A wire is a single electrical conductor (bare or insulated), while a cable is a group of two or more wires bound together inside a single outer sheath or jacket. While this sounds like basic vocabulary, confusing the two on a jobsite or in your garage dictates whether your installation passes inspection, how much heat your conductors can safely dissipate, and whether you will physically be able to pull your run through a raceway. In a real circuit, this distinction changes your conduit fill calculations, your ampacity derating requirements, and the physical protection methods you must employ. The most common confusion? Hobbyists and DIYers routinely call a 12/2 NM-B (Romex) assembly a "wire," or falsely assume that because a cable has an outer jacket, it counts as a single, tiny entity for conduit fill purposes.
The Core Difference: Single Conductors vs. Assembled Cables
When you buy a spool of THHN or THWN-2, you are buying individual wires. Each conductor has its own thin, tough PVC and nylon insulation. When you buy a roll of NM-B (Non-Metallic Sheathed) or UF-B (Underground Feeder), you are buying a cable. The individual conductors inside are bundled together, wrapped in paper or a separator, and extruded with a thick outer PVC jacket.
This physical difference drives entirely different installation rules under the NFPA 70 (National Electrical Code). Individual wires are flexible, dissipate heat well when spaced out, and are designed to be pulled through raceways. Cables are rigid, trap heat internally, and are designed to be stapled to framing or buried directly in the earth (in the case of UF-B).
Never strip the outer jacket off an NM-B cable to use the individual inner wires inside a conduit. While the inner hot and neutral wires are typically THHN, the bare ground wire lacks insulation for a wet conduit environment, and the white neutral jacket is not rated to be used as a re-identified hot wire in a raceway. Always buy dedicated THWN-2 wires for conduit pulls.
Where You Meet This in Practice
You will encounter the wire vs. cable divide at two specific choke points in any residential or workshop build:
- Inside the Panel Enclosure: You must use individual wires (like THHN) here. Cables are far too rigid to route neatly through knockouts and into tight lug terminals. Modern smart panels and high-density neutral bars make the flexibility of individual wires mandatory for clean, code-compliant dressing.
- Branch Circuits in Walls: You will use NM-B cables. Pulling three individual wires through the bored holes of 15 wall studs is a nightmare of friction and time. The cable's jacket acts as a pre-assembled sleeve, allowing you to fish the entire circuit through framing in one pull.
The Conduit Fill and Derating Trap
Think of a conduit like a two-lane tunnel. If you send three wide flatbed trucks (cables) through instead of six motorcycles (individual wires), the tunnel is physically blocked, and the heat from the engines has nowhere to dissipate. The NEC strictly limits how much of a conduit's internal cross-sectional area can be filled to prevent jamming and overheating.
Let's look at a worked numeric example using standard 1/2-inch EMT (Electrical Metallic Tubing).
- Total Internal Area: According to NEC Chapter 9, Table 4, 1/2" EMT has an internal area of 0.304 square inches.
- The 40% Rule: For three or more conductors, the NEC limits fill to 40%. That leaves us with a maximum usable area of 0.121 square inches.
- Using Individual Wires: A single #12 AWG THHN wire has an area of 0.0133 sq in. You can safely fit nine #12 THHN wires into this conduit.
- Using a Cable: A 12/2 NM-B cable is roughly an ellipse measuring 0.20" by 0.45". Its cross-sectional area is roughly 0.070 sq in. Mathematically, you can only fit one 12/2 NM-B cable before hitting the 40% limit. Practically, pulling even one flat NM-B cable through 1/2" EMT over any distance will result in a jammed pull and torn jacket.
Real-World Scenario: The Underground NM-B Disaster
To understand what happens when you ignore the physical realities of cables in raceways, let's walk through a classic DIY failure.
The Setup: A homeowner wants to run a 240V circuit to a detached shed 60 feet away. They trench 18 inches down and lay 1-inch Schedule 40 PVC conduit. To "save time and money," they decide to pull a 10/2 UF-B (Underground Feeder) cable through the conduit instead of buying three individual #10 THWN-2 wires.
The Numbers: 10/2 UF-B is approximately 0.65 inches wide and 0.30 inches thick. The 1-inch PVC has an internal diameter of 1.029 inches. While the cable technically fits inside the pipe with room to spare on paper, the friction coefficient of flat UF-B against curved PVC is massive.
The Outcome: The cable jams 25 feet into the run. The builder applies aggressive wire lube and uses brute physical force with a fish tape to yank it the rest of the way. They terminate the shed subpanel, turn on the breaker, and everything works. Three months later, after a heavy spring rainstorm, the GFCI breaker in the main panel begins tripping immediately.
What Went Wrong: The brute-force pull micro-tore the outer UF-B jacket where it scraped against a slight misalignment in the PVC coupling. UF-B is rated for direct burial in dirt, but when pulled through a conduit that inevitably collects condensation and rainwater, that torn jacket allowed water to wick directly into the bare copper ground wire. The wet ground wire created a parallel leakage path, tripping the GFCI. Furthermore, if they had attempted to pull two UF-B cables through that 1-inch pipe, the heat trapped between the flat jackets would have required severe ampacity derating under NEC 310.15(C)(1), potentially melting the insulation under a continuous 20A load.
Step-by-Step: Calculating Your Raceway Fill
When planning a conduit run, follow this sequence to ensure your wires or cables will actually pull and pass inspection. The EC&M National Electrical Code section is a great resource for deep dives into these tables.
- Identify the Raceway Type and Size: Determine if you are using EMT, PVC Schedule 40, or flexible metal conduit, and note the trade size (e.g., 3/4").
- Find the Total Area: Look up NEC Chapter 9, Table 4 for your specific raceway to find the 100% internal area.
- Apply the Fill Percentage: Use 53% for 1 wire, 31% for 2 wires, and 40% for 3 or more wires.
- Calculate Conductor Area: Look up NEC Chapter 9, Table 5 for the exact square-inch area of your specific wire type and gauge (THHN and THWN have different thicknesses).
- Divide and Verify: Divide your usable raceway area by the single wire area. Round down to the nearest whole number. That is your maximum wire count.
Frequently Asked Questions
Does a multi-conductor cable count as one wire or multiple wires for derating?
This is where the NEC gets highly specific. For conduit fill percentage (physical space), a cable counts as a single conductor based on its major diameter. However, for ampacity derating (heat dissipation when bundling), you must count the individual current-carrying conductors inside the cable. If you pull two 12/2 NM-B cables through a single bored hole in a stud, you have four current-carrying conductors, which triggers an 80% derating factor on your 20A ampacity.
Can I use individual THHN wires for direct burial without a conduit?
No. Individual THHN/THWN-2 wires are only rated for use inside a raceway or cable assembly. If you are burying power without a conduit, you must use a cable specifically rated for direct burial, such as UF-B or USE-2. The Southwire Building Wire Catalog clearly delineates which jackets are approved for direct earth contact.
Why do electricians prefer individual wires in conduit over pulling Romex?
Friction and heat. Individual THHN wires are round, slick, and slide past each other easily. They also have air gaps between them inside the conduit, allowing heat to dissipate. Romex (NM-B) is flat, sticky, and traps heat against itself. Pulling NM-B through conduit is reserved only for short, straight "sleeve" runs (like dropping down a wall to a panel), never for long, multi-bend underground or overhead runs.
