A wire is a single electrical conductor (bare or insulated), while a cable is an assembly of two or more insulated wires bound together inside a unified outer jacket. If you are pulling individual THHN conductors through EMT conduit, you are working with wires; if you are stapling yellow Romex through wall studs, you are working with cables. While hardware stores and casual DIYers use the terms interchangeably, the National Electrical Code (NEC) treats them as fundamentally different installation methods with distinct routing, support, and ampacity rules.
Common confusion: People routinely point to a 14/2 NM-B assembly and call it '14-gauge wire.' In code-speak, that is a cable containing two insulated wires and a bare ground. Calling it a wire on a permit application or materials list will get you corrected by an inspector or a seasoned journeyman.
The Anatomy of Cables and Wires in Home Wiring
To size and route correctly, you need to know exactly what you are holding in your hand. Single wires rely on external protection (like conduit or a cable tray) to prevent physical damage, whereas cables have built-in physical protection via their outer jacket or armor.
| Feature | Single Wire (e.g., THHN/THWN-2) | Nonmetallic Cable (e.g., NM-B / Romex) | Metal-Clad Cable (e.g., MC) |
|---|---|---|---|
| Construction | Single copper/aluminum strand with thin nylon/PVC insulation | 2+ insulated wires + bare ground wrapped in paper and a PVC jacket | Insulated wires wrapped in a continuous interlocking metal armor |
| Routing Method | Must be inside a raceway (EMT, PVC, flexible conduit) | Run directly through framing cavities, bored holes, or exposed joists | Run directly through framing or surface-mounted; no separate conduit needed |
| Temperature Column | Use 90°C column for derating, 75°C for termination limits | Strictly limited to 60°C column for final ampacity (NEC 334.80) | Depends on internal wire rating, typically 90°C for derating |
| Best Application | Long runs in conduit, panel feeders, commercial/industrial wiring | Standard residential branch circuits (outlets, lighting, switches) | Commercial exposed runs, residential areas requiring physical protection |
According to the Cerrowire ampacity charts, the base current-carrying capacity of the copper inside is identical across these types for a given AWG size. The difference lies entirely in how the installation method restricts heat dissipation.
Worked Numeric Example: Ampacity and Bundling Derating
The most critical practical difference between pulling individual wires in conduit and running cables through a wall is bundling derating. When you pack multiple current-carrying conductors tightly together, they cannot shed heat effectively. The NEC requires you to reduce (derate) their allowable ampacity.
The Scenario: You are wiring a subpanel and need to pull nine current-carrying 10 AWG conductors through a single piece of 1-inch EMT conduit. You want to protect them with a standard breaker.
Step 1: Find Base Ampacity
Looking at NEC Table 310.16, a 10 AWG THHN copper wire in the 90°C column has a base ampacity of 40A.
Step 2: Apply the Derating Factor
NEC Table 310.15(C)(1) states that for 7 to 9 current-carrying conductors in a single raceway, you must apply a 70% adjustment factor.
Calculation: 40A × 0.70 = 28A.
Step 3: Select the Overcurrent Device
Your derated ampacity is 28A. Per NEC 240.4(B), you can round up to the next standard breaker size, but 30A is the next standard size up, which exceeds 28A. Therefore, you must step down to the next standard size below your calculated value: a 25A breaker.
Contrast with NM-B Cable:
If you were instead running a 10/3 NM-B cable (which contains 3 current-carrying conductors) through standard wall studs, bundling derating does not apply in the same way because the studs naturally space the cables apart. However, NM-B is strictly limited to the 60°C column per NEC 334.80. A 10 AWG conductor in the 60°C column is rated for exactly 30A. Therefore, a 10/3 NM-B cable gets a 30A breaker, while nine 10 AWG THHN wires in a conduit get capped at 25A. As noted in this EC&M guide on conductor derating, ignoring these temperature column limits is one of the most common causes of failed rough-in inspections.
Where You Meet This in Practice
The distinction between cables and wires dictates your physical workflow on the jobsite or at the bench.
- Stripping and Prep: When terminating NM-B cable, you use a cable ripper or a careful utility knife slit to remove the outer jacket without nicking the internal wire insulation, then fold the paper separator back. When terminating THHN wire, you simply use a standard wire stripper to remove the thin insulation directly from the copper, as there is no outer jacket to deal with.
- Support and Stapling: NM-B cable must be secured within 12 inches of every electrical box and at intervals not exceeding 4.5 feet (NEC 334.30). Individual wires in conduit do not need internal support; the conduit itself is strapped to the framing every 3 to 10 feet depending on the conduit material and size.
- Box Fill Calculations: When calculating junction box fill, a cable with a unified jacket that enters the box and is clamped counts differently than individual wires entering through a conduit fitting. A single cable clamp counts as one conductor fill, while individual wires require you to count every single insulated conductor that enters the box.
- Pulling Tension: Pulling individual wires through conduit requires fish tape, wire pulling compound (lubricant), and careful tension management to avoid stretching the copper. Pushing NM-B cable through bored stud holes requires no lube, but demands care to avoid twisting the cable, which can separate the internal ground wire from the current-carrying conductors.
Frequently Asked Questions About Cables and Wires
Can I mix cables and wires in the same junction box?
Yes, you can transition from a cable to individual wires in the same junction box, provided the box is large enough to accommodate the combined box fill requirements. A common example is bringing 12/2 NM-B cable into a large metal junction box, and then transitioning to individual 12 AWG THHN wires inside EMT conduit to run to a distant motor or appliance. The box must have both NM-B cable clamps and conduit fittings, and you must calculate the box fill based on the largest conductor size present and the total number of current-carrying wires entering and leaving.
Why is my 12 AWG NM-B cable rated for 20A when the copper can handle 30A?
This is a direct result of NEC 334.80. The individual insulated wires inside a standard NM-B (Romex) cable are typically rated for 90°C, which would normally allow 12 AWG copper to carry 30A. However, the PVC outer jacket and the paper wrapping inside the cable cannot dissipate heat as effectively as a single wire in an open conduit. To prevent the jacket from melting or degrading over time, the NEC forces you to use the 60°C ampacity column for NM-B. In the 60°C column, 12 AWG copper is strictly limited to 20A, regardless of the internal wire's higher temperature rating.
Is it safe to run individual wires without conduit behind drywall?
No. Running individual THHN or THWN wires loose behind drywall, drop ceilings, or inside wall cavities is a severe code violation and a major fire/shock hazard. Single wires lack the physical protection required to survive the environment inside a wall. They can easily be nicked by drywall screws, crushed by insulation, or chewed by rodents. If you are not using a unified cable assembly like NM-B, UF-B, or MC, your individual wires must be pulled through a continuous, properly supported raceway (conduit) from box to box.
