An electrical wire is a single solid or stranded metal conductor, while an electrical cable is a bundled assembly of two or more wires wrapped together inside a single outer protective jacket. While hardware store aisles and casual job-site chatter use the terms interchangeably, confusing them leads to failed inspections, improper routing, and burned-out equipment. This distinction changes your physical labor, your material costs, and the specific National Electrical Code (NEC) articles your inspector will use to grade your work. The most common mistake DIYers make is pointing at a yellow sheath of 12/2 NM-B and calling it '12-gauge wire' when it is, in fact, a cable, and subsequently sizing it based purely on breaker ampacity while ignoring voltage drop.
The Anatomy: Single Conductors vs. Bundled Assemblies
To choose the right material, you have to understand what you are physically pulling through your walls or conduit.
- Electrical Wire (Single Conductor): Typically THHN or THWN-2. It consists of a single copper (or aluminum) core with a thin, tough thermoplastic insulation. It has no outer jacket. Because it lacks bundled protection, the NEC requires single wires to be installed inside a raceway (like EMT metal conduit or PVC) once they leave an enclosure.
- Electrical Cable (Bundled Assembly): NM-B (commonly called Romex), UF-B, or MC (Metal Clad). These contain multiple insulated conductors (hot, neutral, and a bare or insulated ground) wrapped in a secondary outer sheath. This outer jacket allows the cable to be stapled directly to wooden framing and run through bored holes in studs without a conduit raceway.
Where You Meet This in Practice
The choice between pulling individual wires in conduit versus running a pre-assembled cable dictates your entire installation workflow and compliance path.
- Routing and Support: If you use NM-B cable, you must staple it within 8 inches of every electrical box and at intervals not exceeding 4.5 feet (NEC 334.30). If you use individual THHN wires, you cannot staple them to studs; you must first install a continuous conduit raceway and support the conduit itself.
- Conduit Fill vs. Cable Fill: When pulling individual wires, you must calculate conduit fill percentages (NEC Chapter 9, Table 1) to ensure you don't overheat the wires or jam them during the pull. You generally cannot push a full NM-B cable into a conduit for long distances; NM-B is only permitted inside conduit for short physical protection (like running down a wall to a panel).
- Wet Locations: Standard NM-B cable is strictly for dry, indoor locations. If you are routing power to an outdoor post or burying it to a detached garage, you must switch to UF-B cable (direct burial) or pull individual THWN-2 wires (the 'W' stands for water-resistant) inside a watertight conduit.
The Math: Ampacity vs. Voltage Drop
People commonly confuse a wire's ampacity (its ability to handle heat without melting the insulation) with its voltage drop (the loss of electrical pressure over distance). A wire might be perfectly safe from a fire perspective, but entirely inadequate for the equipment it powers.
Worked Numeric Example:
Let's size a 120V, 20-Amp branch circuit for a continuous load (16A actual draw) located 100 feet from the main panel. We will test 12 AWG copper.
- Formula: Voltage Drop (VD) = (2 × K × I × L) / CM
- K (Copper constant): 12.9 ohms per mil-foot
- I (Current): 16 Amps
- L (One-way length): 100 feet
- CM (Circular Mils for 12 AWG): 6,530
Calculation:
VD = (2 × 12.9 × 16 × 100) / 6,530
VD = 41,280 / 6,530 = 6.32 Volts
The Result: 6.32V is a 5.26% drop from the 120V source. The NEC recommends a maximum 3% drop for branch circuits. While the 12 AWG wire will not overheat (it is rated for 20A), the equipment at the end of the run will only see ~113.6V under load. To fix this, you must bump the wire size to 10 AWG (CM = 10,380), which drops the VD to 3.97V (3.3%), or 8 AWG to get under the strict 3% threshold.
Scenario Walkthrough: The Shed Compressor Failure
To see why this distinction matters, let's look at a real-world bench and job-site failure.
- The Setup: A homeowner runs 100 feet of standard 12/2 NM-B cable from the main house panel to a detached backyard shed to power a 1.5HP air compressor. The compressor nameplate lists a running current of 12 Amps.
- The Numbers: The homeowner correctly sizes the breaker (20A) and the wire (12 AWG, rated for 20A). Under normal running load, the voltage drop is about 4.5%. However, when an AC motor starts, it experiences 'Locked Rotor Amps' (LRA), which can be 5 to 7 times the running current. For a brief second, the compressor pulls 70 Amps.
- The Outcome: When the homeowner turns on the compressor, it emits a loud hum, fails to spin, and trips its internal thermal overload switch after 10 seconds. After repeated attempts, the motor's start winding burns out.
- What Went Wrong: The homeowner sized the cable purely for continuous ampacity (heat), completely ignoring the voltage drop during startup. At 70 Amps of inrush current, the 12 AWG copper experienced a massive momentary voltage drop, delivering less than 90V to the motor. The motor lacked the electromagnetic torque to overcome the piston's inertia at that low voltage, stalled, and converted the electrical energy directly into destructive heat.
The Fix: For long runs with high-inrush motors, you must calculate voltage drop based on the startup characteristics, or simply oversize the cable to 8 AWG or 6 AWG to ensure the motor receives adequate voltage during the critical first second of operation.
Quick-Reference: Choosing the Right Assembly
| Assembly Type | Best Application | Max Ampacity (Typical) | Routing & Code Rules |
|---|---|---|---|
| THHN/THWN-2 Wire | Conduit runs, panel wiring, commercial | Up to 75°C/90°C column limits | Must be inside a continuous raceway; calculate conduit fill. |
| NM-B Cable (Romex) | Indoor residential framing, dry locations | 60°C column limits (e.g., 14AWG=15A, 12AWG=20A) | Staple every 4.5 ft; cannot be used in wet locations or masonry. |
| UF-B Cable | Direct burial, outdoor posts, damp locations | 60°C column limits | Can be buried directly (24" depth) or run outdoors; UV resistant. |
| MC Cable (Metal Clad) | Commercial, exposed residential, EMI protection | 75°C column limits (depending on terminations) | Requires specific MC fittings; armor acts as ground if listed. |
Frequently Asked Questions
Can I strip the outer jacket off NM-B cable and pull the individual wires inside a conduit?
No. The individual conductors inside NM-B are not marked with the required THHN/THWN printing, and the paper filler inside the cable is not rated for the heat dissipation environment of a conduit. If you need wires in a conduit, you must buy individual THHN/THWN-2 conductors on spools.
Why is my 12 AWG cable limited to 20A when the copper itself can handle 30A?
The copper might handle the heat, but the PVC outer jacket of NM-B cable cannot dissipate heat as efficiently as a single wire in an open conduit. Furthermore, standard residential receptacles and breakers are rated for 60°C terminations. The NEC forces you to use the lowest temperature rating in the entire circuit chain to prevent the termination points from melting.
Is stranded wire better than solid wire for home electrical?
For standard home branch circuits (15A and 20A), solid wire (like standard 14/2 or 12/2 NM-B) is preferred because it is easier to strip, pushes into back-wire receptacles more reliably, and is cheaper. Stranded wire is superior for pulling through long conduit runs with multiple bends because it is more flexible and less likely to snap under pulling tension.
