- I = 16A (the actual continuous load)
- D = 150 feet (one-way distance)
- CM = Circular Mils of the wire (12 AWG = 6,530 CM)
Calculation for 12 AWG NM-B:
VD = (2 × 12.9 × 16 × 150) / 6,530
VD = 61,920 / 6,530 = 9.48 Volts
A 9.48V drop on a 120V system is a 7.9% voltage drop. The NEC recommends a maximum of 3% for branch circuits to prevent motor overheating and dimming lights. Standard 12 AWG wire is dangerously undersized for this distance, even though it is perfectly rated for 20A on a short run.
The Fix:
Upgrade to 8 AWG NM-B cable (CM = 16,510).
VD = 61,920 / 16,510 = 3.75 Volts (a 3.12% drop).
This brings the circuit into compliance with NEC informational note recommendations, ensuring your workshop tools receive adequate voltage under load. Always consult OSHA and local electrical safety guidelines when sizing long feeder or branch runs.
Frequently Asked Questions
Can I mix solid and stranded wire in the same wire nut?
Yes, but only if the wire connector is specifically UL-listed for mixing solid and stranded conductors (most modern twist-on wire nuts are, but always check the manufacturer's spec sheet). When twisting them together, strip the stranded wire slightly longer than the solid wire so the stranded strands wrap around the solid core, preventing the stranded wire from slipping out of the nut under tension.
Why is my 12 AWG THHN wire only good for 20A when the 90°C column says 30A?
This is the termination temperature rule (NEC 110.14(C)). While the THHN insulation itself will not melt until 90°C, the brass and steel terminals inside your standard residential breakers and receptacles are only tested and rated to dissipate heat safely up to 60°C or 75°C. If you push 30A through a 12 AWG wire, the wire will survive, but the breaker terminal will overheat, potentially causing a fire inside the panel. Always size your overcurrent protection based on the lowest temperature rating in the entire circuit chain.
Is Romex (NM-B) allowed in exposed garage walls?
NEC Article 334.15 states that NM-B must be protected from "physical damage" where exposed. In a finished garage where the cable is run through bored holes in studs and plates, it is fine. However, if you are surface-mounting the cable directly across the face of the studs or on a block wall where it could be struck by a car door, tools, or shelving, the AHJ (Authority Having Jurisdiction) will likely fail the inspection. For exposed runs in garages or workshops, use MC (Metal Clad) cable or pull THHN wires through EMT conduit.
