Understanding Cables and Wiring: Differences, Sizing, and Installation Rules

A wire is a single solid or stranded metal conductor, while a cable is a bundled assembly of two or more insulated wires enclosed in an outer protective sheath. This physical distinction dictates your installation method, physical protection requirements, and thermal derating limits in a real circuit. Most DIYers confuse the two by using the word "wire" to describe a multi-conductor cable like yellow 12/2 Romex, which leads to critical errors such as miscalculating conduit fill or applying the wrong National Electrical Code (NEC) article during an inspection.

The Physics and Code Behind Cables vs. Individual Wiring

When you buy a spool of 12 AWG THHN, you are buying individual wires. When you buy a 250-foot roll of 12/2 NM-B, you are buying a cable that contains two 12 AWG current-carrying wires (hot and neutral) and one bare copper ground wire, all wrapped in a PVC jacket. The NEC treats these two formats entirely differently because of heat dissipation.

Individual wires installed inside a metal or PVC conduit (governed by NEC Article 300 and 310) rely on the air space inside the raceway to shed heat. Because you can pull individual wires through long, complex conduit runs with sweeping bends, they are the standard for exposed commercial work, underground feeds, and finished basement ceilings where physical protection is required.

Cables like NM-B (Non-Metallic Sheathed Cable, governed by NEC Article 334) are designed to be stapled directly to wooden studs behind drywall. The outer jacket provides the physical protection that the drywall and framing will eventually complete. However, because the wires inside a cable are tightly bound together with no air gap, they trap heat more readily than wires spaced out in a conduit.

This thermal reality forces a strict ampacity limitation. NM-B cable is strictly limited to the 60°C ampacity column in NEC Table 310.16, regardless of the fact that the individual THHN wires inside it might be rated for 90°C. This means a 12 AWG NM-B cable is permanently capped at 20 amps. If you were to pull three individual 12 AWG THHN wires into a conduit, you could technically use the 90°C column for derating calculations, though the final overcurrent protection device (breaker) is still typically limited to the 60°C/75°C terminal ratings of your standard receptacles.

Worked Numeric Example: 20A Kitchen Circuit Cost and Labor

To understand how the choice between cables and wiring impacts your project budget and timeline, let us look at a real-world scenario. You need to run a new 20-amp small-appliance branch circuit for a kitchen countertop. The run is 50 feet from the main panel to the first receptacle box, passing through an unfinished utility room before entering the finished kitchen wall.

Here is the material and labor breakdown comparing 12/2 NM-B cable versus individual 12 AWG THHN wires pulled through 1/2-inch EMT (Electrical Metallic Tubing) conduit. Pricing reflects average 2026 retail costs from major suppliers like Southwire and big-box hardware stores.

The Verdict: For the finished kitchen wall cavity, Option A (NM-B cable) is the undisputed winner. It is cheaper, faster, and perfectly code-compliant when protected by drywall. However, for the 15-foot section crossing the exposed utility room ceiling, local code will likely prohibit exposed NM-B due to the risk of physical damage. In that exposed section, you must transition to Option B (THHN in EMT) or use armored cable (MC/AC).

Where You Meet This In Practice

Understanding the boundary between cables and wiring prevents the three most common jobsite mistakes:

• The Conduit Stuffing Error: A DIYer tries to push 50 feet of 12/2 NM-B cable through a 1-inch PVC conduit to protect it under a concrete slab. This violates NEC conduit fill rules because the flat, oval shape of the cable takes up vastly more cross-sectional area than round individual wires, and the friction will tear the cable jacket. Fix: Always pull individual THHN/THWN-2 wires through long conduit runs.
• The Panel Dressing Mess: When terminating circuits in a subpanel, leaving the outer cable jacket extending an inch past the cable clamp and into the panel box. The jacket takes up valuable space and makes it impossible to route the individual wires neatly to the bus bars. Fix: Strip the cable jacket exactly flush with the inside edge of the panel knockout or cable clamp.
• The Derating Blindspot: Bundling four separate 12/2 NM-B cables tightly together through a single bored hole in a top plate, then insulating over them with spray foam. Because there are more than three current-carrying conductors bundled together without air circulation, NEC 310.15(C)(1) requires you to derate their ampacity to 80%. A 20A circuit on 12 AWG suddenly becomes a 16A circuit, creating a fire hazard if the breaker isn't downsized. Fix: Space cables apart in framing holes or upgrade to 10 AWG if tight bundling is unavoidable.